Showing papers by "Di Li published in 2007"

The Transition from Atomic to Molecular Hydrogen in Interstellar Clouds: 21 cm Signature of the Evolution of Cold Atomic Hydrogen in Dense Clouds

Abstract: We have investigated the timescale for formation of molecular clouds by examining the conversion of H I to H2 using a time-dependent model with H2 photodissociation including self-shielding. H2 formation on dust grains and cosmic-ray destruction are also included in one-dimensional model slab clouds that incorporate time-independent density and temperature distributions. We calculate 21 cm spectral line profiles seen in absorption against a background provided by general Galactic H I emission and compare the model spectra with H I narrow self-absorption (HINSA) profiles absorbed in a number of nearby molecular clouds. The time evolution of the H I and H2 densities is dramatic, with the atomic hydrogen disappearing in a wave propagating from the central, denser regions, which have a shorter H2 formation timescales, to the edges, where the density is lower and the timescales for H2 formation longer. The model 21 cm spectra are characterized by very strong absorption at early times. Emission at early times produced by the warm edges of the cloud is difficult to separate from variations in the background spectrum, when the background temperature is low. The minimum time for cloud evolution based on the model spectra is set by the requirement that most of the H I in the outer portions of the cloud be removed. The characteristic time that has elapsed since cloud compression and initiation of the H I → H2 conversion is a few ×1014 s, or 107 yr. This sets a minimum time for the age of these molecular clouds and thus for star formation that may take place within them.

Massive Quiescent Cores in Orion. II. Core Mass Function

Abstract: We have surveyed submillimeter continuum emission from relatively quiescent regions in the Orion molecular cloud to determine how the core mass function in a high-mass star-forming region compares to the stellar initial mass function. Such studies are important for understanding the evolution of cores to stars, and for comparison to formation processes in high- and low-mass star-forming regions. We used the SHARC II camera on the Caltech Submillimeter Observatory telescope to obtain 350 μm data having angular resolution of about 9'', which corresponds to 0.02 pc at the distance of Orion. Further data processing using a deconvolution routine enhances the resolution to about 3''. Such high angular resolution allows a rare look into individually resolved dense structures in a massive star-forming region. Our analysis combining dust continuum and spectral line data defines a sample of 51 Orion molecular cores with masses ranging from 0.1 to 46 M☉ and a mean mass of 9.8 M☉, which is 1 order of magnitude higher than the value found in typical low-mass star-forming regions, such as Taurus. The majority of these cores cannot be supported by thermal pressure or turbulence, and are probably supercritical. They are thus likely precursors of protostars. The core mass function for the Orion quiescent cores can be fitted by a power law with an index equal to -0.85 ± 0.21. This is significantly flatter than the Salpeter initial mass function and is also flatter than the core mass function found in low and intermediate star-forming regions. When compared with other massive star-forming regions such as NGC 7538, this slope is flatter than the index derived for samples of cores with masses up to thousands of M☉. Closer inspection, however, indicates slopes in those regions similar to our result if only cores in a similar mass range are considered. Based on the comparison between the mass function of the Orion quiescent cores and those of cores in other regions, we find that the core mass function is flatter in an environment affected by ongoing high-mass star formation. Thus, it is likely that environmental processes play a role in shaping the stellar IMF later in the evolution of dense cores and the formation of stars in such regions.

The Five College Radio Astronomy Observatory CO Mapping Survey of the Taurus Molecular Cloud

Abstract: The FCRAO Survey of the Taurus molecular cloud observed the 12CO and 13CO J = 1–0 emission from 98 deg2 of this important, nearby, star-forming region. This set of data with 45'' resolution comprises the highest spatial dynamic range image of an individual molecular cloud constructed to date and provides valuable insights to the molecular gas distribution, kinematics, and the star formation process. In this contribution, we describe the observations, calibration, data processing, and characteristics of the noise and line emission of the survey. The angular distribution of 12CO and 13CO emission over 1 km s−1 velocity intervals and the full velocity extent of the cloud are presented. These reveal a complex, dynamic medium of cold, molecular gas.