arXiv subjects

Philip C. Myers

Publications and source records attributed to Philip C. Myers.

At least 127 records · Page 7Linked to original sources

arXiv2005

Observations of Global and Local Infall in NGC 1333

We report ``infall asymmetry'' in the HCO$^+$ (1--0) and (3--2) lines toward NGC 1333, extended over $\sim 0.39 {\rm pc}^2$, a larger extent than has been reported be fore, for any star-forming region. The infall asymmetry extends over a major portion of the star-forming complex, and is not limited to a single protostar, or to a single dense core, or to a single spectral line. It seems likely that the infall asymmetry represents inward motions, and that these motions are physically associated with the complex. Both blue-asymmetric and red-asymmetric lines are seen, but in both the (3--2) and (1--0) lines of HCO$^+$ the vast majority of the asymmetric lines are blue, indicating inward motions. The (3--2) line, tracing denser gas, has the spectra with the strongest asymmetry and these spectra are associated with the protostars IRAS 4A and 4B, which most likely indicates a warm central source is affecting the line profiles. The (3--2) and (1--0) lines usually have the same sense of asymmetry in common positions, but their profiles differ significantly, and the (1--0) line appears to trace motions on much larger spatial scales than does the (3--2) line. Line profile models fit the spectra well, but do not strongly constrain their parameters. The mass accretion rate of the inward motions is of order 10$^{-4}$ M$_\odot$/yr, similar to the ratio of stellar mass to cluster age.

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arXiv2005

Discovery of a Low Mass Bipolar Molecular Outflow from L1014-IRS with the Submillimeter Array

Using the Submillimeter Array we report the discovery of a compact low mass bipolar molecular outflow from L1014-IRS and confirm its association with the L1014 dense core at 200 pc. Consequently, L1014-IRS is the lowest luminosity (L \~0.09 Lsun) and perhaps the lowest mass source known to be driving a bipolar molecular outflow, which is one of the smallest known in size (~500 AU), mass (< 10^{-4} Msun), and energetics (e.g., force < 10^{-7} Msun km/s/yr).

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arXiv2005

Centrally Condensed Collapse of Starless Cores

Models of self-gravitating gas in the early stages of pressure-free collapse are compared for initial states which are equilibrium layers, cylinders, and Bonnor-Ebert spheres. For each geometrical case the density profile has an inner region of shallow slope surrounded by an outer region of steep slope, and the profile shape during early collapse remains similar to the profile shape of the initial equilibrium. The two-slope density structure divides the spherical collapse history into a starless infall phase and a protostellar accretion phase. The similarity of density profiles implies that Bonnor-Ebert fits to observed column density maps may not distinguish spherical cores from oblate or prolate cores, and may not distinguish static cores from collapsing cores. The velocity profiles discriminate better than the density profiles between initial geometries and between collapse ages. The infall velocity generally has a subsonic maximum value, which is approximately equal to the initial velocity dispersion times the ratio of collapse age to central free-fall time. Observations of starless core line profiles constrain collapse models. Collapse from initial states which are strongly condensed and slightly prolate is consistent with infall asymmetry observed around starless cores, and is more consistent than collapse from initial states which are weakly condensed, and/or oblate. Spherical models match observed inward speeds 0.05-0.09 km/s over 0.1-0.2 pc, if the collapse has a typical age 0.3-0.5 free fall times, and if it began from a centrally condensed state which was not in stable equilibrium. In a collapsing core, optically thin line profiles should broaden and develop two-peak structure as seen in L1544.

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arXiv2004

Molecular Line Profile Fitting with Analytic Radiative Transfer Models

We present a study of analytic models of starless cores whose line profiles have ``infall asymmetry,'' or blue-skewed shapes indicative of contracting motions. We compare the ability of two types of analytical radiative transfer models to reproduce the line profiles and infall speeds of centrally condensed starless cores whose infall speeds are spatially constant and range between 0 and 0.2 km s-1. The model line profiles of HCO+ (J=1-0) and HCO+ (J=3-2) are produced by a self-consistent Monte Carlo radiative transfer code. The analytic models assume that the excitation temperature in the front of the cloud is either constant (``two-layer'' model) or increases inward as a linear function of optical depth (``hill'' model). Each analytic model is matched to the line profile by rapid least-squares fitting. The blue-asymmetric line profiles with two peaks, or with a blue shifted peak and a red shifted shoulder, can be well fit by the ``HILL5'' model (a five parameter version of the hill model), with an RMS error of 0.02 km s-1. A peak signal to noise ratio of at least 30 in the molecular line observations is required for performing these analytic radiative transfer fits to the line profiles.

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arXiv2004

IRAC Colors of Young Stellar Objects

We compare the infrared colors predicted by theoretical models of protostellar envelopes and protoplanetary disks with initial observations of young stellar objects made with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope (Werner et al. 2004, Fazio et al. 2004). Disk and envelope models characterized by infall and/or accretion rates found in previous studies can quantitatively account for the range of IRAC colors found in four young embedded clusters: S 140, S 171, NGC 7129, and Cep C. The IRAC color-color diagram ([3.6]-[4.5] vs. [5.8]-[8.0]) can be used to help to distinguish between young stars with only disk emission and protostars with circumstellar envelopes.

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arXiv2004

A Survey For Infall Motions Toward Starless Cores. III. CS (3-2) and DCO+(2-1) Observations

We present CS(3-2) and DCO+(2-1) observations of 94 starless cores and compare the results with previous CS(2-1) and N2H+(1-0) observations to study inward motions in starless cores. The velocity shifts of the CS(3-2) and (2-1) lines with respect to N2H+ correlate well with each other and have similar distributions. This implies that, in many cores, systematic inward motions of gaseous material may occur over a range of density of at least a factor 4. We identify 18 infall candidates based on observations of CS(3-2), CS(2-1), DCO+(2-1) and N2H+(1-0). The eight best candidates, L1355, L1498, L1521F, L1544, L158, L492, L694-2, and L1155C-1, each show at least four indications of infall asymmetry and no counter-indications. Fits of the spectra to a 2-layer radiative transfer model in ten infall candidates suggest that the median effective line-of-sight speed of the inward-moving gas is ~0.07 km/s for CS (3-2) and ~0.04 km/s for CS(2-1). Considering that the optical depth obtained from the fits is usually smaller in CS(3-2) than in (2-1) line, this may imply that CS(3-2) usually traces inner denser gas in higher inward motions than CS(2-1). However, it is also possible that this conclusion is not representative of all starless core infall candidates, due to the statistically small number analyzed here. Further line observations will be useful to test this conclusion.

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arXiv2003

Envelope Structure of Starless Core L694-2 Derived from a Near-Infrared Extinction Map

We present a near-infrared extinction study of the dark globule L694-2, a starless core that shows strong evidence for inward motions in molecular line profiles. The J,H, and K band data were taken using the European Southern Observatory New Technology Telescope. The best fit simple spherical power law model has index p=2.6 +/- 0.2, over the 0.036--0.1 pc range in radius sampled in extinction. This power law slope is steeper than the value of p=2 for a singular isothermal sphere, the initial condition of the inside-out model for protostellar collapse. Including an additional extinction component along the line of sight further steepens the inferred profile. Fitting a Bonnor-Ebert sphere results in a super-critical value of the dimensionless radius xi_max=25 +/- 3. The unstable configuration of material may be related to the observed inward motions. The Bonnor-Ebert model matches the shape of the observed profile, but significantly underestimates the amount of extinction (by a factor of ~4). This discrepancy in normalization has also been found for the nearby protostellar core B335 (Harvey et al. 2001). A cylindrical density model with scale height H=0.0164+/- 0.002 pc viewed at a small inclination to the cylinder axis provides an equally good radial profile as a power law model, and reproduces the asymmetry of the core remarkably well. In addition, this model provides a basis for understanding the discrepancy in the normalization of the Bonnor-Ebert model, namely that L694-2 has prolate structure, with the full extent (mass) of the core being missed by assuming symmetry between the profiles in the plane of the sky and along the line-of-sight. If the core is sufficiently magnetized then fragmentation may be avoided, and later evolution might produce a protostar similar to B335.

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arXiv2003

A Catalog of Young Stellar Groups and Clusters Within 1kpc of the Sun

We present a catalog of near-infrared surveys of young (<= a few 10^6 yr) stellar groups and clusters within 1 kpc from the Sun, based on an extensive search of the literature from the past ten years. We find 143 surveys from 69 published articles, covering 73 different regions. The number distribution of stars in a region has a median of 28 and a mean of 100. About 80% of the stars are in clusters with at least 100 members. By a rough classification of the groups and clusters based on the number of their associated stars, we show that most of the stars form in large clusters. The spatial distribution of cataloged regions in the Galactic plane shows a relative lack of observed stellar groups and clusters in the range 270 deg < l < 60 deg of Galactic longitude, reflecting our location between the Local and Sagittarius arms. This compilation is intended as a useful resource for future studies of nearby young regions of multiple star formation.

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arXiv2003

Disk Properties and Density Structure of the Star-Forming Dense Core B335

We present subarcsecond resolution dust continuum observations of the protostellar collapse candidate B335 made with the IRAM Plateau de Bure Interferometer at wavelengths of 1.2 and 3.0 mm. These observations probe to < 100 AU size scales and reveal a compact source component that we identify with a circumstellar disk. We analyze these data in concert with previous lower resolution interferometer observations and find a best fit density structure for B335 that consists of a power law envelope with index p=1.55 +/- 0.04 (r < 5000 AU) together with a disk (r < 100 AU) of flux F_{1.2 mm}=21 +/-2 mJy. We estimate a systematic uncertainty in the power law index delta(p) < 0.15, where the largest error comes from the assumed form of the dust temperature falloff with radius. This determination of the inner density structure of B335 has a precision unique amongst protostellar cores, and it is consistent with the r^{-1.5} profile of gravitational free-fall, in accord with basic expectations for the formation of a star. The flux (and implied mass) of the compact component in B335 is typical of the disks around T Tauri stars.

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arXiv2003

Inner Structure of Starless Core L694--2 Derived from Millimeter-Wave Interferometry

We study the density structure of candidate contracting starless core L694-2 using 1.3 mm dust continuum observations from the IRAM Plateau de Bure Interferometer and the Berkeley-Illinois-Maryland Array, probing spatial scales from 10000-500 AU. The long baseline PdBI observations detect no emission, and limit the contamination from a compact component F_c < 2.7 mJy. This limit corresponds to a very small disk mass, M_disk < 5e-4 M_sun x (60 K / T_disk), and bolsters the ``starless'' interpretation of the core. The shorter baseline BIMA data are compared to density models using a physically motivated temperature distribution with a central minimum. This analysis provides clear evidence for a turn-over from the steep outer density profile observed in dust extinction to much more shallow behavior in the inner regions (<7500 AU). The best fit Bonnor-Ebert, Plummer-like, broken power law, and end-on cylinder models produce very similar flattened profiles and cannot be distinguished. We quantify the sensitivity of the inferred structure to various uncertainties, including the temperature distribution, the central position, and the presence of a weak unresolved central component. The largest uncertainty comes from the temperature assumption; an isothermal model modifies the best fit parameters by \~2 sigma, with the inferred profiles more shallow. Dust emission and extinction profiles are reproduced by an embedded cylinder with scale height H=13.5'' inclined at a small angle to the line of sight. The flat central density profile suggests that pressure forces still support the core, despite the extended inward motions inferred from molecular line observations (Lee, Myers, & Tafalla 2001). In the context of the cylindrical density model, these inward motions may represent the contraction of a prolate core along its major axis.

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arXiv2003

Infrared Photometry of Starless Dense Cores

Deep JHKs photometry was obtained towards eight dense molecular cores and J-H vs. H-Ks color-color plots are presented. Our photometry, sensitive to the detection of a 1 solar mass, 1 X 10^6 year old star through approx. 35 - 50 magnitudes of visual extinction, shows no indication of the presence of star/disk systems based on J-H vs. H-Ks colors of detected objects. The stars detected towards the cores are generally spatially anti-correlated with core centers suggesting a background origin, although we cannot preclude the possibility that some stars detected at H and Ks alone, or Ks alone, are not low mass stars or brown dwarfs (< 0.3 Solar Masses) behind substantial amounts of visual extinction (e.g. 53 magnitudes for L183B). Lower limits to optical extinctions are estimated for the detected background stars, with high extinctions being encountered, in the extreme case ranging up to at least Av = 46, and probably higher. The extinction data are used to estimate cloud masses and densities which are comparable to those determined from molecular line studies. Variations in cloud extinctions are consistent with a systematic nature to cloud density distributions and column density variations and extinctions are found to be consistent with submillimeter wave continuum studies of similar regions. The results suggest that some cores have achieved significant column density contrasts (approx. 30) on sub-core scales (approx. 0.05 pc) without having formed known stars.

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arXiv2002

Dense Cores in Dark Clouds. XIV. N2H+(1-0) maps of dense cloud cores

We present results of an extensive mapping survey of N2H+(1-0) in about 60 low mass cloud cores already mapped in the NH3(1,1) inversion transition line. The survey has been carried out at the FCRAO antenna with an angular resolution about 1.5 times finer than the previous ammonia observations. Cores with stars typically have map sizes about a factor of two smaller for N2H+ than for NH3, indicating the presence of denser and more centrally concentrated gas compared to starless cores. Significant correlations are found between NH3 and N2H+ column densities and excitation temperatures in starless cores, but not in cores with stars, suggesting a different chemical evolution of the two species. Velocity gradients range between 0.5 and 6 km/s/pc, similar to what has been found with NH3 data. ``Local'' velocity gradients show significant variation in both magnitude and direction, suggesting the presence of complexmotions not interpretable as simple solid body rotation. Integrated intensity profiles of starless cores present a ``central flattening'' and are consistent with a spherically symmetric density law n ~ r^{-1.2} for r < ~0.03 pc and n ~ r^{-2} at larger r. Cores with stars are better modelled with single density power laws with n ~ r^{-2}. Line widths change across the core but we did not find a general trend. The deviation in line width correlates with the mean line width, suggesting that the line of sight contains ~ 10 coherence lengths. The corresponding value of the coherence length, ~ 0.01 pc, is similar to the expected cutoff wavelength for MHD waves. This similarity may account for the increased ``coherence'' of line widths on small scales. Despite of the finer angular resolution, the majority of N2H+ and NH3 maps show a similar ``simple'' structure, with single peaks and no elongation.

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arXiv2001

HST/NICMOS Imaging Survey of the Ophiuchus (Lynds 1688) Cluster

We present a catalogue of near-infrared photometry of young stars associated with the Ophiuchus molecular cloud, based on observations made with the Hubble Space Telescope NICMOS-3 camera at 1.1 $μ$m and 1.6 $μ$m. Our survey covers 0.02 square degrees centered on the dense molecular cores in Lynds 1688. We detect 165 sources at 1.6 $μ$m and 65 sources at 1.1 $μ$m, within our estimated completeness limits of 21.0 mag and 21.5 mag, respectively. An analysis of the cloud extinction, based on existing molecular line maps, suggests that most of the sources lying within the 40 $\rm{A_V}$ extinction contour of the cloud are probable cloud members. Approximately half (58/108) of these sources are previously unpublished. The faint embedded sources revealed by these observations are spatially concentrated in three regions of high stellar space density (N$> 10^4$ stars pc$^{-3}$). While the spatial distribution of these sources reflects that of the brighter, well--known population of young stars in Ophiuchus, it is distinctly different from the distribution of cool concentrations seen in the submillimeter. Seven new brown dwarf candidates are identified, based on their infrared colors and their projected locations on high column-density regions of the molecular cloud. Eight new candidate binary and five new candidate triple systems, having separations between 0\farcs2 to 10\arcsec (29 to 1450 AU) are reported. The spatial resolution and sensitivity of these observations reveal five apparent disk/envelope systems seen via scattered light, and four nebulous objects with complex morphologies.

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arXiv2001

Structure of Protostellar Collapse Candidate B335 Derived from Near-Infrared Extinction Maps

We present a near-infrared extinction study of the dark globule B335, a protostellar collapse candidate, using data from HST/NICMOS and the W.M. Keck Observatory. These data allow a new quantitative test of the "inside-out" collapse model previously proposed to explain molecular line profiles observed toward this region. We find that the shape of the density profile is well matched by the collapse model, but that the amount of extinction corresponds to larger column densities than predicted. An unstable Bonnor-Ebert sphere with dimensionless outer radius xi_max=12.5+/-2.6 provides an equally good description of the density profile, and is indistinguishable from the collapse model over the range in radius sampled by the data. The bipolar outflow driven by the embedded YSO has an important effect on the extinction through the core, and modeling the outflow as a hollowed-out bipolar cone of constant opening angle provides a good match to theobservations. The complete exinction map is well reproduced by a model that includes both infall and outflow, and an extra 20% dispersion that likely results from residual turbulent motions. This fitted model has an infall radius of R_inf=26"+/-3"(0.031pc for 250pc distance), and an outflow cone semi-opening angle of alpha=41+/-2 degrees. The fitted infall radius is consistent with those derived from molecular line observations and supports the inside-out collapse interpretation of the density structure. The fitted opening angle of the outflow is slightly larger than observed in CO emission, perhaps because the full extent of the outflow cone in CO becomes confused with ambient core emission at low velocities.

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arXiv2001

Infall, Outflow, Rotation, and Turbulent Motions of Dense Gas within NGC 1333 IRAS 4

Millimeter wavelength observations are presented of NGC 1333 IRAS 4, a group of highly-embedded young stellar objects in Perseus, that reveal motions of infall, outflow, rotation, and turbulence in the dense gas around its two brightest continuum objects, 4A and 4B. These data have finest angular resolution of approximately 2" (0.0034 pc) and finest velocity resolution of 0.13 km/s. Infall motions are seen from inverse P-Cygni profiles observed in H2CO 3_12-2_11 toward both objects, but also in CS 3-2 and N2H+ 1-0 toward 4A, providing the least ambiguous evidence for such motions toward low-mass protostellar objects. Outflow motions are probed by bright line wings of H2CO 3_12-2_11 and CS 3-2 observed at positions offset from 4A and 4B, likely tracing dense cavity walls. Rotational motions of dense gas are traced by a systematic variation of the N2H+ line velocities, and such variations are found around 4A but not around 4B. Turbulent motions appear reduced with scale, given N2H+ line widths around both 4A and 4B that are narrower by factors of 2 or 3 than those seen from single-dish observations. Minimum observed line widths of approximately 0.2 km/s provide a new low, upper bound to the velocity dispersion of the parent core to IRAS 4, and demonstrate that turbulence within regions of clustered star formation can be reduced significantly. A third continuum object in the region, 4B', shows no detectable line emission in any of the observed molecular species.

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arXiv2001

A Survey for Infall Motions toward Starless Cores. II. $CS (2-1)$ and $N_2H^+ (1-0)$ Mapping Observations

We present the results of an extensive mapping survey of 53 `starless' cores in the optically thick line of CS 2-1 and the optically thin lines of N2H+ 1-0 and C18O 1-0. The purpose of this survey was to search for signatures of extended inward motions. This study finds 10 `strong' and 9 `probable' infall candidates, based on $δV_{CS}$ analysis and on the spectral shapes of CS lines. From our analysis of the blue-skewed CS spectra and the $δV_{CS}$ parameter, we find typical infall radii of 0.06-0.14 pc. Also, using a simple two layer radiative transfer model to fit the profiles, we derive one-dimensional infall speeds, half of whose values lie in the range of 0.05-0.09 km s$^{-1}$. These values are similar to those found in L1544 by Tafalla et al., and this result confirms that infall speeds in starless cores are generally faster than expected from ambipolar diffusion in a strongly sub-critical core. In addition, the observed infall regions are too extended to be consistent with the `inside-out' collapse model applied to a very low-mass star. In the largest cores, the spatial extent of the CS spectra with infall asymmetry is larger than the extent of the $\rm N_2H^+$ core by a factor of 2-3. All these results suggest that extended inward motions are a common feature in starless cores, and that they could represent a necessary stage in the condensation of a star-forming dense core.

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arXiv2001

New OH Zeeman measurements of magnetic field strengths in molecular clouds

We present the results of a new survey of 23 molecular clouds for the Zeeman effect in OH undertaken with the ATNF Parkes 64-m radio telescope and the NRAO Green Bank 43-m radio telescope. The Zeeman effect was clearly detected in the cloud associated with the HII region RCW 38, with a field strength of 38+/-3 micro-Gauss, and possibly detected in a cloud associated with the HII region RCW 57, with a field strength of -203+/-24 micro-Gauss. The remaining 21 measurements give formal upper limits to the magnetic field strength, with typical 1-sigma sensitivities <20 micro-Gauss. For 22 of the molecular clouds we are also able to determine thecolumn density of the gas in which we have made a sensitive search for the Zeeman effect. We combine these results with previous Zeeman studies of 29 molecular clouds, most of which were compiled by Crutcher (1999), for a comparsion of theoretical models with the data. This comparison implies that if the clouds can be modeled as initially spherical with uniform magnetic fields and densities that evolve to their final equilibrium state assuming flux-freezing then the typical cloud is magnetically supercritical, as was found by Crutcher (1999). If the clouds can be modeled as highly flattened sheets threaded by uniform perpendicular fields, then the typical cloud is approximately magnetically critical, in agreement with Shu et al. (1999), but only if the true values of the field for the non-detections are close to the 3-sigma upper limits. If instead these values are significantly lower (for example, similar to the 1-sigma limits), then the typical cloud is generally magnetically supercritical.

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arXiv2001

Modes of Multiple Star Formation

This paper argues that star forming environments should be classified into finer divisions than the traditional isolated and clustered modes. Using the observed set of galactic open clusters and theoretical considerations regarding cluster formation, we estimate the fraction of star formation that takes place within clusters. We find that less than 10% of the stellar population originates from star forming regions destined to become open clusters, confirming earlier estimates. The smallest clusters included in the observational surveys (having at least N=100 members) roughly coincide with the smallest stellar systems that are expected to evolve as clusters in a dynamical sense. We show that stellar systems with too few members N < N_\star have dynamical relaxation times that are shorter than their formation times (1-2 Myr), where the critical number of stars N_\star \approx 100. Our results suggest that star formation can be characterized by (at least) three principal modes: I. isolated singles and binaries, II. groups (N N_\star). Many -- if not most -- stars form through the intermediate mode in stellar groups with 10<N<100. Such groups evolve and disperse much more rapidly than open clusters; groups also have a low probability of containing massive stars and are unaffected by supernovae and intense ultraviolet radiation fields. Because of their short lifetimes and small stellar membership, groups have relatively little effect on the star formation process (on average) compared to larger open clusters.

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