Publications (8)0 Total impact
-
Article: A Simple Perspective on the Mass-Area Relationship in Molecular Clouds
[show abstract] [hide abstract]
ABSTRACT: Despite over 30 years of study, the mass-area relationship within and among clouds is still poorly understood both observationally and theoretically. Modern extinction datasets should have sufficient resolution and dynamic range to characterize this relationship for nearby molecular clouds, although recent papers using extinction data seem to yield different interpretations regarding the nature and universality of this aspect of cloud structure. In this paper we try to unify these various results and interpretations by accounting for the different ways cloud properties are measured and analyzed. We interpret the mass-area relationship in terms of the column density distribution function and its possible variation within and among clouds. We quantitatively characterize regional variations in the column density PDF. We show that structures both within and among clouds possess the same degree of "universality", in that their PDF means do not systematically scale with structure size. Because of this, mass scales linearly with area.04/2012; -
Article: 2MASS wide field extinction maps: IV. The Orion, Mon R2, Rosette, and Canis Major star forming regions
[show abstract] [hide abstract]
ABSTRACT: We present a near-infrared extinction map of a large region (approximately 2200 deg^2) covering the Orion, the Monoceros R2, the Rosette, and the Canis Major molecular clouds. We used robust and optimal methods to map the dust column density in the near-infrared (NICER and NICEST) towards ~19 million stars of the Two Micron All Sky Survey (2MASS) point source catalog. Over the relevant regions of the field, we reached a 1-sigma error of 0.03 mag in the K-band extinction with a resolution of 3 arcmin. We measured the cloud distances by comparing the observed density of foreground stars with the prediction of galactic models, thus obtaining d_{Orion A} = (371 +/- 10) pc, d_{Orion B} = (398 +/- 12) pc, $d_{Mon R2} = (905 +/- 37) pc, $d_{Rosette} = (1330 +/- 48) pc, and $d_{CMa} = (1150 +/- 64) pc, values that compare very well with independent estimates.07/2011; -
Article: Insights on molecular cloud structure
[show abstract] [hide abstract]
ABSTRACT: Stars form in the densest regions of clouds of cold molecular hydrogen. Measuring structure in these clouds is far from trivial as 99% of the mass of a molecular cloud is inaccessible to direct observation. Over the last decade we have been developing an alternative, more robust density tracer technique based on dust extinction measurements towards background starlight. The new technique does not suffer from the complications plaguing the more conventional molecular line and dust emission techniques, and when used with these can provide unique views on cloud chemistry and dust grain properties in molecular clouds. In this brief communication we summarize the main results achieved so far using this technique.Proceedings of the International Astronomical Union 04/2010; 6:99 - 102. -
Chapter: THE ORIGIN OF THE INITIAL MASS FUNCTION IS IN THE CLOUD STRUCTURE
[show abstract] [hide abstract]
ABSTRACT: Detailed knowledge of the initial distribution of stellar masses at birth (known as the initial mass function or IMF) is necessary to predict and understand the evolution of stellar systems, such as clusters and galaxies. Unfortunately, stellar evolution theory cannot predict the form of this critical function and the origin of the stellar IMF remains one of the major unsolved problems of modern astrophysics.Stars form in the cold dense cores of interstellar molecular clouds and the detailed knowledge of the spectrum of masses of such cores is clearly a key for the understanding of the origin of the IMF. To date, observations have presented somewhat contradictory evidence relating to this issue. Here we present a new and more robust determination of the dense core mass function. We find the core mass function to be surprisingly similar to the stellar IMF, modified by a uniform star formation efficiency of about 30%. This suggests that the distribution of stellar birth masses is a direct product of the fragmentation/coalescence process in a molecular cloud.12/2006: pages 417-422; -
Article: The Formation of Brown Dwarfs: Observations
[show abstract] [hide abstract]
ABSTRACT: We review the current state of observational work on the formation of brown dwarfs, focusing on their initial mass function, velocity and spatial distributions at birth, multiplicity, accretion, and circumstellar disks. The available measurements of these various properties are consistent with a common formation mechanism for brown dwarfs and stars. In particular, the existence of widely separated binary brown dwarfs and a probable isolated proto-brown dwarf indicate that some substellar objects are able to form in the same manner as stars through unperturbed cloud fragmentation. Additional mechanisms such as ejection and photoevaporation may play a role in the birth of some brown dwarfs, but there is no observational evidence to date to suggest that they are the key elements that make it possible for substellar bodies to form.09/2006; -
Article: 24-micron Detections of Disks around Very Low-mass Stars and Brown Dwarfs in IC 348: Grain Growth/Settling and Inner Holes?
[show abstract] [hide abstract]
ABSTRACT: We present observations of six late-type members of the young cluster IC 348 detected at 24 microns with the Multiband Imaging Photometer for Spitzer(MIPS). At least four of the objects are probably substellar. Combining these data with ground-based optical and near-infrared photometry and complementary observations with the Infrared Array Camera (IRAC), we have modeled the spectral energy distributions using detailed models of irradiated accretion disks. We are able to fit the observations with models using a range of maximum grain sizes from ISM-type dust to grains as large as 1 millimeter. Two objects show a lack of excess emission at wavelengths shortward of 5.8-8 microns but significant excess at longer wavelengths, indicative of large optically thin or evacuated inner holes. Our models indicate a inner hole of radius ~ 0.5-0.9 AU for the brown dwarf L316; this is the first brown dwarf with evidence for an AU-scale inner disk hole. We examine several possible mechanisms for the inner disk clearing in this case, including photoevaporation and planet formation.04/2006; -
Chapter: Molecular Cloud Structure: The VLT View
[show abstract] [hide abstract]
ABSTRACT: Despite 30 years of molecular spectroscopy of dark clouds little is understood about the internal structure of these objects and consequently about the initial conditions that give rise to star and planet formation. This is largely due to the fact that molecular clouds are primarily composed of molecular hydrogen, which is virtually inaccessible to direct observation. The traditional methods used to derive the basic physical properties of these clouds therefore make use of observations of trace H2 surrogates, namely those rare molecules with sufficient dipole moments to be easily detected by radio spectroscopic techniques (e.g., Lada 1996, Myers 1999), and interstellar dust, whose thermal emission can be detected by radio continuum techniques (e.g., André et al. 2000). However, as discussed in the previous article in this book by M. Walmsley and collaborators, the interpretation of results derived from these methods is not always straightforward (see also Alves, Lada, & Lada 1999 and Zucconi, Walmsley, & Galli 2001). Several poorly constrained effects inherent in these techniques (e.g., deviations from local thermodynamic equilibrium, opacity variations, chemical evolution, small-scale structure, depletion of molecules, unknown emissivity properties of the dust, unknown dust temperature) make the construction of an unambiguous picture of the physical structure of these objects a very difficult task. There is a clear need for a less complicate and more robust tracer of H2 to access not only the physical structure of these objects but also to accurately calibrate molecular abundances and dust emissivity inside these clouds. The deployment of sensitive, large format infrared array cameras on large telescopes however, has fulfilled this need by enabling the direct measurement of the dust extinction toward thousands of individual background stars observed through the densest regions of a molecular cloud. Such measurements are free from the complications that plague molecular-line or dust emission data and enable detailed maps of cloud density structure to be constructed.02/2006: pages 37-44; -
Article: Evidence for Circumstellar Disks around Young Brown Dwarfs in the Trapezium Cluster
[show abstract] [hide abstract]
ABSTRACT: We report the results of deep infrared observations of brown dwarf candidates in the Trapezium cluster in Orion. Analysis of the JHK color-color diagram indicates that a large fraction (approx 65% +/- 15%) of the observed sources exhibit infrared excess emission. This suggests the extreme youth of these objects and in turn, provides strong independent confirmation of the existence of a large population of substellar objects in the cluster. Moreover, this suggests that the majority of these substellar objects are presently surrounded by circumstellar disks similar to the situation for the stellar population of the cluster. This evidence for a high initial disk frequency (> 50%) around cluster members of all masses combined with the smooth continuity of the cluster's initial mass function across the hydrogen burning limit suggests that a single physical mechanism is likely responsible for producing the entire cluster mass spectrum down to near the deuterium burning limit. The results may also indicate that even substellar objects are capable of forming with systems of planetary companions. Comment: 6 pages, 2 figures. Accepted to ApJ Letters07/2001;
Top co-authors
Top Journals
Institutions
-
2006–2011
-
Harvard-Smithsonian Center for Astrophysics
Cambridge, MA, USA
-
