Article

# A near-infrared imaging survey of NGC 2282

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## Abstract

We present the first near-infrared (JHK) imaging and photometry of a young cluster associated with NGC 2282, a reflection nebula in Monoceros. Our observations reveal that the cluster is centrally concentrated with a surface density that falls as r(-1) . The cluster has a radius of roughly 1.6 pc and contains at least 100 members, approximately 9% of which exhibit infrared excess emission characteristic of young stellar objects. Infrared extinction maps suggest that the cluster is located at the edge of a molecular cloud and is not heavily reddened. We construct the K-band luminosity function (KLF) of the cluster and find that it increases with magnitude up to the completeness limit of our observations (mK = 15.0). The shape of the KLF is similar to those of other young clusters, such as IC 348 and the Trapezium, which suggest that the cluster contains a significant population of pre-main sequence stars. However, at the distance of NGC 2282 (1.7 kpc) our observations are not deep enough to sample the low mass end of the cluster IMF\@. Consequently, our KLF does not provide meaningful constraints on either the age of the cluster or the duration of star formation within it. On the other hand, the low extinction toward the cluster, its location at the edge of a molecular cloud, and the relatively small fraction of infrared excess sources suggest that it is a relatively evolved cluster of pre-main sequence stars with an age of 5-10 x 10(6) years.

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... NGC 2282 (α 2000 = 06 h 46 m 50.4 s ; δ 2000 = +01 0 18 m 50 s ) is a reflection nebula in the Monoceros constellation, 3 degree away from the Mon OB2 complex and may be associated with it. The cluster parameters of NGC 2282 were first studied by Horner et al. (1997) using NIR data. Subsequently, Dutta et al. (2015) (hereafter Paper I) studied more elaborately the PMS member candidates and cluster parameters of NGC 2282 using deep optical BV IHα, UKIDSS JHK and Spitzer-IRAC data. ...
... Majority of the variables are concentrated in the core region of the cluster in the Fig. 1, while a significant scattered population is seen towards the north-east part. Such distribution suggests that the cluster might be extended towards that direction as predicted by Horner et al. (1997). ...
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We report here CCD I-band time-series photometry of a young (2-5 Myr) cluster NGC 2282 to identify and understand the variability of pre-main-sequence (PMS) stars. The $I$-band photometry, down to $\sim$ 20.5 mag, enables us to probe the variability towards the lower mass end ($\sim$ 0.1 M$_\odot$) of the PMS stars. From the light curves of 1627 stars, we identified 62 new photometric variable candidates. Their association with the region was established from H$\alpha$ emission and infrared (IR) excess. Among 62 variables, 30 young variables exhibit H$\alpha$ emission, near-IR (NIR)/mid-IR (MIR) excess or both, and they are candidate members of the cluster. Out of 62 variables, 41 are periodic variables with the rotation rate ranging from 0.2 to 7 days. The period distribution exhibits a median period at $\sim$ 1-day as in many young clusters (e.g., NGC~2264, ONC, etc.), but it follows a uni-modal distribution unlike others having bimodality with the slow rotators peaking at $\sim$ 6$-$8 days. To investigate the rotation-disk and variability-disk connection, we derived NIR excess from $\Delta$(I$-$K) and MIR excess from $Spitzer$ [3.6]$-$[4.5] $\mu$m data. No conclusive evidence of slow rotation with the presence of disks around stars and fast rotation for diskless stars is obtained from our periodic variables. A clear increasing trend of the variability amplitude with the IR excess is found for all variables.
... BFS54 is cataloged in surveys for outer Galactic H II regions (Blitz et al. 1982;Avedisova & Kondratenko 1984;Fich 1993;Kislyakov & Turner 1995) and is also listed as a reflection nebula (NGC 2282;van den Bergh 1966;Racine 1968;Kutner et al. 1980;Chini et al. 1984). BFS54 hosts a star cluster which was first studied by Horner et al. (1997) with near-IR (NIR) data. Based on the optical and NIR color-magnitude diagrams and disk fraction (∼58 percent) of stars, the age of the BFS54 star cluster was determined to be 2-5 Myr (Dutta et al. 2018) and the masses of the YSOs are 0.1-2.0 ...
... We start with summarizing the mass and radius evolution of observed young star clusters. These observations are presented in Figure 8, in particular for observed embedded clusters, classical open star clusters, young massive (starburst) clusters and associations (Lada & Lada 2003;Piskunov et al. 2008;Winston et al. 2009;Luhman et al. 2003;Andersen et al. 2006;Fang et al. 2009;Levine et al. 2006;Flaherty & Muzerolle 2008;Bonatto & Bica 2011;Horner et al. 1997;Drew et al. 1997;Hodapp & Rayner 1991;Pfalzner 2009;Portegies Zwart et al. 2010). For clarity we bin the clusters in age in intervals of t age = 1-5 Myr, 5-20 Myr, and 20-100 Myr. ...
Article
Recent observations have revealed a variety of young star clusters, including embedded systems, young massive clusters, and associations. We study the formation and dynamical evolution of these clusters using a combination of simulations and theoretical models. Our simulations start with a turbulent molecular cloud that collapses under its own gravity. The stars are assumed to form in the densest regions in the collapsing cloud after an initial free-fall times of the molecular cloud. The dynamical evolution of these stellar distributions are continued by means of direct $N$-body simulations. The molecular clouds typical for the Milky Way Galaxy tend to form embedded clusters which evolve to resemble open clusters. The associations were initially considerably more clumpy, but lost their irregularity in about a dynamical time scale due to the relaxation process. The densest molecular clouds, which are absent in the Milky Way but are typical in starburst galaxies, form massive young star clusters. They indeed are rare in the Milky Way. Our models indicate a distinct evolutionary path from molecular clouds to open clusters and associations or to massive star clusters. The mass-radius relation for both types of evolutionary tracks excellently matches the observations. According to our calculations the time evolution of the half-mass radius for open clusters and associations follows $r_{\rm h}/{\rm pc}=2.7(t_{\rm age}/{\rm pc})^{2/3}$, whereas for massive star clusters $r_{\rm h}/{\rm pc}=0.34(t_{\rm age}/{\rm Myr})^{2/3}$. Both trends are consistent with the observed age-mass-radius relation for clusters in the Milky Way.
... C. Lada et al. 1991; Hanson et al. 1997; Chini & Wargau 1998 L1630 E. Lada et al. 1991; E. Lada 1992; Li et al. 1997 Trapezium OMC2 Ali & Depoy 1995 Mon R2 Carpenter et al. 1997 Rosette Phelps & Lada 1997 NGC 281 Henning et al. 1994; Megeath & Wilson 1997 NGC 1333 Aspin et al. 1994; C. Lada et al. 1996 NGC 2264 C. Lada et al. 1993; Piche 1993 NGC 2282 Horner et al. 1997 NGC 3576 Persi et al. 1994 NGC 6334 Tapia et al. 1996 IC 348 Lada & Lada 1995 W3 IRS5 Megeath et al. 1996 S106 Hodapp & Rayner 1991 S255 Howard et al. 1997 S269 Eiroa & Casali 1995 BD 40 • 4124 Hillenbrand et al. 1995 LkHα101 Aspin & Barsony 1994 G35.20-1.74 Persi et al. 1997 H 2 O and OH maser sources Testi et al. 1994 19 IRAS sources Carpenter et al 1993 – 32 – Fig. 1.— Star field southwest of 30 Dor in the Large Magellanic Clouds, showing hierarchical structure in the stellar groupings. ...
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Young stars form on a wide range of scales, producing aggregates and clusters with various degrees of gravitational self-binding. The loose aggregates have a hierarchical structure in both space and time that resembles interstellar turbulence, suggesting that these stars form in only a few turbulent crossing times with positions that map out the previous gas distribution. Dense clusters, on the other hand, are often well mixed, as if self-gravitational motion has erased the initial fine structure. Nevertheless, some of the youngest dense clusters also show sub-clumping, so it may be that all stellar clustering is related to turbulence. Some of the densest clusters may also be triggered. The evidence for mass segregation of the stars inside clusters is reviewed, along with various explanations for this effect. Other aspects of the theory of cluster formation are reviewed as well, including many specific proposals for cluster formation mechanisms. The conditions for the formation of bound clusters are discussed. Critical star formation efficiencies can be as low as 10% if the gas removal process is slow and the stars are born at sub-virial speeds. Environmental conditions, particularly pressure, may affect the fraction and masses of clusters that end up bound. Globular clusters may form like normal open clusters but in conditions that prevailed during the formation of the halo and bulge, or in interacting and starburst galaxies today. Various theories for the formation of globular clusters are summarized.
... The core-halo structure of clusters has been studied quantitatively through azimuthally smoothed radial density profiles (Muench et al. 2003 ). Although these density profiles can be fit by power laws, King models, or exponential functions (Hillenbrand and Hartmann, 1998; Lada and Lada, 1995; Horner et al., 1997; Gutermuth, 2005 ), the resulting fits and their physical implications can be misleading . As pointed out by Hartmann (2004) , azimuthally averaged density profiles can be significantly steepened by elongation (Hartmann (2004) argues this for molecular cores, but the same argument applies to clusters). ...
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We simulate the formation and evolution of young star clusters from turbulent molecular clouds using smoothed-particle hydrodynamics and direct N -body methods. We find that the shape of the cluster mass function that originates from an individual molecular cloud is consistent with a Schechter function with power-law slopes of β = −1.73. The superposition of mass functions turn out to have a power-law slope of < −2. The mass of the most massive cluster formed from a single molecular cloud with mass Mg scales with 6.1 M0.51g . The molecular clouds that tend to form massive clusters are much denser than those typical found in the Milky Way. The velocity dispersion of such molecular clouds reaches 20km s ⁻¹ and it is consistent with the relative velocity of the molecular clouds observed near NGC 3603 and Westerlund 2, for which a triggered star formation by cloud-cloud collisions is suggested.
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The- question of the origin of stars is one of the most fundamental of astronomy. Yet, despite thousands of years of stellar observation, and early speculations by Newton and Laplace, it has only been in the latter part of the present century that the investigation of star formation has become an active discipline of astrophysical research. That the origin of stars has remained so mysterious for so long is largely due to the fact that the process of star formation has never been directly observed either with the naked eye or the with most powerful telescopes. Moreover, prior to the twentieth century neither the energy source nor the bulk composition of stars were known. Indeed, without knowledge of the physical nature of stars it was very difficult to develop an understanding of their origin. At the end of the nineteenth century, the nature of the mystery surrounding stellar origins was nicely summed up by the fictional character Huck Finn (in the book The Adventures of Huck Finn by Mark Twain) when he observed the stars in the night sky and wondered “…did they just happen or was they made?”
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We present the analysis of the stellar content of NGC 2282, a young cluster in the Monoceros constellation, using deep optical BVI and INT Photometric Hα Survey (IPHAS) photometry along with infrared (IR) data from UKIRT Infrared Deep Sky Survey and Spitzer-IRAC. Based on the stellar surface density analysis using nearest neighbourhood method, the radius of the cluster is estimated as ∼3.15 arcmin. From optical spectroscopic analysis of eight bright sources, we have classified three early B-type members in the cluster, which includes, HD 289120, a previously known B2V-type star, a Herbig Ae/Be star (B0.5 Ve) and a B5 V star. From spectrophotometric analyses, the distance to the cluster has been estimated as ∼1.65 kpc. The K-band extinction map is estimated using nearest neighbourhood technique, and the mean extinction within the cluster area is found to be AV ∼ 3.9 mag. Using IR colour–colour criteria and Hα-emission properties, we have identified a total of 152 candidate young stellar objects (YSOs) in the region, of which, 75 are classified as Class II, nine are Class I YSOs. Our YSO catalogue also includes 50 Hα-emission line sources, identified using slitless spectroscopy and IPHAS photometry data. Based on the optical and near-IR colour–magnitude diagram analyses, the cluster age has been estimated to be in the range of 2–5 Myr, which is in agreement with the estimated age from disc fraction (∼58 per cent). Masses of these YSOs are found to be ∼0.1–2.0 M⊙. Spatial distribution of the candidate YSOs shows spherical morphology, more or less similar to the surface density map.
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We report the discovery of a new young stellar cluster in the outer Galaxy located at the position of an IRAS Point Source Catalog source that has been previously misidentified as an external galaxy. The cluster is seen in our near-infrared imaging towards IRAS 04186+5143 and in archive Spitzer images confirming the young stellar nature of the sources detected. There is also evidence of subclustering seen in the spatial distributions of young stars and of gas and dust. Near- and mid-infrared photometry indicates that the stars exhibit colours compatible with reddening by interstellar and circumstellar dust and are likely to be low- and intermediate-mass young stellar objects (YSOs) with a large proportion of Class I YSOs. Ammonia and CO lines were detected, with the CO emission well centred near the position of the richest part of the cluster. The velocity of the CO and NH3 lines indicates that the gas is Galactic and located at a distance of about 5.5 kpc, in the outer Galaxy. Herschel data of this region characterize the dust environment of this molecular cloud core where the young cluster is embedded. We derive masses, luminosities, and temperatures of the molecular clumps where the young stars reside and discuss their evolutionary stages.
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We report the discovery of a small aggregate of young stars seen in high-resolution, deep near-infrared (JHKS) images towards IRAS 06345−3023 in the outer Galaxy and well below the mid-plane of the Galactic disc. The group of young stars is likely to be composed of low-mass stars, mostly Class I young stellar objects. The stars are seen towards a molecular cloud whose CO map peaks at the location of the IRAS source. The near-infrared images reveal, additionally, the presence of nebular emission with rich morphological features, including arcs in the vicinity of embedded stars, wisps and bright rims of a butterfly-shaped dark cloud. The location of this molecular cloud as a new star formation site well below the Galactic plane in the outer Galaxy indicates that active star formation is taking place at vertical distances larger than those typical of the (thin) disc.
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The young stellar cluster illuminating the W40 H II region, one of the nearest massive star-forming regions, has been observed with the ACIS detector on board the Chandra X-ray Observatory. Due to its high obscuration, this is a poorly studied stellar cluster with only a handful of bright stars visible in the optical band, including three OB stars identified as primary excitation sources. We detect 225 X-ray sources, of which 85% are confidently identified as young stellar members of the region. Two potential distances of the cluster, 260 pc and 600 pc, are used in the paper. Supposing the X-ray luminosity function to be universal, it supports a 600 pc distance as a lower limit for W40 and a total population of at least 600 stars down to 0.1 M ☉ under the assumption of a coeval population with a uniform obscuration. In fact, there is strong spatial variation in Ks -band-excess disk fraction and non-uniform obscuration due to a dust lane that is identified in absorption in optical, infrared, and X-ray. The dust lane is likely part of a ring of material which includes the molecular core within W40. In contrast to the likely ongoing star formation in the dust lane, the molecular core is inactive. The star cluster has a spherical morphology, an isothermal sphere density profile, and mass segregation down to 1.5 M ☉. However, other cluster properties, including a 1 Myr age estimate and ongoing star formation, indicate that the cluster is not dynamically relaxed. X-ray diffuse emission and a powerful flare from a young stellar object are also reported.
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Young massive clusters are as young as open clusters but more massive and compact compared with typical open clusters. The formation process of young massive clusters is still unclear, and it is an open question whether the formation process is the same as typical open clusters or not. We perform a series of $N$-body simulations starting from initial conditions constructed from the results of hydrodynamical simulations of turbulent molecular clouds. In our simulations, both open clusters and young massive clusters form when we assume a density-dependent star formation efficiency. We find that a local star formation efficiency higher than 50 % is necessary for the formation of young massive clusters, but open clusters forms from less dense regions with a local star formation efficiency of $<50$ %. We confirm that the young massive clusters formed in our simulations have mass, size, and density profile similar to those of observed young massive clusters such as NGC 3603 and Trumpler 14. We also find that these simulated clusters evolve via hierarchical mergers of sub-clusters within a few Myr, as is suggested by recent simulations and observations. Although we do not assume initial mass segregation, we observe that the simulated massive clusters show a shallower slope of the mass function ($\Gamma\sim-1$) in the cluster center compared to that of the entire cluster ($\Gamma\sim-1.3$). These values are consistent with those of some young massive clusters in the Milky Way such as Westerlund 1 and Arches.
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During the past century, astronomers have discovered that star formation is a continuous, ongoing process, occurring over the lifetime of our Galaxy and the universe. However, how stars form, still remains a mystery. Unraveling the process of stellar birth is of fundamental importance for understanding not only the origin of stars but also the origin and evolution of planets, life and even galaxies. For example, since stars are the basic constituents of galaxies, stellar formation and evolution are the engines that drive galaxy evolution. Once stars form, astronomers can use the elegant theory of stellar structure and evolution to predict the life histories of galaxies. However a comparable theory of star formation does not currently exist and therefore limits any future progress toward understanding galaxy evolution.
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Star formation is a continuous ongoing process occurring over the lifetime of our Galaxy and the universe. However understanding how stars form from their pre-natal clouds of gas and dust remains a mystery. During the last two decades we have made remarkable progress toward unraveling this mystery mainly due to advances in observational technology especially at infrared and millimeter wavelengths which allow direct observation of the sites of star birth. Such observations suggest that embedded clusters may be the fundamental units of star formation in molecular clouds. Low star formation efficiency and rapid gas dispersal make these clusters disperse to provide the field star population. Consequently embedded clusters provide important laboratories for investigating fundamental issues of star formation within our Galaxy.
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We present a photometric and spectroscopic study of the compact H ii region Sh 138 and its associated stellar cluster. The positions and BVRIJHK magnitudes are obtained for more than 400 stars over a field of about 4arcmin square centred on the H ii region. Sh 138 is excited by a cluster of young massive stars. At the cluster's very centre are at least four O-B2 stars separated by less than 4arcsec . The brightest of these, both in the visible and the near infrared, exhibits a spectrum similar to those of the more massive Herbig Ae/Be stars. This star, our No. 183, is overluminous by a factor of 2.5 in the visible and four in the near IR with respect to the O9.5 V star required to account for the ionization level of the H ii region. However star 183's position in the J - H versus H - K diagram does not indicate a near-IR excess. We suggest that this star is a young massive object belonging to a binary or multiple system. The stellar cluster associated with Sh 138 is very reminiscent of the Orion Trapezium cluster: it is centrally peaked around several massive stars, and is dense - more than 550 stars pc(-2) at its centre. The visual extinction in the cluster varies between 5 mag and more than 35 mag; large variations are observed over very small scales (for example, more than 20 mag over less than 4arcsec among the central massive stars). Based on observations done at the Observatoire de Haute Provence and the Observatoire du Pic du Midi, France, and at the Canada-France-Hawaii Telescope. The CFHT is operated by the Centre National de la Recherche Scientifique of France, the National Research Council of Canada and the University of Hawaii. Table 1 is available in electronic form at the CDS (via anonymous ftp to cdsarc.u-strasbg.fr or via http://cdsweb.u-strasbg.fr/Abstract.html) as well as at http://www-obs.cnrs-mrs.fr/matiere/sh138-t1.html.
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Over the past decade embedded clusters have become recognized as the primary sites of star formation in giant molecular clouds. Although most stars form in such systems and such systems are the progenitors of classical open clusters, their basic physical properties are not well established. Here I present a brief summary of the current status of knowledge concerning such fundamental properties as the mass function, birthrate, mortality and structure of these protocluster systems.
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We have computed the evolving ultraviolet-millimeter spectral energy distribution (SED) produced by protogalactic starbursts at high redshift, incorporating the chemical evolution of the interstellar medium in a consistent manner. Dust extinction is calculated in a novel way that is not based on empirical calibrations of extinction curves, but rather on the lifetime of molecular clouds which delays the emergence of each successive generation of stars at ultraviolet wavelengths by typically 15 Myr. The predicted rest-frame far-infrared-to-millimeter-wave emission includes the calculation of molecular emission-line luminosities (12CO and O2 among other molecules) consistent with the evolving chemical abundances. Here we present details of this new model along with the results of comparing its predictions with several high-redshift observables, namely, the ultraviolet SEDs of Lyman limit galaxies, the high-redshift radio galaxies 4C 41.17 and 8C 1435, the SCUBA submillimeter survey of the Hubble Deep Field (HDF), and the SEDs of intermediate-redshift elliptical galaxies. With our new reddening method, we are able to fit the spectrum of the Lyman limit galaxy 1512-cB58, and we find an extinction of about 1.9 mag at 1600 Å. This extinction applies to starbursts with spectral slope α in the range 0 α 1.5. The model also predicts that most Lyman limit galaxies should have a value of α inside that range, as is observed. The 850 μm flux density of a typical Lyman limit galaxy is expected to be only 0.5 mJy, and therefore the optical counterparts of the most luminous submillimeter sources in the HDF (or any other currently feasible submillimeter survey) are unlikely to be Lyman break galaxies. The passive evolution of our starburst model is also compared with Keck observations of the reddest known elliptical galaxy at z ~ 1.5 and with the SED of a typical nearby elliptical galaxy. The SED of the high-redshift elliptical is nicely matched by the starburst model with an age of 4 Gyr and the SED of the nearby elliptical galaxy with an age of 13 Gyr.
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We consider from a general point of view the problem of determining the extinction in dense molecular clouds. We use a rigorous statistical approach to characterize the properties of the most widely used optical and infrared techniques, namely the star count and the color excess methods. We propose a new maximum-likelihood method that takes advantage of both star counts and star colors to provide an optimal estimate of the extinction. Detailed numerical simulations show that our method performs optimally under a wide range of conditions and, in particular, is significantly superior to the standard techniques for clouds with high column-densities and affected by contamination by foreground stars. Comment: 20 pages; A&A in press
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This paper analyzes optical and infrared photometry of pre-main-sequence stars in the Taurus-Auriga molecular cloud. More than half of the stars in our sample have excess near-infrared emission. The near-infrared excesses correlate very well with other measures of activity, such as Ho emission, ultraviolet excess emission, millimeter continuum emission, and the presence of reflection nebulae and molecular outflows. The infrared colors and the ratio of far-infrared to bolometric luminosity display a smooth progression from the most deeply embedded protostars to optically visible T Tauri stars. Infalling envelope models account for the colors of protostars; simple disk models similarly reproduce the colors of many T Tauri stars. Both the stellar birth line and a l05 yr isochrone provide a reasonable upper envelope to the luminosity distribution of optically visible stars in the H-R diagram. Only a few stars in the cloud have apparent ages exceeding 2- 3 × 106 yr, as derived from detailed stellar evolution calculations. The distribution of stars in the H-R diagram indicates that the cloud has formed stars at a roughly constant rate for the past 1-2 × 106 yr. Analyses of the J and K-luminosity functions support this conclusion. Within the uncertainties, the observed mass distribution for optically visible stars agrees with a Miller-Scalo initial mass function. Source statistics imply a lifetime of 1-2 × l05 yr for the typical protostar in Taurus-Auriga. There is no evidence, however, that these sources lie on the stellar birth line. Indeed, the protostellar luminosity function is essentially identical to the luminosity function derived for optically visible T Tauri stars in the cloud. These results provide some support for the evolutionary sequence-embedded protostar → T Tauri star with a circumstellar disk → T Tauri star without a circumstellar disk-currently envisioned in standard models of low-mass star formation. Source statistics and infrared color-color diagrams demonstrate that pre-main-sequence stars develop bluer colors and display less evidence for circumstellar material with time. The data show little evidence, however, for the luminosity evolution expected along the proposed evolutionary sequence. Time-dependent accretion during the infall phase may account for the low luminosity of Taurus-Auriga protostars; this hypothesis requires more tests.
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Broad and narrow band 1-4-micron and red CCD images of the star formation complex associated with the 'cometary' nebula GM 24 are presented together with a medium-resolution 2.16-micron spectrum of the most conspicuous source. A young IR cluster of more than 23 stars is embedded in the densest and hottest parts of the dense molecular cloud. Most of the embedded stellar objects show large infrared excesses at lambda equal to or greater than 2.2 microns. The obscuration toward the center of the cluster (Irs3) is found to be AV = 55. A compact 'blister' H II region is associated with Irs3.
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IRIM on the KPNO 1.3-m telescope is used to map an 8-ft x 12-ft area, corresponding to a 1.9- x 2.8-pc field centered on the Herbig Ae/Be star, LkH-alpha 101, in the near-infrared J and K bandpasses. The discovery of an embedded cluster of more than 100 young stellar objects within this area is reported. The present completeness limits are estimated at 16.5 mag at J and 14.5 mag at K. Four types of cluster members, defined by their location in the K versus J-K color magnitude diagram, are identified. The embedded cluster population of the LkH-alpha 101 cloud core is found to consist of 16 Class I sources, 39 probable Class I sources, 40 young embedded low-mass PMS stars, and 46 young, embedded brown dwarf candidates. From the relative numbers of sources of each type, an upper limit for the accreting protostar phase of 400,000 yr is estimated. The spatial distribution of the sources in the cloud core is highly nonuniform, such that the youngest sources are the most centrally concentrated.
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J H K images and photometry of sources associated with the HII region NGC 3576 are presented together with an 8-13 micrometers CVF spectrum of the most conspicuous source in the region, Irs 1. A total of 135 sources were detected in K in the surveyed area of 340 x 340 square arc seconds. From the spatial distribution and photometry we deducted the presence of a very young massive star cluster deeply embedded in the molecular cloud. Comparison of the apparent K luminosity distribution of the cluster region with that of surrounding areas, suggests that the cluster contains more than fifty members brighter than K approxmiately = 13, the majority of which (greater than 70%) show significant IR excess. A steep gradient in the near-infrared color is seen from the north-east to the south-west of the cluster indicating that the formation of stars started further out of the cloud and as the older population expands, it has progressed to deeper parts into the cloud where the more recent events are taking place. In the core, the star formation efficiency is found to be 0.19. The distance to this cluster is estimated to be 2.4 kpc.
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We have conducted near-IR spectroscopic observations of young stellar objects (YSOs) in the ρ Ophiuchi dark cloud (L1688), many of which are optically invisible. We present equivalent widths of K-band spectral features for 34 YSOs, and we determine their spectral types, stellar luminosities, and extinctions. A subset of 19 YSOs with small near-IR excesses are placed in the H-R diagram, and we estimate their masses and ages via comparisons to pre-main-sequence (PM S) stellar models. These objects are apparently very young, low- mass, PMS stars. The median stellar age is about 3 × 105 yr according to modern PMS models. Most stars in the cloud core are certainly less than 3 × 106 yr old and probably less than 106 yr old. The sample spans a mass range 0.1-2.5 Msun, with a median mass of 0.4-0.5 M0. Combining these results with previous photometric studies, we find the distribution of masses in the ρ Oph cloud core to be consistent with the initial mass function of field stars in the solar neighborhood. In addition, we find that this IR population is significantly younger and more coeval than the optically visible weak-line T Tauri stars found in a much larger region encompassing the core. This study quantitatively confirms previous results that suggested that a very young, low-mass cluster is forming in the ρ Oph cloud core. The technique of IR spectroscopy has proven valuable in compiling a detailed picture of star formation in this deeply embedded cluster of young stars.
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This is a catalog of 242 molecular cloud complexes which are associated with optical H II regions. CO observations were made toward all but five of the H II regions in the Sharpless catalog and toward 62 additional suspected H II regions, 33 of which are previously uncataloged. Radial velocities are tabulated for each molecular cloud complex found to be associated with an H II region. The CO antenna temperature and line width are given for the most intense CO line seen toward each source. The catalog also summarizes previous CO observations as well as the optical distances to the stars exciting the H II regions. Radio-quiet H II regions (those with 1.4 GHz flux densities less than 100 mJy) are found to be well correlated with objects having no associated CO. A list of kinematically distinct complexes is tabulated to facilitate investigations of the motions of the complexes.
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About one-eighth of a well-sampled 850 deg**2 region of Orion and Monoceros shows CO emission coming from either local clouds (d less than 1 kpc) lying as much as 25 degree from the galactic plane or from more distant objects located within a few degrees of the plane. Local giant clouds associated with Orion A and B have enhanced temperatures and densities near their western edges possibly due to compression by a high pressure region created by approx. 10 supernovae that occurred in the Orion OB association. Another giant cloud associated with Mon R2 may be related to the Orion clouds. Two filamentary clouds (one possibly 300 pc long but 10 pc wide) may represent a new class of object. An expanding ring of clouds concentric with the H II region ionized by lambda Ori probably constitute fragments of the original cloud from which lambda Ori formed; the gas pressure of the H II region and the rocket effect probably disrupted the original cloud.
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Results are reported of a deep near-IR imaging survey which samples 0.77 sq deg of the L1641 star-forming complex and reaches 5 sigma limits at J (1.25 micron,), H (1.65 micron), and K (2.2 microns) of 16.8, 15.8, and 14.7 mag, respectively. A population of about 1500 stars spread throughout the cloud, seven small aggregates comprised of 10-50 stars whose typical projected surface densities exceed that of the distributed population by factors of 4-10, and a heretofore unknown, partially embedded dense cluster comprised of about 150 stars are identified. Analysis of the distribution of (J-H), (H-K), and (R-I) colors for these groups suggest that, in all cases, the stellar populations are dominated by solar-type PMS stars which appear to contain a mix of objects analogous to weak-line T Tauri stars. The present observed luminosity functions are based on reddening-corrected J-band magnitude.
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Results are presented from a systematic search for CO emission from regions around 34 young open clusters in the outer Galaxy. The clusters have well-determined distances ranging from about 1 to 5 kpc and ages not greater than about 100 Myr. It was found that some moderately young clusters have no associated CO emission. All the surveyed clusters younger than about 5 Myr have associated with them at least one molecular cloud more massive than 10,000 solar mass, while the molecular clouds associated with clusters older than about 10 Myr are not more massive than a few thousands solar masses. It was also found that molecular clouds are receding from young clusters at a rate of about 10 km/sec, and that they seem to be destroyed by their interaction with the stars. Sites of ongoing star formation were identified in a number of clouds associated with young clusters.
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We have conducted an imaging survey of 1.4 sq pc of the Rho Ophiuchi dark cloud in the J, H, and K near-infrared photometric bands. Approximately 337 of our 481 detected sources are associated with the cloud, and we estimate that 48 percent of these have near-infrared excesses, indicative of disks or circumstellar material surrounding these young stellar objects (YSOs). The K-band luminosity function is significantly different in different regions of our survey area, suggesting that YSOs in these regions have different ages or mass functions. We estimate that the entire survey area has a high star-formation efficiency, at roughly 23 percent. Finally, our many newly detected sources provide a relatively large, uniformly sensitive sample of objects for study at longer wavelengths to better determine true source luminosities and evolutionary lifetimes.
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The spatial distribution of stars in the Milky Way is modeled using an exponential disk (from a previous model) with the addition of a de Vaucouleurs spheroid. The model is compared to 2-2.5 µm star counts and surface-brightness data and to IRAS 12 µm source counts. With an R1/4 spheroid and Ro/Re ~ 3, the model fits the data inside 10° from the Galactic center fairly well but is too extended in latitude, suggesting the bulge is flattened by approximately 4:1. Extrapolation of this bulge component to the solar neighborhood yields a local spheroid-to-disk population ratio of 1:150, well above the accepted value. Either the local density of stars in the spheroidal component is higher than expected or the inner bulge does not maintain a simple R1/1 form as far out as the distance of the Sun. A much shorter bulge scale length would remove the discrepancy with the solar-neighborhood density but is difficult to reconcile with observations of the inner bulge. Comparison of the model with IRAS 12 µm source counts was limited by our uncertainty in the stellar population sampled at 12 µm by IRAS. However, the data are consistent with a flattened bulge. As shown by others, the IRAS data also suggest that some fraction of the bulge population has a considerable, but not extreme, infrared excess.
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Initial results of an extensive IR imaging survey of the young cluster NGC 2264 and a nearby galactic control field are presented. A large portion of the cluster was imaged in each of the three standard near-IR colors (J, H, and K) with an IR array camera. On the basis of a comparison of these observations, the size of the cluster population is estimated, and the nature of its members is investigated. The cluster is found to contain 360 +/- 130 members. The slope of the K luminosity function of the cluster is significantly steeper than that expected for a cluster of ZAMS stars, and appears to flatten out or turn over at an apparent K magnitude of roughly 13.0-14.0. An analysis of the JHK color-color diagrams of the cluster and control fields shows that approximately 170 sources observed toward the cluster have colors indicative of intrinsic excess IR emission.
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An 8′ × 12′ area, corresponding to a 1.9 × 2.8 pc field centered on the Herbig Ae/Be star, LkHα 101, has been mapped in the near-infrared J and K (1.2 and 2.2 μm) bandpasses at 1″.35 pixel-1 resolution using IRIM on the KPNO 1.3 m telescope. We report the discovery of an embedded cluster of more than 100 young stellar objects within this area. We estimate our completeness limits to be 16.5 mag at J and 14.5 mag at K, although sources as faint as K = 16.0 and J= 17.5 were detected. We have identified four types of cluster members, defined by their location in the K versus J-K color magnitude diagram: (1) accreting protostars (the Class I sources of Lada and Wilking, which are the sources with J-K > 4), (2) probable accreting protostars (sources with 3 < J-K < 4), (3) embedded, low-mass, pre-main-sequence (PMS) stars younger than 106 yr (sources with K < 13.4 and 2 < J-K < 3), and (4) young (≈ 106 yr), embedded, brown dwarf candidates (sources with K > 13.4 and 2 < J-K < 3). According to this classification scheme, the embedded cluster population of the LkHa 101 cloud core consists of 16 Class I sources, 39 probable Class I sources, 40 young, embedded, low-mass, PMS stars, and 46 young, embedded, brown dwarf candidates. Only eight of the latter have J counterparts. From the relative numbers of sources of each type, we estimate an upper limit for the accreting protostar phase of 4 × 105 yr. The spatial distribution of the sources in the cloud core is highly nonuniform, such that the youngest sources are the most centrally concentrated. In keeping with this trend, it appears that the most massive star of the cluster, LkHα 101, is probably the youngest. There is also a hint of mass segregation in the cluster, such that ≈58% of the brown dwarf candidate sources are concentrated in 17% of the observed area. The star formation efficiency (SFE) in the central 2′ × 2′ of this cluster is ≈18%. In the densest parts of the cluster, the observed stellar density places a limit of 1 star per ≈(2.7 × 1017 cm)3 for the initial volume of gas which can eventually form a star. This volume corresponds to initial masses of 0.045, 0.45, and 4.5 M⊙ for uniform densities of pH2 = 103, 104, and 105 cm-3, respectively. We find an upper limit of 4 × 105 yr for the protostellar collapse phase in this cluster and infall rates of 1.1 × 10-7, 1.1 × 10-6, and 1.1 × 10-5 M⊙ yr-1, corresponding to the above-quoted stellar masses if the collapse time is assumed independent of the final stellar mass.
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We present the results of an extensive near-infrared (JHK) imaging survey of the NGC 1333 star forming region. Our survey covers an area more than 4 times larger than the previous imaging survey of this cloud reported by Aspin et al. [A&AS, 106, 165 (1994)] and is sufficiently sensitive to render an accurate census of the embedded stellar population in the cloud. We detected 275 sources with m(K) < 14.5 magnitudes within the 432 square arcminute region surveyed. The spatial distribution of these sources is found to be significantly clustered with approximately 45% of the sources contained within two adjacent stellar clusters which together occupy an area less than 16% of the entire region surveyed. From comparison with observations of nearby control fields we estimate that roughly 143 of the sources detected over the entire region are physically associated with or embedded in the NGC 1333 molecular cloud. The majority (94 or 66%) of these are also found within the boundaries of the double cluster. From analysis of the JHK colors of the stars in the NGC 1333 region we find that roughly 30% of all sources display detectable infrared excess. This corresponds to more than 50% of all the sources embedded in the cloud. The infrared-excess sources display a higher degree of clustering than the general K band source population with roughly 75% of all excess sources located within the boundaries of the double cluster. Moreover, infrared excess sources comprise roughly 60% of the sources within the cluster suggesting that it is extremely young (less than or equal to 1-2 x 10(6) years). We also find that the stars in the cluster suffer from significant amounts of differential extinction indicating that the cluster is deeply embedded in molecular material. We construct the K luminosity function (KLF) for the NGC 1333 cloud and compare it to that of the control fields off the cloud. We find that background field stars dominate the KLF at faint magnitudes and that the vast majority of stars associated with the cloud are brighter than our completeness limit. We use the infrared colors to de-redden the stars in the cluster and construct their (de-reddened) K luminosity function. The resulting KLF of the NGC 1333 cluster is compared to the KLFs of the Trapezium cluster in Orion and IC 348, a rich young cluster located within the same GMC complex as NGC 1333. The KLFs of NGC 1333 and the Trapezium cluster are found to be very similar in shape and extent while the KLFs of NGC 1333 and IC 348 are found to differ. We attribute this to the effects of luminosity evolution in these young clusters and suggest that the NGC 1333 cluster is similar in age to the Trapezium cluster (i.e., less than or equal to 10(6) years) and significantly younger than IC 348 (i.e., <5-7X10(6) years). This is consistent with both the large population of excess sources and outflow sources [Hodapp & Ladd (ApJ, 1995) (in Press)] contained in the NGC 1333 cluster. However, we derive a star formation rate of 4 x 10(-5) M. yr(-1) for NGC 1333 which is essentially the same as that characterizing IC 348 and nearly an order of magnitude lower than that found for the Trapezium cluster. The two most active star forming regions in the Perseus cloud complex have now been throughly surveyed with comparable sensitivities at near-infrared wavelengths. Both regions have produced embedded stellar clusters, which are forming stars at a similar rate, but which appear to be in very different stages of evolution. (C) 1996 American Astronomical Society.
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High-sensitivity IRAS coadded survey data, coupled with new high-sensitivity near-IR observations, are used to investigate the nature of embedded objects over an 4.3-sq-pc area comprising the central star-forming cloud of the Ophiuchi molecular complex; the area encompasses the central cloud of the Rho Ophiuchi complex and includes the core region. Seventy-eight members of the embedded cluster were identified; spectral energy distributions were constructed for 53 objects and were compared with theoretical models to gain insight into their evolutionary status. Bolometric luminosities could be estimated for nearly all of the association members, leading to a revised luminosity function for this dust-embedded cluster. 53 refs.
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Measurements from 1 to 13 mum are reported for omicr Sco and for stars in the galactic center. The interstellar extinction law toward these sources and toward VI Cyg No. 12 is the same from 1 to 13 mum. An improved estimate of the extinction law beyond 3 mum is presented, including an improved ratio of total to selective extinction, R &ap; 3.09±0.03, and an improved ratio of total extinction to optical depth in the 10 mum silicate absorption, Av/tauSi = 16.6±2.1.
Article
The relations between colors of the JHKL systems of SAAO, ESO, CIT/CTIO, MSO, AAO and Arizona have been examined and linear relations derived to enable transformation between the J-K, J-H, H-K, and K-L colors in the different systems. A homogenized system, essentially the Johnson-Glass system is proposed and its absolute calibration derived based on the Bell model atmosphere fluxes for Alpha Lyrae. The homogenized colors of the standard stars were used to derive intrinsic colors for stars with spectral types between B7V and M6V, and G7III and MSIII. The JHKL passbands of the MSO IR system, derived from measured filter passbands and estimated atmospheric transmission values, were used to compute synthetic colors from relative absolute fluxes of some stars (including the sun). The reasonable agreement with the standardized JHKL colors indicates that these passbands can be adopted as representing the homogeneous system, and used to compute broad band IR colors from theoretical or observed fluxes. The passbands of other IR systems were similarly estimated from published data, and the synthetic colors were intercompared using black-body and stellar fluxes. These passbands were then adjusted in wavelength to produce agreement with the observed relations between different systems, enabling the effective wavelengths of the different natural systems to be established. Better effective wavelengths could be determined were spectrophotometry available for the very red stars with known broad band colors. The full text of this paper is published in Bessell and Brett (1988).
Article
The early-type stars exciting 220 diffuse nebulae were found from extensive literature survey, as well as from extensive study of the magnetic tape catalogues of the Soviet astronomical data center. The photometric distances of the stars and nebulae were determined from UBV photometry and spectral classification of the stars. The transformation of the spectral classification into absolute visual magnitudes were made by means of the averaged calibration constructed by the authors. The distribution of the diffuse nebulae in the galactic plane shows the internal, local and two external arms of the Galaxy.
Article
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Article
We present JHKnbL and 'ice' images of the Serpens molecular cloud core. A total of 163 stellar objects are detected, which represents an increase of about 140 objects with respect to the number of stars detected at wavelengths shorter than 1 micron. By using different criteria, 51 stars are identified as members of the Serpens young stellar cluster. The rest of the objects remain unidentified. They could be either background stars or additional embedded Serpens objects of very low luminosity. In a near-infrared color-magnitude diagram most of the identified Serpens stars are bright and red and seem to be reddened T Tauri-like stars. A turnover at K of about 13.5 mag in the K-distribution and also in the empirical luminosity function of the Serpens cluster occurs in the case that a significant part of the unidentified stars are indeed background stars. A lower limit of about 450/cu pc of the stellar density and a SFE in the range 8-28 percent are estimated. Finally, the infrared images reveal several new cometary nebulae embedded in the Serpens core.
Article
Radio continuum observations of 109 optically visible Galactic H II regions were obtained with the Very Large Array. Contour plots of the images of those that were well resolved and tables of radio continuum flux density for all detected objects are presented. The measured total flux densities matched well with the results of single-dish surveys, and thus it is expected that the problem of missing 'zero-spacing flux' has not contributed greatly to the morphologies seen in the resulting images. The relative numbers of objects of each morphological type were very similar to the numbers seen in the survey of ultracompact H II regions by Wood and Churchwell (1989) even though the objects in this new survey are on average 50 times larger (in radius) and therefore presumably more evolved.
Article
We present the results of a sensitive near-infrared (JHK) imaging survey of the young cluster IC 348 and a nearby control field. From comparison of the cluster and control field observations we estimate that 380 sources, the majority of the stars observed in the cluster field, are members of the cluster. The spatial density of these stars is found to be significantly larger than that typical of the classical open clusters but comparable to that which characterizes young embedded clusters such as NGC 2024 and the Trapezium. Overall, we find the surface density distribution of stars in IC 348 to be centrally concentrated and to decrease inversely with the distance from the inner (r approximately 0.1 pc) to the outer (r approximately 1.0 pc) regions of the cluster. In detail the stellar surface density distribution of this cluster exhibits significant structure. Roughly half the stars are contained within a central subcluster with a radius of 0.5 pc. Outside this half-mass radius we identify eight small subclusters which contain 10-20 stars and have radii 0.1-0.2 pc in extent. We construct the K luminosity function (KLF) for IC 348 and find it to increase with magnitude in a nonlinear, power-law fashion in the range 8 less than or equal mK less than or equal 11 mag. We construct evolutionary models for the near-infrared luminosity functions of young (taucl less than or equal 107 yr) star clusters containing pre-main-sequence stars. We find that the KLFs of very young synthetic clusters evolve in a systematic and predictable manner as the clusters age. In general we find that the luminosity functions of young clusters broaden with age. For coeval models (i.e., tausf much less than taucl) the slopes of the power-law portion of the KLFs exhibit significant variation with time, while models with continuous star fromation (i.e., tausf approximately taucl) maintain more or less constant slopes as they age. From comparison of four models with our observations of IC 348 we conclude that star formation in IC 348 has been a continuous process over the last 5-7 X 106 yr and that the overall rate of star formation and the rate of star formation as a function of mass has been constant over the cluster lifetime. From a comparative analysis of published observations of the Trapezium cluster with our models and observations of IC 348, we find that the underlying mass function of both clusters is similar to the IMF for field stars down to the hydrogen burning limit with little evidence for a significant population of single, lower mass objects (brown dwarfs). In addition we also find that despite the similarities in their mass functions, stellar densities, and sizes, IC 348 and the Trapezium have been characterized by significantly different rates of star formation over their lifetimes. The rate of star formation in the younger trapezium cluster has been a factor of 20 greater than that in IC 348. Finally, analysis of the JHK colors of the stars in IC 348 reveals that approximately 20% of the cluster sources are infrared excess sources.
Article
Classical and weak-line T Tauri stars, Hernig AeBe stars, infrared protostars, and classical Be stars are found to occupy well-defined and different regions in the JHK infrared color-color diagram, with some overlap. The location of each of these young stellar objects on the diagram is determined to a large extent by its evolutionary state. The pattern of colors displayed by classical T Tauri stars with near-infrared excesses can be reproduced very well with standard circumstellar disk models. But standard disk models cannot account for the large infrared excesses and the distribution of AeBe stars unless (1) models contain central holes and (2) the physical parameters of the disks and central stars always conspire to produce temperatures in the narrow range between 2000-3000 K at the inner edges of the disks. The fact that this temperature range is near the sublimation temperature of interstellar dust suggests that dust is the primary source of opacity at infrared wavelengths in circumstellar disks around AeBe stars.
Article
The paper presents 2.6 mm wavelength CO and (C-13)O observations of 130 molecular clouds associated with reflection nebulae. Enhanced CO emission was found in the vicinity of the illuminating star in about half the objects studied. There is a tendency for the CO peak to be slightly displaced from the star. Many examples of peaks that appear to result from heating of the cloud by the nearby star are found, while others appear to be associated with independent concentrations of material.
Article
The author continues the description of his unification and reparametrization of results of various authors on near-infrared photometry, by deriving intrinsic colours and absolute calibration values for the photometric system described in a previous paper. The author also supplies a representative set of interstellar extinction parameters. Comparisons between the observed stellar colours and black bodies as well as current stellar models are included when helpful or illustrative. The present series of papers enhances the Arizona photometric system as defined by Johnson in the early nineteen-sixties.
Article
A K-band image, near-infrared photometry, and I-band polarization data on the bipolar H II region S106 are presented. When a distance of 600 pc is assumed, S106 is found to be associated with an 0.3-pc radius protocluster of about 160 stars embedded in the molecular cloud surrounding S106. Only a few of the stars in the present cluster show reflection nebulosity surrounding them, which is interpreted as a sign of very young age. An age estimate of 1 x 10 to the 6th - 2 x 10 to the 6th yr is derived for the cluster. The K-band brightness function in this cluster rises smoothly to the completeness limit of 14.0 mag. Only within the last 100,000 yr has a star formed that is sufficiently massive to remove significant amounts of gas from the protocluster and end further star formation there. Polarization data on embedded and background stars near S106 confirm earlier results about the magnetic field orientation in its parent molecular cloud.
Article
Infrared imaging of embedded clusters provides new information on the frequency with which circumstellar accretion disks are found around extremely young stars spanning the mass range 0.1 < M ⊙ < 1: at ages t ≪ 1 Myr, the fraction of stars in this mass range surrounded by disks approaches 100%. Infrared photometric surveys of optically-revealed young stellar clusters and associations provide important new constraints on the lifetimes of circumstellar disks surrounding stars of differing mass. By an age t ∼ 1 Myr, stars with masses M > 2 M ⊙ show no evidence of infrared excesses of a magnitude consistent with the presence of circumstellar accretion disks. However, for stars with M < 0.5 M ⊙, accretion disks may persist for ages in excess of 10 Myr. New techniques promise major advances in our ability to trace the evolution of disks beyond the accretion phase. ISO should have sufficient sensitivity in the mid-and far- infrared to detect (1) emission from the outer regions of disks in transition between massive (M ≫ 0.01 M ⊙), optically thick accretion disks and postaccretion structures; and (2) emission from "secondary" disks, analogous to that surrounding β Pictoris. The recent discovery of small (100 < r < 1000 AU) regions of ionized gas surrounding young, low mass stars located in clusters associated with O stars may provide another tool for identifying candidate post-accretion disks. Recent observations suggest that although these regions in most cases appear to be alternative "signposts" for accretion disks manifest as well through their infrared signatures, some appear to be associated with pre-main sequence stars which lack near-infrared excesses. Stars in this latter group may be surrounded by gas-dominated disks, in which nearly all solid material is in the form of bodies much larger than micron-size grains.
Article
The Green Bank 4.85 GHz (λ ~ 6 cm) survey was made with the NRAO seven-beam receiver on the (former) 91 m telescope during 1986 November and 1987 October. The final set of sky maps covering the declination band 0^deg^ < δ < +75^deg^ was constructed with data from both epochs. Its noise and position errors are nearly a factor of2^1/2^ smaller than in the epoch 1987 maps, from which the 87GB catalog of 54,579 sources stronger than S ~ 25 mJy was extracted. Therefore, we used the new maps to make the GB6 catalog of 75,162 discrete sources with angular sizes φ <= 10.5' and flux densities S >= 18 mJy. This catalog is available in machine-readable versions with either B1950 or J2000 positions and as a printed book with B1950 positions. The GB6 weighted differential source counts S^5/2^n(S) between 18 mJy and 7 Jy agree well with evolutionary models based on independent data.
Article
Consideration is given to results of a near-infrared (J, H, and K bands) and CS (J = 2-1) survey of 20 star-forming regions selected from the IRAS Point Source Catalog that lie in the second and third quadrants of the Galaxy. Data obtained show that nineteen of the 20 IRAS sources contain a significant number of stars in excess of that observed in the reference fields and, undoutedly, each are newly formed clusters. To a limiting magnitude of 15.5 mag, the number of observed cluster members varies from 15 to 91. Radio continuum, FIR, and NIR observations confirm the notion that high-mass, B type stars are forming within these clusters. The CS observations revealed dense cores in all 20 IRAS sources, with the core's masses ranging from 8 to 270 solar masses. The CS cores exhibit a variety of cloud morphologies.
Article
A 256 x 256 near-IR camera at NOAO to image a large portion of the M17 star formation complex. A mosaic image of total size 9 x 9 arcmin is constructed in each of three infrared colors-J, H, and K. The observations determine the size and extent of the obscured cluster of stars which excites the powerful H II region and heats the luminous dust in the adjacent molecular cloud. More than 100 OB stars are found in a clustering several parsecs in diameter within an obscured portion of the H II region. The K luminosity function of this cluster differs significantly from that of the field stars, and has the form expected from the Salpeter initial mass function. The present level of star formation activity in the adjacent molecular clouds M17 SW is found to be considerably lower than that which produced the OB cluster in the H II region. Most of the cluster members are found to have infrared colors which are not consistent with those of purely reddened stellar photospheres. These stars that display IR excesses may be surrounded by circumstellar disks.
Article
In this paper we describe a powerful method for mapping the distribution of dust through a molecular cloud using data obtained in large-scale, multiwavelength, infrared imaging surveys. This method combines direct measurements of near-infrared color excess and certain techniques of star counting to derive mean extinctions and map the dust column density distribution through a cloud at higher angular resolutions and greater optical depths than those achieved previously by optical star counting. We report the initial results of the application of this method to a dark cloud complex near the cluster IC 5146, where we have performed coordinated, near-infrared, JHK imaging and (13)CO, C(18)O, and CS millimeter-wave, molecular-line surveys of a large portion of the complex. More than 4000 stars were detected in our JHK survey of the cloud. Of these, all but about a dozen appear to be field stars not associated with the cloud. Star count maps at J band show a striking and detailed anticorrelation between the surface density of J-band sources and CO and CS molecular-line emission. We used the (H-K) colors and positions of nearly 1300 sources to directly measure and map the extinction and thus trace the dust column density through the cloud at an effective angular resolution of 1 min .5. We report an interesting correlation between the measured dispersion in our extinction determinations and the extinction. Modeling this relation indicates that effects of small-scale cloud structure dominate the uncertainties in our measurements. Moreover, we demonstrate that such observations can be used to place constraints on the nature of the spatial distribution of extinction on scales smaller than our resolution. In particular, we show that models in which the dust is distributed uniformly or in discrete high-extinction clumps on scales smaller than (1 min .5) are inconsistent with the observations. We have derived extinctions at the same positions and at the same angular resolution (1 min .7) as our molecular-line observations. This enabled a direct comparison of (13)CO, C(18)O, and CS integrated intensities and column densities with A(sub V) for more than 500 positions in the cloud, corresponding to a range in A(sub V) between 0 to 32 mag of extinction. We found the integrated intensities of (13)CO, C(18)O, and CS to be roughly linearly correlated with extinction over different ranges of extinction. However, for all three molecules we find the scatter in the observed relations to be larger than can be accounted for by instrumental error, suggesting that there are large intrinsic variations in the abundances or excitation of the molecules through the cloud. Mean abundances for all the molecules relative to hydrogen were directly derived from the data. The ratio of (13)CO to C(18)O abundances was found to be significantly higher than the terrestrial ratio in regions where extinction is less than 10 mag. In the same region, the dispersion in the abundance ratio is also found to be very large, suggesting that the abundances of one or both molecules are very unstable even at relatively large cloud optical depths. Beyond 10 mag of extinction the abundances of both species appear very stable with their ratio close to the terrestrial value.
Article
We present high spatial resolution (0.35 and 0.65 arcsec full width at half maximum (FWHM)) near-infrared images of the central 0.2x0.2 pc of the Trapezium Cluster in the Orion Nebula, centered on the Trapezium OB stars. These images provide the most complete census of stars in this region, and we give accurate positions and near-infrared (2.1 microns) magnitudes for 123 stars. After accounting for line-of-sight projection, we estimate a stellar density for the cluster of approximately 4.7 X 10(exp 4) stars per cubic parsec in the approximately 0.1 pc diameter core. We identify stellar counterparts to virtually all the known dense knots of ionized gas seen at optical and radio wavelengths, strongly supporting the hypothesis that many are either intrinsic stellar radio emitters or circumstellar disks around stars, and not simply dense clumps of gas and dust. We derive approximate masses for the stars associated with the compact knots, finding that the majority are relatively low mass.
Article
A 2.2 micron survey of a significant portion of the L1630 molecular cloud was carried out using the NOAO infrared array camera on the Kitt Peak 1.3 m telescope. This survey is estimated to be complete to 13th magnitude at K, which corresponds to a main-sequence star of about 0.6 solar mass. Therefore this survey has provided a sensitive census of the young stellar objects within this molecular cloud. As a result, 912 sources having m(K) less than 13.0 have been detected. It is estimated that 50 percent of these sources are associated with the molecular cloud. These sources are not distributed uniformly throughout the surveyed region but rather appear grouped or clustered. In fact, four embedded infrared clusters were identified. These clusters contain at least 58 percent but possibly as much as 96 percent of the total number of sources associated with this molecular cloud suggesting that the dominant mode of star formation in this region is in clusters.
Article
Current theories for the origin of stellar masses and the initial mass function are surveyed, with emphasis on the formation of stars in clusters, and the possibility that several different formation mechanisms are at work in the Galaxy. Models are presented for the evolution of the 2.2-micron premain-sequence luminosity function of a young coeval star cluster, and the implications the results have for the conversion of an observed luminosity function into the underlying mass function are discussed. A description of theories which allow spectral energy distributions of young stellar objects to be used as diagnostics of their evolutionary states is given. The question of the existence of substellar objects (brown dwarfs) in nearby young clusters and regions of star formation is addressed.
Article
Star counts are discussed as an aid in studies of galactic structure. Iterative computations using a method-of-color-bins to match numerical models with observational data are described. The star counts provide bounds for the luminosity function and the color-magnitude diagram and allow spheroid normalization in a copycat Galaxy model which posits a galaxy with a Population I disk and a Population II spheroid. Good agreement has been obtained between star counts predicted by the model and observed distributions, including color and magnitude ranges and direction. Modifications being considered for the model include consideration of the halo, a galactic bulge and a thick disk. Several galactic structural features which have yet to be modeled and which may be tractable to the copycat model corrected with star counts are identified.
Article
We present a near-infrared survey of approximately 2 sq deg covering parts of L1537, L1538, and Heiles cloud 2 in the Taurus-Auriga molecular cloud. Although this study is more sensitive than previous attempts to identify pre-main sequence stars in Taurus-Auriga, our survey regions contain only one new optically visible, young star. We did find several candidate embedded protostars; additional 10 micrometer photometry is necessary to verify the pre-main sequence nature of these sources. Our results--combined with those of previous surveys--show that the L1537/L1538 clouds contain no pre-main sequence stars. These two clouds are less dense than the active star formation sites in Taurus-Auriga, which suggests a cloud must achieve a threshold density to form stars.