S. E. Dahm

W. M. Keck Observatory, Hilo, Hawaii, United States

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Publications (21)80.75 Total impact

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    ABSTRACT: We present results from our ongoing radial velocity search for planets in open clusters. The occurrence rate of 2 planets among 53 F- and G-dwarfs surveyed in the metal-rich Praesepe open cluster, when corrected for incompleteness, indicates a short-period giant planet frequency of 3.96 (+5.02)(-2.51)%, which is consistent with the hot Jupiter frequency around metal-rich field stars; contrary to previous suggestions, giant planet formation and migration may not be inhibited in open clusters. Moreover, these discoveries demonstrate that giant planets can migrate to small orbital separations within 600 Myr. We also show results from a more complete survey of the Hyades than was previously carried out and our initial investigation of Alpha Persei. Finally, we briefly discuss the potential impact of planets in clusters on theories of giant planet migration.
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    ABSTRACT: We present the first near-IR scattered light detection of the transitional disk associated with the Herbig Ae star MWC 758 using data obtained as part of the Strategic Exploration of Exoplanets and Disks with Subaru, and 1.1 micron HST/NICMOS data. While sub-millimeter studies suggested there is a dust-depleted cavity with r=0.35, we find scattered light as close as 0.1 (20-28 AU) from the star, with no visible cavity at H, K', or Ks. We find two small-scaled spiral structures which asymmetrically shadow the outer disk. We model one of the spirals using spiral density wave theory, and derive a disk aspect ratio of h ~ 0.18, indicating a dynamically warm disk. If the spiral pattern is excited by a perturber, we estimate its mass to be 5+3,-4 Mj, in the range where planet filtration models predict accretion continuing onto the star. Using a combination of non-redundant aperture masking data at L' and angular differential imaging with Locally Optimized Combination of Images at K' and Ks, we exclude stellar or massive brown dwarf companions within 300 mas of the Herbig Ae star, and all but planetary mass companions exterior to 0.5. We reach 5-sigma contrasts limiting companions to planetary masses, 3-4 MJ at 1.0 and 2 MJ at 1.55 using the COND models. Collectively, these data strengthen the case for MWC 758 already being a young planetary system.
    The Astrophysical Journal 12/2012; 762(1). DOI:10.1088/0004-637X/762/1/48 · 6.28 Impact Factor
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    ABSTRACT: We present the discovery of two giant planets orbiting stars in Praesepe (also known as the Beehive Cluster). These are the first known hot Jupiters in an open cluster and the only planets known to orbit Sun-like, main-sequence stars in a cluster. The planets are detected from Doppler shifted radial velocities; line bisector spans and activity indices show no correlation with orbital phase, confirming the variations are caused by planetary companions. Pr0201b orbits a V=10.52 late F dwarf with a period of 4.4264 +/- 0.0070 days and has a minimum mass of 0.540 +/- 0.039 Mjup, and Pr0211b orbits a V=12.06 late G dwarf with a period of 2.1451 +/- 0.0012 days and has a minimum mass of 1.844 +/- 0.064 Mjup. The detection of 2 planets among 53 single members surveyed establishes a lower limit on the hot Jupiter frequency of 3.8 (+5.0)(-2.4) % in this metal-rich open cluster. Given the precisely known age of the cluster, this discovery also demonstrates that, in at least 2 cases, giant planet migration occurred within 600 Myr after formation. As we endeavor to learn more about the frequency and formation history of planets, environments with well-determined properties -- such as open clusters like Praesepe -- may provide essential clues to this end.
    The Astrophysical Journal Letters 07/2012; 756(2). DOI:10.1088/2041-8205/756/2/L33 · 5.60 Impact Factor
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    ABSTRACT: We present Keck/NIRC2 $K_{s}$ band high-contrast coronagraphic imaging of the luminous debris disk around the nearby, young A star HD 32297 resolved at a projected separation of $r$ = 0.3-2.5\arcsec{} ($\approx$ 35-280 AU). The disk is highly warped to the north and exhibits a complex, "wavy" surface brightness profile interior to $r$ $\approx$ 110 AU, where the peaks/plateaus in the profiles are shifted between the NE and SW disk lobes. The SW side of the disk is 50--100% brighter at $r$ = 35-80 AU, and the location of its peak brightness roughly coincides with the disk's mm emission peak. Spectral energy distribution modeling suggests that HD 32297 has at least two dust populations that may originate from two separate belts likely at different locations, possibly at distances coinciding with the surface brightness peaks. A disk model for a single dust belt including a phase function with two components and a 5-10 AU pericenter offset explains the disk's warped structure and reproduces some of the surface brightness profile's shape (e.g. the overall "wavy" profile, the SB peak/plateau shifts) but more poorly reproduces the disk's brightness asymmetry. Although there may be alternate explanations, agreement between the SW disk brightness peak and disk's peak mm emission is consistent with an overdensity of very small, sub-blowout-sized dust and large, 0.1-1 mm-sized grains at $\approx$ 45 AU tracing the same parent population of planetesimals. New near-IR and submm observations may be able to clarify whether even more complex grain scattering properties or dynamical sculpting by an unseen planet are required to explain HD 32297's disk structure.
    The Astrophysical Journal 06/2012; 757(1). DOI:10.1088/0004-637X/757/1/28 · 6.28 Impact Factor
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    S. E. Dahm, J. E. Lyke
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    ABSTRACT: We present high angular resolution, near-infrared imaging, and spectroscopy of a low-mass companion to the lithium-depleted, double-line spectroscopic binary HBC 425 (St 34) obtained using NIRSPEC and the Keck II adaptive optics system. Positioned ∼1.23″ southeast of the primary pair, the companion, HBC 425C, is ∼2.4 mag fainter at 2.2 μm. Moderate-resolution ( ∼ 2500 ) - and -band spectroscopy reveals HBC 425C to have an M5.5 ( ± 0.5 ) spectral type. Comparisons with pre–main-sequence evolutionary models imply a mass of ∼0.09 and ages of 8–10 Myr, assuming the nominal distance of Taurus-Auriga ( ∼140 pc ), or ∼25 Myr if placed at ∼90 pc . We also present high-dispersion optical spectra of HBC 425 and HBC 425C obtained using the high-resolution echelle spectrometer (HIRES) on Keck I. We detect strong Li i λ6708 absorption in the spectrum of HBC 425C. Using curves of growth for the Li i λ6708 doublet, we estimate its abundance level to lie between log (Li) = 1.9 and 3.1 dex. The spectrum of HBC 425 exhibits Ca ii H and K lines, He i λλ5876, 6678, and strong Balmer line emission, consistent with accretion. We place more restrictive upper limits on the surface abundance of lithium and find that HBC 425 retains less than ∼0.1% of its primordial abundance. The presence of lithium in the photosphere of HBC 425C does not resolve the discrepancy between isochronal and lithium-depletion ages for the primary pair. However, if lithium were depleted relative to interstellar abundance levels, even minimally, considerable support would be gained for the more advanced age of this hierarchical triple system.
    Publications of the Astronomical Society of the Pacific 12/2011; 123(910):1383-1390. DOI:10.1086/663724 · 3.23 Impact Factor
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    ABSTRACT: We present projected rotational velocities for 20 early-type (B8-A9) and 74 late-type (F2-M8) members of the ~5 Myr old Upper Scorpius OB Association derived from high dispersion optical spectra obtained with the High Resolution Echelle Spectrometer (HIRES) on Keck I and the Magellan Inamori Kyocera Echelle (MIKE) on the Magellan Clay telescope. The spectroscopic sample is composed of stars and brown dwarfs with infrared signatures of circumstellar disks, both primordial and debris, and non-excess sources of comparable spectral type. We merge projected rotational velocities, accretion diagnostics, and Spitzer Space Telescope Infrared Array Camera (IRAC) and Multiband Imaging Photometer for Spitzer (MIPS) 24 micron photometry to examine the relationship between rotation and circumstellar disks. The rotational velocities are strongly correlated with spectral type, a proxy for mass, such that the median vsini for B8-A9 type stars is: 195(+/-)70 km/s, F2-K4: 37.8(+/-)7.4 km/s, K5-K9: 13.8(+21.3/-8.2) km/s, M0-M5: 16.52(+/-)5.3 km/s, and M5.5-M8: 17.72(+/-)8.1 km/s. We find with a probability of >0.99 that M-type stars and brown dwarfs having infrared excess suggestive of circumstellar disks rotate more slowly than their non-excess counterparts. A similar correlation is present among F2-K9 type stars, but only at the ~97% confidence level. Among the early-type (B8-A9) members, rotational velocities of the debris-disk and non-disk populations are indistinguishable. Considering the late-type (F2-M8) stars and brown dwarfs, we find a low fraction of slowly rotating, non-excess sources relative to younger star forming regions, suggesting that most have spun up following disk dissipation. The few late-type (F2-M5) debris disk sources, which may be representative of stars that have recently dispersed their inner disks, are evenly divided between slow and moderate rotators.
    The Astrophysical Journal 10/2011; 745(1). DOI:10.1088/0004-637X/745/1/56 · 6.28 Impact Factor
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    ABSTRACT: IC 1274 is a faintly luminous nebula lying on the near surface of the Lynds 227 (L227) molecular cloud. Four luminous, early-type (B0-B5) stars are located within a spherical volume ~5' in diameter that appears to be clear of heavy obscuration. Approximately centered in the cleared region is the B0 V star HD 166033, which is thought to be largely responsible for the cavity's excavation. Over 80 H-alpha emission sources brighter than V~21 have been identified in the region. More than half of these are concentrated in IC 1274 and are presumably members of a faint T Tauri star population. Chandra Advanced CCD Imaging Spectrometer (ACIS) imaging of a nearby suspected pulsar and time-variable gamma-ray source (GeV J1809-2327) detected 21 X-ray sources in the cluster vicinity, some of which are coincident with the early-type stars and H-alpha emitters in IC 1274. Deep (V~22) optical BVRI photometry has been obtained for the cluster region. A distance of 1.82 +/- 0.3 kpc and a mean extinction of Av ~1.21 +/- 0.2 mag follow from photometry of the early-type stars. Using pre-main-sequence evolutionary models, we derive a median age for the H-alpha emitters and X-ray sources of ~1 Myr; however, a significant dispersion is present. Our interpretation of the structure of IC 1274 is that the early-type stars formed recently and are in the process of dispersing the molecular gas on the near surface of L227. The displaced material was driven against what remains of the molecular cloud to the east, enabling the formation of the substantial number of T Tauri stars found there. We identify a V~21.5 star very near the position of X-ray source 5, the assumed gamma-ray source and young pulsar candidate. The lack of distinctive characteristics for this source, however, coupled with the density of faint stars in this region suggest that this may be a random superposition.
    The Astronomical Journal 09/2011; 143(1). DOI:10.1088/0004-6256/143/1/3 · 4.05 Impact Factor
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    S. E. Dahm
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    ABSTRACT: Moderate-resolution, near-infrared spectra between 0.8 and 5.2 microns were obtained for 12 late-type (K0-M3) disk-bearing members of the ~5 Myr old Upper Scorpius OB association using SpeX on the NASA Infrared Telescope Facility. For most sources, continuum excess emission first becomes apparent between ~2.2 and 4.5 microns and is consistent with that produced by single-temperature blackbodies having characteristic temperatures ranging from ~500 to 1300 K. The near-infrared spectra for 5 of 12 Upper Scorpius sources exhibit Pa-gamma, Pa-beta and Br-gamma emission, indicators of disk accretion. Using a correlation between Pa-beta and Br-gamma emission line luminosity and accretion luminosity, mass accretion rates (Mdot) are derived for these sources that range from Mdot = 3.5 X 10^{-10} to 1.5 X 10^{-8} MSun per yr. Merging the SpeX observations with Spitzer Space Telescope mid-infrared (5.4-37.0 micron) spectroscopy and 24 and 70 micron broadband photometry, the observed spectral energy distributions are compared with those predicted by two-dimensional, radiative transfer accretion disk models. Of the 9 Upper Scorpius sources examined in this analysis, 3 exhibit spectral energy distributions that are most consistent with models having inner disk radii that substantially exceed their respective dust sublimation radii. The remaining Upper Scorpius members possess spectral energy distributions that either show significant dispersion among predicted inner disk radii or are best described by models having inner disk rims coincident with the dust sublimation radius. Comment: 35 pages, 5 figures, accepted for publication in the Astronomical Journal
    The Astronomical Journal 09/2010; DOI:10.1088/0004-6256/140/5/1444 · 4.05 Impact Factor
  • Scott E. Dahm, B. Bowler, G. H. Herbig
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    ABSTRACT: We present initial results from an Halpha emission survey and a deep optical (BVRI) photometric survey of the IC 1274 emission/reflection nebula and the L227 molecular cloud. Over 50 Halpha emission stars brighter than V 21 have been identified in the region, presumably members of a faint T Tauri star population. A striking concentration of Halpha emitters is found in the south-eastern quadrant of the nebula's luminous circular cavity. A Chandra ACIS image of a nearby suspected pulsar and time-variable gamma-ray source detected 21 X-ray sources in the vicinity of IC 1274, many of which are coincident with the early-type cluster members and Halpha emission stars. A distance of 1.82 +/- 0.3 kpc follows from photometry of the early-type stars, which is consistent with previous estimates derived for the associated H II region Sh 2-32. Using pre-main sequence evolutionary models, we find a median age of the Halpha emitters and X-ray sources to be 1 Myr, however, significant dispersion is present. Based upon the strength of Halpha emission, a large fraction of T Tauri stars in IC 1274 is accreting, implying the presence of optically thick circumstellar disks. This finding is supported by significant H-KS
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    S. E. Dahm, John M. Carpenter
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    ABSTRACT: We present mid-infrared spectra between 5.2 and 38 microns for 26 disk-bearing members of the ~5 Myr old Upper Scorpius OB association obtained with the Infrared Spectrograph onboard the Spitzer Space Telescope. We find clear evidence for changes in the spectral characteristics of dust emission between the early (B+A) and late-type (K+M) infrared excess stars. The early-type members exhibit featureless continuum excesses that become apparent redward of ~8 microns. In contrast, 10 and 20 micron silicate features are present in all but one of the late-type excess members of Upper Scorpius. The strength of silicate emission among late-type Upper Scorpius members is spectral type dependent, with the most prominent features being associated with K5-M2 type stars. By fitting the spectral energy distributions (SED) of a sample of low-mass stars with accretion disk models, we find that the SEDs are consistent with models having inner disk radii ranging from ~0.2 to 1.2 AU. Complementary high resolution optical spectra for the Upper Scorpius excess stars were examined for signatures of gaseous accretion. Of the 35 infrared excess stars identified in Upper Scorpius, only 7 (all late-type) exhibit signatures of accretion. Mass accretion rates for these stars range from ~1e-11 to 1e-9 solar masses/yr. Compared to Class II sources in Taurus-Auriga, the disk population in Upper Scorpius exhibits reduced levels of near and mid-infrared excess emission and an order of magnitude lower mass accretion rates. These results suggest that the disk structure has changed significantly over the 2-4 Myr in age separating these two stellar populations. The ubiquity of depleted inner disks in the Upper Scorpius excess sample implies that such disks are a common evolutionary pathway that persists for some time. Comment: Accepted by AJ: 26 pages, 16 figures, 5 tables
    The Astronomical Journal 01/2009; DOI:10.1088/0004-6256/137/4/4024 · 4.05 Impact Factor
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    ABSTRACT: An H2 emission filament is found in close proximity to the unique object KH 15D using the adaptive optics system of the Subaru Telescope. The morphology of the filament, the presence of spectroscopic outflow signatures observed by Hamilton et al., and the detection of extended H2 emission from KH 15D by Deming, Charbonneau, & Harrington suggest that this filament arises from shocked H2 in an outflow. The filament extends about 15'' to the north of KH 15D.
    The Astrophysical Journal 12/2008; 601(1):L91. DOI:10.1086/382701 · 6.28 Impact Factor
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    S. E. Dahm
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    ABSTRACT: NGC 2264 is a young Galactic cluster and the dominant component of the Mon OB1 association lying approximately 760 pc distant within the local spiral arm. The cluster is hierarchically structured, with subclusters of suspected members spread across several parsecs. Associated with the cluster is an extensive molecular cloud complex spanning more than two degrees on the sky. Star formation is ongoing within the region as evidenced by the presence of numerous embedded clusters of protostars, molecular outflows, and Herbig-Haro objects. The stellar population of NGC 2264 is dominated by the O7 V multiple star, S Mon, and several dozen B-type zero-age main sequence stars. X-ray imaging surveys, H-alpha emission surveys, and photometric variability studies have identified more than 600 intermediate and low-mass members distributed throughout the molecular cloud complex, but concentrated within two densely populated areas between S Mon and the Cone Nebula. Estimates for the total stellar population of the cluster range as high as 1000 members and limited deep photometric surveys have identified 230 substellar mass candidates. The median age of NGC 2264 is estimated to be 3 Myr, but an apparent age dispersion of at least 5 Myr can be inferred from the broadened sequence of suspected members. Infrared and millimeter observations of the cluster have identified two prominent sites of star formation activity centered near NGC 2264 IRS1, a deeply embedded early-type star, and IRS2, a star forming core and associated protostellar cluster. Given its relative proximity, well-defined stellar population, and low foreground extinction, the cluster will remain a prime candidate for star formation studies throughout the foreseeable future.
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    S. E. Dahm
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    ABSTRACT: NGC 2362 is a richly populated Galactic cluster, devoid of natal molecular gas and dust. The cluster represents the final product of the star forming process and hosts an unobscured and near-complete initial mass function. NGC 2362 is dominated by the O9 Ib multiple star, tau CMa, as well as several dozen unevolved B-type stars. Distributed throughout the cluster are several hundred suspected intermediate and low-mass pre-main sequence members. Various post-main sequence evolutionary models have been used to infer an age of 5 Myr for the one evolved member, tau CMa. These estimates are in close agreement with the ages derived by fitting pre-main sequence isochrones to the contracting, low-mass stellar population of the cluster. The extremely narrow sequence of stars, which extends more than 9 mag in the optical color-magnitude diagram, suggests that star formation within the cluster occurred rapidly and coevally across the full mass spectrum. Ground-based near infrared and H-alpha emission surveys of NGC 2362 concluded that most (~90%) of the low-mass members have already dissipated their optically-thick, inner(< 1 AU) circumstellar disks. Spitzer IRAC observations of the cluster have confirmed these results, placing an upper limit on the primordial, optically thick disk fraction of the cluster at 7(+/-)2%. The presence of circumstellar disks among candidate members of NGC 2362 is also strongly mass-dependent, such that no stars more massive than 1.2 Msun exhibit significant infrared excess shortward of 8 microns. The well-defined upper main sequence of NGC 2362, its large population of low-mass stars, and the narrow age spread evident in the color-magnitude diagram ensure its role as a standard model of cluster as well as stellar evolution.
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    S. E. Dahm
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    ABSTRACT: High-resolution optical and moderate-resolution near-infrared spectra were obtained for 40 near solar mass (~2.0-0.5 M⊙) members of the 2-3 Myr old cluster IC 348 in order to examine established accretion diagnostics and the coupling between inner disk gas and hot, micron-sized dust grains inferred from thermal and mid-infrared excesses. The stellar sample was drawn from the cluster census of Luhman in 2003 with membership being confirmed by radial velocity analysis and the presence of strong Li I λ6708 absorption. Of the stars included in this survey, 12 were classified by Lada in 2006 as hosting primordial, optically thick circumstellar disks, 5 as weak or transition disk systems, and 23 as non-excess stars using the measured slope of the stellar spectral energy distribution (SED) through the four Infrared Array Camera channels (3.6-8.0 μm) of Spitzer Space Telescope. Using the velocity width of Hα as an accretion indicator, we find that 11 primordial disk candidates are suspected accretors, suggesting a strong correlation between gaseous inner disks and optically thick dust emission. Of the five weak or transition disk systems observed, two (L21 and L67) exhibit spectroscopic features indicative of accretion. The presence of gas within the inner disk of these systems, which are free of infrared excess emission shortward of ~4.5 μm, may place constraints upon the physical mechanism responsible for inner disk clearing. Mass accretion rates (M) were determined for all suspected accretors using continuum excess measurements near λ6500 and established relationships between He I λ5876, Hα, Ca II λ8542, Paβ, and Brγ line fluxes and accretion luminosity. M values were found to range from log M to –7.2 M⊙ yr^–1, with a median value of –8.1 M⊙ yr^–1. Magnetospheric accretion models of Hα, Paβ, and Brγ emission by Muzerolle et al. and Kurosawa et al. are found to be in relative agreement with observed fluxes and derived M estimates. He I λ10830 with its metastable lower level is confirmed to be a critical indicator of magnetospheric accretion, although deep subcontinuum absorption profiles often associated with its emission are not fully understood. No statistically significant correlation is found between M and the slope of the SED from 3.6 to 8.0 μm or from 8.0 to 24.0 μm; however, the small number of suspected accretors examined does not allow broader conclusions to be drawn.
    The Astronomical Journal 08/2008; DOI:10.1088/0004-6256/136/2/521 · 4.05 Impact Factor
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    S. E. Dahm
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    ABSTRACT: An Hα emission survey of the young cluster NGC 2362 resulted in the detection of 130 Hα emission stars in an 11' × 11' field approximately centered on the fourth magnitude O9 Ib multiple star τ CMa. The survey was carried out using the wide-field grism spectrograph on the University of Hawaii 2.2 m telescope and the Gemini Multi-Object Spectrograph (GMOS) on Gemini North. Deep optical VRCIC (to V ~ 23.0) and near-infrared (NIR) photometry (JHK) to K ~ 16 were obtained for several fields within the cluster. Spectra covering the 6000–8000 Å region at a resolution of R ~ 3000 (adequate for the determination of Li I λ6708 line strengths) were also acquired for ~200 pre-main-sequence (PMS) candidates with GMOS. Ages and masses for the Hα emitters in NGC 2362 were inferred from the isochrones and evolutionary tracks of D'Antona & Mazzitelli, as well as those of Baraffe et al. An estimated cluster age of ~1.8 Myr follows from the models of D'Antona & Mazzitelli and 3.5–5.0 Myr from those of Baraffe et al. The fraction of the T Tauri star (TTS) population that is composed of weak-line emitters, f(WTTS), is 0.91, compared with 0.43 for the TTS population of NGC 2264. On the basis of W(Hα) alone, the fraction of TTSs still undergoing accretion is 5%–9%, comparable to the inner disk fraction determined from JHKL-band excesses by Haisch and coworkers (12%). Approximately 15% of the PMS sample in this study exhibits possible NIR excess, having EH-K > 0.1 mag. Given the lack of NIR excess and strong Hα emission from the majority of cluster members, it is inferred that the inner disk regions of the TTS population have dissipated significantly. The mean level of chromospheric activity among the WTTS population of NGC 2362 is log(LHα/Lbol) = -3.65, significantly greater than that of the low-mass population of the 600 Myr old Hyades cluster, log(LHα/Lbol) = -3.90. The total mass of the Hα emitters and the OB stellar population of NGC 2362 defines a lower limit for the cluster mass of ~300 M. Allowance for A- and F-type stars still on the radiative track, multiplicity, outlying members, and the low-mass population lying below the completeness limit of the Hα emission survey increases this lower limit to well over 500 M. The derived relaxation, disruption, and evaporation timescales for the cluster imply that NGC 2362 will likely survive beyond the age of the Pleiades, but statistics of galactic cluster lifetimes favor its disruption well before the age of the Hyades.
    The Astronomical Journal 12/2007; 130(4):1805. DOI:10.1086/433178 · 4.05 Impact Factor
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    G. H. Herbig, S. E. Dahm
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    ABSTRACT: L988 is a large (~05 × 07) dark cloud complex at about 600 pc that contains several bright pre-main-sequence objects (such as V1331 Cyg and LkHα 321), but this paper deals in detail only with a small region on its eastern edge, near the HAeBe star LkHα 324. That star and its distant companion LkHα 324SE lie at the apex of a V-shaped area apparently excavated from the edge of L988, and are the brightest members of a small cluster containing about 50 Hα-emission stars. A median age of about 0.6 Myr (with large dispersion) is inferred from its color-magnitude diagram, constructed from VRI photometry to V = 22. Keck HIRES spectra show that LkHα 324SE is probably also an HAeBe. Its image is nonstellar, and within 3'' to the northwest are three condensations having complex [S II] and [O I] profiles and radial velocities up to -200 km s-1. They probably originate in an outflow from LkHα 324SE. A bright Ap star with strong Si II lines is embedded in the heavy obscuration 8' to the west. It illuminates a small reflection nebulosity, has several faint Hα emitters nearby, and shares the radial velocity of L988, so clearly it was formed in that cloud. It demonstrates again that such chemical peculiarities can be established very early in young stars of moderate mass.
    The Astronomical Journal 12/2007; 131(3):1530. DOI:10.1086/499809 · 4.05 Impact Factor
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    G. H. Herbig, S. E. Dahm
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    ABSTRACT: The B0 V star BD +46°3474 lies near the front surface of a dense molecular cloud and illuminates the emission/reflection nebula IC 5146. The HAeBe variable BD +46°3471 is embedded in the same cloud, about 10' (3.5 pc) away. CCD photometry in BVRI (to V = 22) and in JHK (to about K = 16.5) has been obtained for the young clusters surrounding each of these two bright stars. Some 100 emission-Hα stars brighter than R = 20.5 have been found in the area, most of them in IC 5146. (Among these are two that have spectra resembling a high-excitation Herbig-Haro [HH] object plus a stellar continuum.) A distance of 1.2 kpc follows from the photometry of several late-type IC 5146 cluster members; the average extinction from 38 stars classified spectroscopically is AV = 3.0 ± 0.2 mag. Although optical photometry is available for 700 stars in the IC 5146 field, only about half (including all the Hα emitters) lie above the main sequence, while a substantial fraction of these are estimated to be foreground. A number of such interlopers have been identified on the basis of proper motion or abnormally low AV. The age distribution of the Hα emitters has been estimated by reference to several sets of theoretical isochrones. There is substantial disagreement, but the median age does appear to be near 1 Myr. The spectrum of +46°3474 is unexceptional except for an unusually low v sin i (10 km s-1), but +46°3471 has a complex emission plus absorption spectrum. Our interpretation of the structure of IC 5146 on the basis of optical and radio radial velocities follows a proposal by Roger & Irwin in 1982, namely, that +46°3474 formed near the near surface of the present cloud and evacuated a blister cavity out of which gas and dust are now flowing through a funnel-shaped volume in the approximate direction of the Sun. It is suggested that the IC 5146 cluster stars formed in a dense foreground section of the molecular cloud that was dissipated following the appearance of +46°3474.
    The Astronomical Journal 12/2007; 123(1):304. DOI:10.1086/324638 · 4.05 Impact Factor
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    ABSTRACT: We present results from an X-ray imaging survey of the young cluster NGC 2264, carried out with the European Photon Imaging Cameras (EPIC) on board the XMM-Newton spacecraft. The X-ray data are merged with extant optical and near-infrared photometry, spectral classifications, Hα emission strengths, and rotation periods to examine the interrelationships between coronal and chromospheric activity, rotation, stellar mass, and internal structure for a statistically significant sample of pre-main-sequence stars. A total of 300 distinct X-ray sources can be identified with optical or near-infrared counterparts. The sources are concentrated within three regions of the cluster: in the vicinity of S Mon, within the large emission/reflection nebulosity southwest of S Mon, and along the broad ridge of molecular gas that extends from the Cone Nebula to the NGC 2264 IRS 2 field. From the extinction-corrected color-magnitude diagram of the cluster, ages and masses for the optically identified X-ray sources are derived. A median age of ~2.5 Myr and an apparent age dispersion of ~5 Myr are suggested by pre-main-sequence evolutionary models. The X-ray luminosity of the detected sources appears well correlated with bolometric luminosity, although there is considerable scatter in the relationship. Stellar mass contributes significantly to this dispersion, while isochronal age and rotation do not. X-ray luminosity and mass are well correlated such that LX ∝ (M/M)1.5, similar to the relationship found within the younger Orion Nebula Cluster. No strong evidence is found for a correlation between EH-K, the near-infrared color excess, and the fractional X-ray luminosity, which suggests that optically thick dust disks have little direct influence on the observed X-ray activity levels. Among the X-ray-detected weak-line T Tauri stars, the fractional X-ray luminosity, LX/Lbol, is moderately well correlated with the fractional Hα luminosity, LHα/Lbol, but only at the 2 σ level of significance. The cumulative distribution functions for the X-ray luminosities of the X-ray-detected classical and weak-line T Tauri stars within the cluster are comparable, assuming the demarcation between the two classes is at an Hα equivalent width of 10 Å. However, if the nondetections in X-rays for the entire sample of Hα emitters known within the cluster are taken into account, then the cumulative distribution functions of these two groups are clearly different, such that classical T Tauri stars are underdetected by at least a factor of 2 relative to the weak-line T Tauri stars. Examining a small subsample of X-ray-detected stars that are probable accretors based on the presence of strong Hα emission and near-infrared excess, we conclude that definitive nonaccretors are ~1.6 times more X-ray luminous than their accreting counterparts. In agreement with earlier published findings for the Orion Nebula Cluster, we find a slight positive correlation (valid at the 2 σ confidence level) between LX/Lbol and rotation period in NGC 2264. The lack of a strong anticorrelation between X-ray activity and rotation period in the stellar population of NGC 2264 suggests that either the deeply convective T Tauri stars are rotationally saturated or the physical mechanism responsible for generating magnetic fields in pre-main-sequence stars is distinct from the one that operates in evolved main-sequence stars.
    The Astronomical Journal 12/2007; 134(3):999. DOI:10.1086/519954 · 4.05 Impact Factor
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    S. E. Dahm, Theodore Simon
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    ABSTRACT: An Hα emission survey of the young cluster NGC 2264 in the Mon OB1 association resulted in the detection of 490 Hα emission stars in a 25' × 40' field approximately centered between the O7 V multiple star S Mon and the Cone Nebula. The survey was carried out with the wide-field grism spectrograph (WFGS) on the University of Hawaii 2.2 m telescope on Mauna Kea. X-ray observations made with the European Photon Imaging Camera on board the European Space Agency's XMM-Newton satellite observatory will be discussed in a subsequent paper. Optical (BVRCIC) photometry was obtained for selected fields to supplement similar data from the literature. Spectra covering the 6000–8000 Å region at a resolution of R ~ 3000 (adequate for the determination of Li I λ6708 line strengths) were obtained for 150 Hα and X-ray emission sources with the Gemini Multi-Object Spectrograph. Near-infrared spectra (1–2.5 μm) of a number of T Tauri stars (TTSs), X-ray sources, and LHα25 (W90) were also obtained using SpeX on the Infrared Telescope Facility. Ages and masses for the Hα emitters were inferred from the isochrones and evolutionary tracks of D'Antona & Mazzitelli. The median age for the TTS population is about 1.1 Myr, but a considerable dispersion, from 0.1 to 5 Myr, exists for individual objects. Several fields in the cluster were observed with the WFGS on more than one occasion, permitting an examination of Hα variability over long baselines in time. About 90% of the classical T Tauri stars (CTTSs) showed changes in W(Hα) of at least 10%, while 57% varied at levels of 50% or more. No evidence was found for a significant pool of dormant Hα emitters. Summing the masses of the TTSs and the OB stellar population of NGC 2264, a lower limit for the total stellar mass content of the cluster is about 430 M. This is less than 1% of the total mass of the atomic and molecular gas believed to be associated with NGC 2264. Evidence for hierarchical structure within the cluster is suggested by the spatial distribution of TTSs. Four concentrations of Hα emitters are evident: two near S Mon and two near the Cone Nebula. The median age of the TTSs in the immediate vicinity of S Mon was found to be greater than that of the TTSs near Allen's infrared source (IRS-1), but a significant dispersion is present. From the rotational data of Lamm et al. and Makidon et al., 241 of the TTSs are periodic variables, 150 weak-line T Tauri stars (WTTSs) and 91 CTTSs, while 123 stars are irregular variables (30 WTTSs and 93 CTTSs). A weak-to-moderate positive correlation is found between H - K color and Prot for the CTTSs, in the sense that stars having longer periods tend to have larger H - K colors. A similar positive correlation is found between LHα and Prot among the CTTSs. No statistically significant correlation is found between Prot and theoretical age or between Prot and LX. Other topics discussed include the fraction of Hα emitters that are WTTSs, f(WTTS) = N(WTTS)/N(TTS), for clusters of different ages; the relative detectability of Hα emission using WFGS and narrowband filter imaging techniques; and the correlation of W(Li I) with Te, age, H - K color, and W(Hα).
    The Astronomical Journal 12/2007; 129(2):829. DOI:10.1086/426326 · 4.05 Impact Factor
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    S. E. Dahm, L. A. Hillenbrand
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    ABSTRACT: We present results from a mid-infrared imaging survey of the ~5 Myr old cluster NGC 2362 carried out with the Infrared Array Camera (IRAC) on board the Spitzer Space Telescope. The archival mid-infrared data were merged with extant Hα emission data, optical and near-infrared photometry, and moderate-resolution optical spectroscopy to identify the remnant disk-bearing population of the cluster and to estimate the fraction of stars that still retain primordial circumstellar disks. The principal sample of 232 suspected cluster members with masses ranging from ~10 to 0.3 M_⊙ (B2-M5 spectral types) was drawn from known Hα emission stars, X-ray-detected stars from a single 100 ks archival Chandra observation, and established lithium-rich stars. A second sample of 153 stars over a similar mass range whose membership status was based on optical photometry alone was also examined. Measured fluxes in the optical and infrared passbands were fitted with synthetic, low-resolution spectra created using the NextGen atmospheric models, permitting the detection of infrared excesses relative to predicted stellar photospheric fluxes. Using the measured slope of the stellar spectral energy distribution through the four IRAC channels to characterize disk emission for the 195 out of 232 activity/lithium-selected stars and the 105 out of 153 photometric membership candidates having complete IRAC photometry, we derive an upper limit for the primordial, optically thick disk fraction of NGC 2362 of ~7% ± 2%, with another ~12% ± 3% of suspected members exhibiting infrared excesses indicative of weak or optically thin disk emission. The presence of circumstellar disks among candidate members of NGC 2362 is strongly mass-dependent, such that no stars more massive than ~1.2 M_⊙ exhibit significant infrared excess shortward of 8 μm. An upper limit for the fraction of stars hosting primordial, optically thick disks peaks near 10.7% ± 4% for stars with masses between 1.05 and 0.6 M_⊙, but the Spitzer IRAC survey is sensitivity-limited below ~0.3 M_⊙. From Hα emission-line strengths, an upper limit for the accretion fraction of the cluster is estimated at ~5%, with most suspected accretors associated with primordial, optically thick disks identified with Spitzer. The presence of primordial disk-bearing stars in NGC 2362, some of which are suspected of still experiencing gaseous accretion, may imply that even within dense cluster environments, sufficient numbers of inner disks survive to ages consistent with core accretion models of giant planet formation to account for the observed frequency of exoplanets within 5 AU of all FGKM-type stars.
    The Astronomical Journal 05/2007; DOI:10.1086/512156 · 4.05 Impact Factor