arXiv subjects

P. C. Myers

Publications and source records attributed to P. C. Myers.

63 records · Page 4Linked to original sources

arXiv2003

A 13CO and C18O Survey of the Molecular Gas Around Young Stellar Clusters Within 1kpc of the Sun

As the first step of a multi-wavelength investigation into the relationship between young stellar clusters and their environment we present fully-sampled maps in the J=1--0 lines of 13CO and C18O and the J=2--1 line of C18O for a selected group of thirty young stellar groups and clusters within 1kpc of the Sun. This is the first systematic survey of these regions to date. The clusters range in size from several stars to a few hundred stars. Thirty fields ranging in size from 8'x 8' to 60'x 30' were mapped with 47'' resolution simultaneously in the J=1-0 lines with Five College Radio Observatory. Seventeen sources were mapped over fields ranging in size from 3'x 3' to 13'x 13' in the J=2--1 line with 35'' resolution with the Heinrich Hertz Telescope. We compare the cloud properties derived from each of the three tracers in order to better understand systematic uncertainties in determing masses and linewidths. Cloud masses are determined independently using the 13CO and C18O transitions, these masses range from 30 to 4000 M_sun. Finally, we present a simple morphological classification scheme which may serve as a rough indicator of cloud evolution.

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arXiv2002

Inner Structure of Protostellar Collapse Candidate B335 Derived from Millimeter-Wave Interferometry

We present a study of the density structure of the protostellar collapse candidate B335 using continuum observations from the IRAM Plateau de Bure Interferometer made at wavelengths of 1.2mm and 3.0mm. We analyze these data, which probe spatial scales from 5000 AU to 500 AU, directly in the visibility domain by comparison to synthetic observations constructed from models that assume different physical conditions. This approach allows for much more stringent constraints to be derived from the data than from analysis of images. A single radial power law in density provides a good description of the data, with best fit power law index p=1.65+/-0.05. Through simulations, we quantify the sensitivity of this result to various model uncertainties, including assumptions of temperature distribution, outer boundary, dust opacity spectral index, and an unresolved central component. The largest uncertainty comes from the unknown presence of a centralized point source. A point source with 1.2mm flux of F=12+/-7 mJy reduces the density index to p=1.47+/-0.07. The remaining sources of systematic uncertainty, the most important of which is the temperature distribution, likely contribute a total uncertainty of < 0.2. We therefore find strong evidence that the power law index of the density distribution within 5000 AU is significantly less than the value at larger radii, close to 2.0 from previous studies of dust emission and extinction. These results conform well to the generic paradigm of isolated, low-mass star formation which predicts a power law density index close to p=1.5 for an inner region of gravitational free fall onto the protostar.

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arXiv2002

Limits on Radio Continuum Emission from a Sample of Candidate Contracting Starless Cores

We used the NRAO Very Large Array to search for 3.6 cm continuum emission from embedded protostars in a sample of 8 nearby ``starless'' cores that show spectroscopic evidence for infalling motions in molecular emission lines. We detect a total of 13 compact sources in the eight observed fields to 5 sigma limiting flux levels of typically 0.09 mJy. None of these sources lie within 1' of the central positions of the cores, and they are all likely background objects. Based on an extrapolation of the empirical correlation between the bolometric luminosity and 3.6 cm luminosity for the youngest protostars, these null-detections place upper limits of ~0.1 L_sun (d/140pc)^2 on the luminosities of protostellar sources embedded within these cores. These limits, together with the extended nature of the inward motions inferred from molecular line mapping (Lee et al. 2001), are inconsistent with the inside-out collapse model of singular isothermal spheres and suggest a less centrally condensed phase of core evolution during the earliest stages of star formation.

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arXiv2001

Systematic Molecular Differentiation in Starless Cores

(Abridged) We present evidence that low-mass starless cores, the simplest units of star formation, are systematically differentiated in their chemical composition. Molecules including CO and CS almost vanish near the core centers, where the abundance decreases by one or two orders of magnitude. At the same time, N2H+ has a constant abundance, and the fraction of NH3 increases toward the core center. Our conclusions are based on a study of 5 mostly-round starless cores (L1498, L1495, L1400K, L1517B, and L1544), which we have mappedin C18O(1-0), C17O(1-0), CS(2-1), C34S(2-1), N2H+(1-0), NH3(1,1) and (2,2), and the 1.2 mm continuum. For each core we have built a model that fits simultaneously the radial profile of all observed emission and the central spectrum for the molecular lines. The observed abundance drops of CO and CS are naturally explained by the depletion of these molecules onto dust grains at densities of 2-6 10^4 cm-3. N2H+ seems unaffected by this process up to densities of several 10^5, while the NH3 abundance may be enhanced by reactions triggered by the disappearance of CO from the gas phase. With the help of our models, we show that chemical differentiation automatically explains the discrepancy between the sizes of CS and NH3 maps, a problem which has remained unexplained for more than a decade. Our models, in addition, show that a combination of radiative transfer effects can give rise to the previously observed discrepancy in the linewidth of these two tracers. Although this discrepancy has been traditionally interpreted as resulting from a systematic increase of the turbulent linewidth with radius, our models show that it can arise in conditions of constant gas turbulence.

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arXiv2001

Molecular ions in L1544. I. Kinematics

We have mapped the dense dark core L1544 in H13CO+(1-0), DCO+(2-1), DCO+(3-2), N2H+(1-0), NTH+(3-2), N2D+(2-1), N2D+(3-2), C18O(1-0), and C17O(1-0) using the IRAM 30-m telescope. We have obtained supplementary observations of HC18O+(1-0), HC17O+(1-0), and D13CO+(2-1). Many of the observed maps show a general correlation with the distribution of dust continuum emission in contrast to C18O(1-0) and C17O(1-0) which give clear evidence for depletion of CO at positions close to the continuum peak. In particular N2D+(2-1) and (3-2) and to a lesser extent N2H+(1-0) appear to be excellent tracers of the dust continuum. We find that the tracers of high density gas (in particular N2D+) show a velocity gradient along the minor axis of the L1544 core and that there is evidence for larger linewidths close to the dust emission peak. We interpret this using the model of the L1544 proposed by Ciolek & Basu (2000) and by comparing the observed velocities with those expected on the basis of their model. The results show reasonable agreement between observations and model in that the velocity gradient along the minor axis and the line broadening toward the center of L1544 are predicted by the model. This is evidence in favour of the idea that amipolar diffusion across field lines is one of the basic processes leading to gravitational collapse. However, line widths are significantly narrower than observed and are better reproduced by the Myers & Zweibel (2001) model which considers the quasistatic vertical contraction of a layer due to dissipation of its Alfvenic turbulence, indicating the importance of this process for cores in the verge of forming a star.

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arXiv2001

Molecular ions in L1544. II. The ionization degree

The maps presented in Paper I are here used to infer the variation of the column densities of HCO+, DCO+, N2H+, and N2D+ as a function of distance from the dust peak. These results are interpreted with the aid of a crude chemical model which predicts the abundances of these species as a function of radius in a spherically symmetric model with radial density distribution inferred from the observations of dust emission at millimeter wavelengths and dust absorption in the infrared. Our main observational finding is that the N(N2D+)/N(N2H+) column density ratio is of order 0.2 towards the L1544 dust peak as compared to N(DCO+)/N(HCO+) = 0.04. We conclude that this result as well as the general finding that N2H+ and N2D+ correlate well with the dust is caused by CO being depleted to a much higher degree than molecular nitrogen in the high density core of L1544. Depletion also favors deuterium enhancement and thus N2D+, which traces the dense and highly CO-depleted core nucleus, is much more enhanced than DCO+. Our models do not uniquely define the chemistry in the high density depleted nucleus of L1544 but they do suggest that the ionization degree is a few times 10^{-9} and that the ambipolar diffusion time scale is locally similar to the free fall time. It seems likely that the lower limit which one obtains to ionization degree by summing all observable molecular ions is not a great underestimate of the true ionization degree. We predict that atomic oxygen is abundant in the dense core and, if so, H3O+ may be the main ion in the central highly depleted region of the core.

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arXiv2000

L483: A Protostar In Transition From Class 0 to Class I

We present molecular-line observations toward the dense core in L483 and its bipolar outflow powered by the Class 0 object IRAS 18148-0440. CO (carbon monoxide) maps show that the outflow is well collimated and asymmetric, and that its gas is warmer than the surrounding cloud by at least a factor of 2. H2CO (formaldehyde) lines toward the outflow show prominent high-velocity wings and evidence for an H2CO abundance enhancement of a factor of 20. At ambient velocities, these lines show strong self-absorption and a brighter blue peak, a characteristic signature of inward motions. Finally, and in contrast with the outflows from other Class 0 objects, the CH3OH (methanol) lines in L483 do not show high velocity wings, and no evidence for abundance enhancement is found in this molecule or in SiO (silicon monoxide). Comparing the physical and chemical properties of the outflow in L483 with those of other outflows from Class 0 and Class I sources, we find that the L483 outflow is somewhat intermediate between these types. This suggests that the L483 central source has already started its transition between Class 0 and Class I, and that its mixed properties illustrate how this transition occurs.

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arXiv1997

A Search for Infall Motions Toward Nearby Young Stellar Objects

We report observations of 47 candidate protostars in two optically thick lines (H2CO 212-111 and CS 2-1) and one optically thin line (N2H+ 1-0) using the IRAM 30-m, SEST 15-m, and Haystack 37-m radio telescopes. The sources were selected from the redness of their spectra (Tbol < 200 K) and their near distance (d < 400 pc). Most of the sources have asymmetric optically thick lines. The observed distribution of velocity differences, deltaV = (Vthick-Vthin)/FWHMthin, is skewed toward negative (blue-shifted) velocities for both the H2CO and CS samples. This excess is much more significant for Class 0 than for Class I sources, suggesting that we detect infall motions toward Class 0 and not toward Class I sources. This indicates a difference in the physical conditions in the circumstellar envelopes around Class I and Class 0 sources, but does not rule out the presence of infall onto Class I sources by e.g. lower opacity gas. Bipolar outflows alone, or rotation alone, cannot reproduce these statistics if the sample of sources has randomly oriented symmetry axes. We identify 15 spectroscopic infall candidates, of which 6 are new. Most of these infall candidates have primarily turbulent rather than thermal motions, and are associated with clusters rather than being isolated.

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arXiv1997

Four New Dense Molecular Cores in the Taurus Molecular Cloud (TMC): Ammonia and Cyanodiacetylene Observations

Trying to obtain a more complete picture of star forming regions in the Taurus Molecular Cloud, four newly discovered dense cores (L1521D, L1521F, L1524, L1507A) are identified and mapped through the ammonia (J,K)=(1,1) and (2,2) rotational inversion lines. These cores have sizes of 0.06-0.09 pc, hydrogen densities of order 10000 - 100000 cm-3 and kinetic temperatures between 8 and 10 K. The masses range from 0.2 - 1.0 solar masses, placing the cores with the lowest M values at the lower edge of the mass distribution for ammonia cores in the TMC. Turbulent, thermal and gravitational energies have been estimated. A comparison between these energy terms and considerations related to thermal and turbulent line broadening indicate that these cores are close to equilibrium. Moreover we report the detection of the J=9-8 transition of HC5N towards the four ammonia peak positions. The HC5N column densities are in agreement with values derived for other molecular cores in the TMC.

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