P. C. Myers

Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, United States

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Publications (237)794.65 Total impact

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    ABSTRACT: We have detected bright HC7N J = 21-20 emission toward multiple locations in the Serpens South cluster-forming region using the K-Band Focal Plane Array at the Robert C. Byrd Green Bank Telescope. HC7N is seen primarily toward cold filamentary structures that have yet to form stars, largely avoiding the dense gas associated with small protostellar groups and the main central cluster of Serpens South. Where detected, the HC7N abundances are similar to those found in other nearby star forming regions. Toward some HC7N `clumps', we find consistent variations in the line centroids relative to NH3 (1,1) emission, as well as systematic increases in the HC7N non-thermal line widths, which we argue reveal infall motions onto dense filaments within Serpens South with minimum mass accretion rates of M ~ 2-5 M_sun Myr^-1. The relative abundance of NH3 to HC7N suggests that the HC7N is tracing gas that has been at densities n ~ 10^4 cm^-3, for timescales t < 1-2 x 10^5 yr. Since HC7N emission peaks are rarely co-located with those of either NH3 or continuum, it is likely that Serpens South is not particularly remarkable in its abundance of HC7N, but instead the serendipitous mapping of HC7N simultaneously with NH3 has allowed us to detect HC7N at low abundances in regions where it otherwise may not have been looked for. This result extends the known star-forming regions containing significant HC7N emission from typically quiescent regions, like the Taurus molecular cloud, to more complex, active environments.
    Monthly Notices of the Royal Astronomical Society 09/2013; 436(2). DOI:10.1093/mnras/stt1671 · 5.23 Impact Factor
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    ABSTRACT: We present a study on the spatial distribution of N2D+ and N2H+ in thirteen protostellar systems. Eight of thirteen objects observed with the IRAM 30m telescope show relative offsets between the peak N2D+ (J=2-1) and N2H+ (J=1-0) emission. We highlight the case of L1157 using interferometric observations from the Submillimeter Array and Plateau de Bure Interferometer of the N2D+ (J=3-2) and N2H+ (J=1-0) transitions respectively. Depletion of N2D+ in L1157 is clearly observed inside a radius of ~2000 AU (7") and the N2H+ emission is resolved into two peaks at radii of ~1000 AU (3.5"), inside the depletion region of N2D+. Chemical models predict a depletion zone in N2H+ and N2D+ due to destruction of H2D+ at T ~ 20 K and the evaporation of CO off dust grains at the same temperature. However, the abundance offsets of 1000 AU between the two species are not reproduced by chemical models, including a model that follows the infall of the protostellar envelope. The average abundance ratios of N2D+ to N2H+ have been shown to decrease as protostars evolve by Emprechtinger et al., but this is the first time depletion zones of N2D+ have been spatially resolved. We suggest that the difference in depletion zone radii for N2H+ and N2D+ is caused by either the CO evaporation temperature being above 20 K or an H2 ortho-to-para ratio gradient in the inner envelope.
    The Astrophysical Journal 01/2013; 765(1). DOI:10.1088/0004-637X/765/1/18 · 6.28 Impact Factor
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    ABSTRACT: Most stars in the Galaxy, if not all, form within cold, dense, and filamentary molecular clouds. Many of these clouds are seen in extinction against the bright mid-infrared Galactic background and understanding star formation requires probing the physics within these infrared dark clouds (IRDCs). To tackle this problem, we are currently carrying out a systematic study of the principal characteristics of ~150 IRDCs in the Galaxy using archival data from Herschel and Spitzer satellites, as well as molecular line observations. Data from the Herschel satellite allows us to observe the large scale structure of the star forming infrared dark filaments, in emission, and with unprecedent sensitivity. This study provides the most complete dataset to date where to study the molecular and dusty structure associated to the earliest stages of clustered star formation. Deriving column densities, masses, temperatures, and the young-stellar content of a large sample of these filamentary clouds is a first crucial step to assess: i) the relevance that filamentary geometry has on star formation, determining the initial mass and luminosity distributions, ii) how pertinent are turbulence, large-scale shocks, magnetic fields, and/or tidal forces in shaping this particular morphology. We present the first results of our study, focusing on statistically significant determination of the main physical parameters of IRDCs in the Galaxy, and consistent theoretical interpretations.
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    ABSTRACT: The star formation characteristics of dense massive clusters are the subject of several observational and theoretical efforts, given their importance in unveiling fundamental aspects such as the slope and upper cutoff of the Initial Mass Function (IMF),and the nature of massive star formation. The degree of success of such methods is limited by the fact that most of the studied clusters are spatially blended or unresolved by current facilities at mid- and far-infrared wavelengths. We present a Bayesian method to study these unresolved clusters using a SED fitting approach that uses a library of pre-computed SED models and archival data from multi-wavelength surveys, making special emphasis on the different beam sizes of the observations. We apply the method to the star-forming complex W43, located in the inner spiral arm of the galaxy and analyze the results in terms of the Luminosity Function (LF), and census of massive stars in this region. We then compare our results to theoretical predictions on the shape of the IMF, and its relation with the young stellar objects LM.
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    ABSTRACT: We present a survey of the Orion A and B molecular clouds undertaken with the IRAC and MIPS instruments on board Spitzer. In total, five distinct fields were mapped, covering 9 deg{sup 2} in five mid-IR bands spanning 3-24 {mu}m. The survey includes the Orion Nebula Cluster, the Lynds 1641, 1630, and 1622 dark clouds, and the NGC 2023, 2024, 2068, and 2071 nebulae. These data are merged with the Two Micron All Sky Survey point source catalog to generate a catalog of eight-band photometry. We identify 3479 dusty young stellar objects (YSOs) in the Orion molecular clouds by searching for point sources with mid-IR colors indicative of reprocessed light from dusty disks or infalling envelopes. The YSOs are subsequently classified on the basis of their mid-IR colors and their spatial distributions are presented. We classify 2991 of the YSOs as pre-main-sequence stars with disks and 488 as likely protostars. Most of the sources were observed with IRAC in two to three epochs over six months; we search for variability between the epochs by looking for correlated variability in the 3.6 and 4.5 {mu}m bands. We find that 50% of the dusty YSOs show variability. The variations are typically small ({approx}0.2 mag) with the protostars showing a higher incidence of variability and larger variations. The observed correlations between the 3.6, 4.5, 5.8, and 8 {mu}m variability suggests that we are observing variations in the heating of the inner disk due to changes in the accretion luminosity or rotating accretion hot spots.
    The Astronomical Journal 12/2012; 144(6):192. DOI:10.1088/0004-6256/144/6/192 · 4.05 Impact Factor
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    ABSTRACT: We present a survey of the Orion A and B molecular clouds undertaken with the IRAC and MIPS instruments onboard Spitzer. In total, five distinct fields were mapped covering 9 sq. degrees in five mid-IR bands spanning 3-24 microns. The survey includes the Orion Nebula Cluster, the Lynds 1641, 1630 and 1622 dark clouds, and the NGC 2023, 2024, 2068 and 2071 nebulae. These data are merged with the 2MASS point source catalog to generate a catalog of eight band photometry. We identify 3479 dusty young stellar objects (YSOs) in the Orion molecular clouds by searching for point sources with mid-IR colors indicative of reprocessed light from dusty disks or infalling envelopes. The YSOs are subsequently classified on the basis of their mid-IR colors and their spatial distributions are presented. We classify 2991 of the YSOs as pre-main sequence stars with disks and 488 as likely protostars. Most of the sources were observed with IRAC in 2-3 epochs over 6 months; we search for variability between the epochs by looking for correlated variability in the 3.6 and 4.5 micron bands. We find that 50% of the dusty YSOs show variability. The variations are typically small (0.2 mag.) with the protostars showing a higher incidence of variability and larger variations. The observed correlations between the 3.6, 4.5, 5.8 and 8 micron variability suggests that we are observing variations in the heating of the inner disk due to changes in the accretion luminosity or rotating accretion hot spots.
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    ABSTRACT: We present Spitzer IRAC and MIPS observations of the star-forming region containing intermediate-mass young stellar object (YSO) AFGL 490. We supplement these data with near-IR 2MASS photometry and with deep SQIID observations off the central high extinction region. We have more than doubled the known membership of this region to 57 Class I and 303 Class II YSOs via the combined 1-24 um photometric catalog derived from these data. We construct and analyze the minimum spanning tree of their projected positions, isolating one locally over-dense cluster core containing 219 YSOs (60.8% of the region's members). We find this cluster core to be larger yet less dense than similarly analyzed clusters. Although the structure of this cluster core appears irregular, we demonstrate that the parsec-scale surface densities of both YSOs and gas are correlated with a power law slope of 2.8, as found for other similarly analyzed nearby molecular clouds. We also explore the mass segregation implications of AFGL 490's offset from the center of its core, finding that it has no apparent preferential central position relative to the low-mass members.
    The Astrophysical Journal 04/2012; 752(2). DOI:10.1088/0004-637X/752/2/127 · 6.28 Impact Factor
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    ABSTRACT: We identify protostars in Spitzer surveys of nine star-forming molecular clouds within 1 kpc: Serpens, Perseus, Ophiuchus, Chamaeleon, Lupus, Taurus, Orion, Cep OB3, and Mon R2, which combined host over 700 protostar candidates. Our diverse cloud sample allows us to compare protostar luminosity functions in these varied environments. We combine photometry from 2MASS J, H, and Ks bands and Spitzer IRAC and MIPS 24 micron bands to create 1 - 24 micron spectral energy distributions (SEDs). Using protostars from the c2d survey with well-determined bolometric luminosities (Lbol), we derive a relationship between Lbol, L_MIR (integrated from 1 - 24 microns), and SED slope. Estimations of Lbol for protostar candidates are combined to create luminosity functions for each cloud. Contamination due to edge-on disks, reddened Class II sources, and galaxies is estimated and removed from the luminosity functions. We find that luminosity functions for high mass star forming clouds peak near 1 Lsun and show a tail extending toward luminosities above 100 Lsun. The luminosity functions of the low mass star forming clouds do not exhibit a common peak, however the combined luminosity function of these regions peaks below 1 Lsun. Finally, we examine the luminosity functions as a function of the local surface density of YSOs. In the Orion molecular cloud, we find a significant difference between the luminosity functions of protostars in regions of high and low stellar density, the former of which is biased toward more luminous sources. This may be the result of primordial mass segregation, although this interpretation is not unique. We compare our luminosity functions to those predicted by models and find that our observed luminosity functions are best matched by models which invoke competitive accretion, although we do not find strong agreement of the high mass star forming clouds with any of the models.
    The Astronomical Journal 04/2012; 144(2). DOI:10.1088/0004-6256/144/2/31 · 4.05 Impact Factor
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    ABSTRACT: We report the discovery and characterization of a power law correlation between the local surface densities of Spitzer-identified, dusty young stellar objects and the column density of gas (as traced by near-IR extinction) in eight molecular clouds within 1 kpc and with 100 or more known YSOs. This correlation, which appears in data smoothed over size scales of ~1 pc, varies in quality from cloud to cloud; those clouds with tight correlations, MonR2 and Ophiuchus, are fit with power laws of slope 2.67 and 1.87, respectively. The spread in the correlation is attributed primarily to local gas disruption by stars that formed there or to the presence of very young sub-regions at the onset of star formation. We explore the ratio of the number of Class II to Class I sources, a proxy for the star formation age of a region, as a function of gas column density; this analysis reveals a declining Class II to Class I ratio with increasing column density. We show that the observed star-gas correlation is consistent with a star formation law where the star formation rate per area varies with the gas column density squared. We also propose a simple picture of thermal fragmentation of dense gas in an isothermal, self-gravitating layer as an explanation for the power law. Finally, we briefly compare the star gas correlation and its implied star formation law with other recent proposed of star formation laws at similar and larger size scales from nearby star forming regions.
    The Astrophysical Journal 10/2011; 739(2):84. DOI:10.1088/0004-637X/739/2/84 · 6.28 Impact Factor
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    ABSTRACT: We report the detection of a compact (of order 5 arcsec; about 1800 AU projected size) CO outflow from L1148-IRS. This confirms that this Spitzer source is physically associated with the nearby (about 325 pc) L1148 dense core. Radiative transfer modeling suggests an internal luminosity of 0.08 to 0.13 L_sun. This validates L1148-IRS as a Very Low Luminosity Object (VeLLO; L < 0.1 L_sun). The L1148 dense core has unusually low densities and column densities for a star-forming core. It is difficult to understand how L1148-IRS might have formed under these conditions. Independent of the exact final mass of this VeLLO (which is likely < 0.24 M_sun), L1148-IRS and similar VeLLOs might hold some clues about the isolated formation of brown dwarfs.
    Monthly Notices of the Royal Astronomical Society 06/2011; 416. DOI:10.1111/j.1365-2966.2011.19205.x · 5.23 Impact Factor
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    ABSTRACT: Mid-infrared observations, obtained by the Cores-to-Disks Spitzer Legacy team, revealed several protostars and young stellar objects in the core DC274.2-0.4. With evidence for a nearby distance (130-200 pc), one of these sources qualifies as a Very Low Luminosity Object (VeLLO) and appears to be among the lowest luminosity embedded sources known, with an internal luminosity of 0.01-0.03 Lsun. If this source is indeed a VeLLO, then its discovery is exciting as it may represent the earliest stage protostar or proto-brown dwarf yet detected. However, the internal luminosity critically depends on the distance and some evidence suggests that DC274.2-0.4 is more distant (>400 pc), making this core a more typical site of low-mass star formation. We review the different distance determinations, and present new optical spectra obtained with Magellan/IMACS to help settle the ambiguity. Finally, we discuss the star-forming environment of DC274.2-0.4 in light of additional infrared observations from CTIO/ISPI, Magellan/MMIRS, and HST/WFC3.
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    The Astrophysical Journal Supplement Series 08/2010; 189(2):352. DOI:10.1088/0067-0049/189/2/352 · 14.14 Impact Factor
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    ABSTRACT: We present N2D+ 3-2 (IRAM) and H2D+ 1_11 - 1_10 and N2H+ 4-3 (JCMT) maps of the small cluster-forming Ophiuchus B2 core in the nearby Ophiuchus molecular cloud. In conjunction with previously published N2H+ 1-0 observations, the N2D+ data reveal the deuterium fractionation in the high density gas across Oph B2. The average deuterium fractionation R_D = N(N2D+)/N(N2H+) ~ 0.03 over Oph B2, with several small scale R_D peaks and a maximum R_D = 0.1. The mean R_D is consistent with previous results in isolated starless and protostellar cores. The column density distributions of both H2D+ and N2D+ show no correlation with total H2 column density. We find, however, an anticorrelation in deuterium fractionation with proximity to the embedded protostars in Oph B2 to distances >= 0.04 pc. Destruction mechanisms for deuterated molecules require gas temperatures greater than those previously determined through NH3 observations of Oph B2 to proceed. We present temperatures calculated for the dense core gas through the equating of non-thermal line widths for molecules (i.e., N2D+ and H2D+) expected to trace the same core regions, but the observed complex line structures in B2 preclude finding a reasonable result in many locations. This method may, however, work well in isolated cores with less complicated velocity structures. Finally, we use R_D and the H2D+ column density across Oph B2 to set a lower limit on the ionization fraction across the core, finding a mean x_e, lim >= few x 10^{-8}. Our results show that care must be taken when using deuterated species as a probe of the physical conditions of dense gas in star-forming regions. Comment: ApJ accepted
    The Astrophysical Journal 06/2010; DOI:10.1088/0004-637X/718/2/666 · 6.28 Impact Factor
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    ABSTRACT: We present Chandra X-ray data of the NGC 1333 embedded cluster, combining these data with existing Chandra data, Sptizer photometry and ground based spectroscopy of both the NGC 1333 & Serpens North clusters to perform a detailed study of the X-ray properties of two of the nearest embedded clusters to the Sun. In NGC 1333, a total of 95 cluster members are detected in X-rays, of which 54 were previously identified with Spitzer. Of the Spitzer sources, we detect 23% of the Class I protostars, 53% of the Flat Spectrum sources, 52% of the Class II, and 50% of the Transition Disk YSOs. Forty-one Class III members of the cluster are identified, bringing the total identified YSO population to 178. The X-ray Luminosity Functions (XLFs) of the NGC 1333 and Serpens clusters are compared to each other and the Orion Nebula Cluster. Based on this comparison, we obtain a new distance for the Serpens cluster of 360+22/-13 pc. The X-ray luminosity was found to depend on the bolometric luminosity as in previous studies of other clusters, and that Lx depends primarily on the stellar surface area. In the NGC 1333 cluster, the Class III sources have a somewhat higher X-ray luminosity for a given surface area. We also find evidence in NGC 1333 for a jump in the X-ray luminosity between spectral types of M0 and K7, we speculate that this may result from the presence of radiative zones in the K-stars. The gas column density vs. extinction in the NGC 1333 was found to be N_H = 0.89 +/- 0.13 x 10^22 A_K, this is lower than expected of the standard ISM but similar to that found previously in the Serpens Cloud Core. Comment: 58 pages, 14 figures, accepted by AJ
    The Astronomical Journal 05/2010; DOI:10.1088/0004-6256/140/1/266 · 4.05 Impact Factor
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    ABSTRACT: The NGC 1999 reflection nebula features a dark patch with a size of ~10,000 AU, which has been interpreted as a small, dense foreground globule and possible site of imminent star formation. We present Herschel PACS far-infrared 70 and 160mum maps, which reveal a flux deficit at the location of the globule. We estimate the globule mass needed to produce such an absorption feature to be a few tenths to a few Msun. Inspired by this Herschel observation, we obtained APEX LABOCA and SABOCA submillimeter continuum maps, and Magellan PANIC near-infrared images of the region. We do not detect a submillimer source at the location of the Herschel flux decrement; furthermore our observations place an upper limit on the mass of the globule of ~2.4x10^-2 Msun. Indeed, the submillimeter maps appear to show a flux depression as well. Furthermore, the near-infrared images detect faint background stars that are less affected by extinction inside the dark patch than in its surroundings. We suggest that the dark patch is in fact a hole or cavity in the material producing the NGC 1999 reflection nebula, excavated by protostellar jets from the V 380 Ori multiple system. Comment: accepted for the A&A Herschel issue; 7 pages
    Astronomy and Astrophysics 05/2010; 518. DOI:10.1051/0004-6361/201014612 · 4.48 Impact Factor
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    ABSTRACT: The target sample of the Spitzer survey was primarily selected from the literature compilation of embedded clusters within 1kpc by Porras et al. (2003, Cat. ). The final sample includes all of the embedded clusters in the Lada & Lada (2003ARA&A..41...57L) tabulation within 1kpc, with the exception of Orion and NGC 2264. Table 2 contains the detailed information on the Spitzer data used for each region. (2 data files).
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    ABSTRACT: Before the launch of the Spitzer Space Telescope, a systematic study to identify and characterize mid-infrared variability in young sources was not feasible. Within the last five years, however, the Cores-to-Disks Spitzer Legacy team has observed and identified protostars and young stellar objects (YSOs) in a large sample of nearby, isolated dense cores. Multiple epochs of mid-infrared observations exist for some of these cores and are publicly available in the Spitzer archive. Analysis of the fluxes of protostars and YSOs at different epochs demonstrates that some of these sources exhibit mid-infrared variability. For example, preliminary results show that of 68 candidate protostars and YSOs with two epochs separated by 1-1.5 years, 16 exhibit significant (3-sigma) variability with at least a 10% change in flux, 7 of which show a 20% change in flux. Such mid-infrared variability suggests that the earliest stages of star formation, like the later stages, may be marked by non-steady mass accretion. We will present the results of a more detailed investigation, which includes an analysis of the relative changes in mid-infrared fluxes in the different Spitzer bands. Specific examples of variability will be highlighted. Finally, we will compare the plausibility of variable clumpiness of material along the lines of sight with an alternative explanation of non-steady mass accretion. This analysis is funded by a grant from the NASA Astrophysics Data Analysis program.
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    ABSTRACT: Stars form out of the gravitational collapse of centrally condensed cores of dense molecular gas. Systematic observations of molecular emission lines which are excited at the high densities and cold temperatures found in star forming regions have revealed the physical and chemical structure of solar-mass size cores which have formed or may soon form a single star or stellar binary, and form the basis of theories of single-star formation. The initial conditions of star formation in this `isolated' mode have consequently become well-understood. Most stars, however, form in more clustered environments. With the goal of characterizing the physical conditions of dense gas in cluster-forming environments, we present the results of systematic mapping of expected dense gas tracers (including NH3, N2H+, N2D+ and H2D+) towards the small cluster-forming Ophiuchus B Core. We find strong evidence that NH3 and N2H+ do not trace the coldest and densest locations of the core, including significant offsets between objects identified in continuum emission and in NH3 and N2H+ (in contrast with typical good correspondence found in isolated regions), relatively low fractional NH3 and N2H+ abundance at the positions of continuum objects, and a general trend of decreasing fractional abundance of both species with increasing H2 column density. Observations across Oph B2 of the deuterated species N2D+ and H2D+ show no abundance trends with column density, density or gas temperature. Also, though some similarity with the respective line emission with continuum is seen, the line emission is surprisingly weak towards embedded protostars in Oph B2. Destruction mechanisms for deuterated molecules require gas temperatures greater than previously determined for Oph B through NH3 emission.
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    ABSTRACT: Fully sampled degree-scale maps of the 13CO 2-1 and CO 4-3 transitions toward three members of the Lupus Molecular Cloud Complex - Lupus I, III, and IV - trace the column density and temperature of the molecular gas. Comparison with IR extinction maps from the c2d project requires most of the gas to have a temperature of 8-10 K. Estimates of the cloud mass from 13CO emission are roughly consistent with most previous estimates, while the line widths are higher, around 2 km/s. CO 4-3 emission is found throughout Lupus I, indicating widespread dense gas, and toward Lupus III and IV. Enhanced line widths at the NW end and along the edge of the B228 ridge in Lupus I, and a coherent velocity gradient across the ridge, are consistent with interaction between the molecular cloud and an expanding HI shell from the Upper-Scorpius subgroup of the Sco-Cen OB Association. Lupus III is dominated by the effects of two HAe/Be stars, and shows no sign of external influence. Slightly warmer gas around the core of Lupus IV and a low line width suggest heating by the Upper-Centaurus-Lupus subgroup of Sco-Cen, without the effects of an HI shell. Comment: 54 pages, 27 figures, 5 tables. To appear in ApJS. Preprint also available (with full-size figures) from http://www.astro.ex.ac.uk/people/nfht/publications.html Datacubes available from http://www.astro.ex.ac.uk/people/nfht/resources.html
    The Astrophysical Journal Supplement Series 10/2009; 185(1). DOI:10.1088/0067-0049/185/1/98 · 14.14 Impact Factor
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    ABSTRACT: The observations here were all obtained by the c2d project or by GTO observations that we have included in our data. They have been described in the publications given hereafter: Five large clouds were selected for the c2d project: Serpens (Eiroa et al., 2008hsf2.book..693E), Perseus (Bally et al., 2008hsf1.book..308B), Ophiuchus (Wilking et al., 2008hsf2.book..351W), Lupus (Comeron, 2008hsf2.book..295C), and Chamaeleon (Luhman, 2008hsf2.book..169L). (5 data files).

Publication Stats

8k Citations
794.65 Total Impact Points

Institutions

  • 1992–2013
    • Harvard-Smithsonian Center for Astrophysics
      • Smithsonian Astrophysical Observatory
      Cambridge, Massachusetts, United States
  • 2010
    • University of Michigan
      • Department of Astronomy
      Ann Arbor, Michigan, United States
  • 2009–2010
    • University of Toledo
      • Department of Physics and Astronomy
      Toledo, Ohio, United States
  • 1988–2007
    • Harvard University
      • Department of Physics
      Cambridge, Massachusetts, United States
  • 2004
    • National Institute of Astrophysics
      Roma, Latium, Italy
    • University of Florence
      Florens, Tuscany, Italy
  • 2003
    • The University of Arizona
      Tucson, Arizona, United States
  • 2002
    • University of Rochester
      • Department of Physics and Astronomy
      Rochester, NY, United States
  • 1994
    • University of Wisconsin–Madison
      Madison, Wisconsin, United States
  • 1989–1993
    • University of California, Berkeley
      • Radio Astronomy Laboratory
      Berkeley, California, United States
  • 1987–1993
    • Wellesley College
      Уэлсли, Massachusetts, United States
  • 1990
    • Université de Montréal
      Montréal, Quebec, Canada
  • 1979–1987
    • University of Massachusetts Amherst
      • Department of Astronomy
      Amherst Center, Massachusetts, United States
  • 1975–1983
    • Massachusetts Institute of Technology
      • Department of Physics
      Cambridge, Massachusetts, United States
  • 1980
    • Space Studies Institute
      MHV, California, United States
  • 1978
    • Columbia University
      New York, New York, United States