Mary Barsony

Submillimeter continuum observations of rho Ophiuchi A: The candidate protostar VLA 1623 and prestellar clumps

We have mapped the submillimeter continuum emission from the rich star-forming core, ρ Oph A, at 350, 450, 800, and 1300 μm using the JCMT and IRAM 30 m telescopes. In addition to the diffuse emission from the cloud core itself, these observations reveal four well-defined clumps, which, suprisingly, are barely visible in the single-dish molecular maps obtained so far in this region. The gas may have partially frozen out onto the dust grains, due to low temperature and/or high density in the clumps. We discuss the possibility that these clumps are prestellar or extremely young, low-mass protostars

The detailed optical light curve of GRB 030329

We present densely sampled BVRI light curves of the optical transient associated with the gamma-ray burst (GRB) 030329, the result of a coordinated observing campaign conducted at five observatories. Augmented with published observations of this GRB, the compiled optical data set contains 2687 photometric measurements, obtained between 78 minutes and 79 days after the burst. This data set allows us to follow the photometric evolution of the transient with unprecedented detail. We use the data to constrain the light curve of the underlying supernova (SN) 2003dh and show that it evolved faster than and was probably somewhat fainter than the Type Ic SN 1998bw, associated with GRB 980425. We find that our data can be described by a broken power-law decay perturbed by a complex variable component. The early- and late-time decay slopes are determined to be alpha(1) approximate to 1.1 and alpha(2) approximate to 2. Assuming this single-break power-law model, we constrain the break to lie between similar to3 and similar to8 days after the burst. This simple, singly broken power-law model, derived only from the analysis of our optical observations, may also account for available multiband data, provided that the break happened similar to8 days after the burst. The more complex double-jet model of Berger et al. provides a comparable fit to the optical, X-ray, millimeter, and radio observations of this event. The unique early coverage available for this event allows us to trace the color evolution of the afterglow during the first hours after the burst. We detect a significant change in optical colors during the first day. Our color analysis is consistent with a cooling-break frequency sweeping through the optical band during the first day. The light curves of GRB 030329 reveal a rich array of variations, superposed over the mean power-law decay. We find that the early variations (less than or similar to8 days after the burst) are asymmetric, with a steep rise followed by a relatively slower ( by a factor of about 2) decline. The variations maintain a similar timescale during the first 4 days and then get significantly longer. The structure of these variations is similar to those previously detected in the afterglows of several GRBs.

A Near-Infrared Imaging Survey of the ρ Ophiuchi Cloud Core

We present results of the largest three-color near-infrared (NIR) imaging survey of the ρ Ophiuchi star-forming cloud core to date. The survey covers 1 deg2, which corresponds to a projected area of ~2.2 pc × 2.2 pc at the distance of the Ophiuchus clouds (125 ± 25 pc). Mapping was carried out in the J (1.25 μm), H (1.63 μm), and K (2.22 μm) bands, to 90%, 10 σ completeness limits of J = 15.0, H = 14.5, and K = 14.0. We have cataloged 4495 sources, of which 2070 have K ≤ 14.0 and 2425 more either have 5 σ or better detections in the K band (for 14.0 ≤ K ≤ 14.5) or have 4 σ or better detections in at least two of the three survey bands (for K ≥ 14.5). We present accurate astrometry and photometry for all identified sources. We also tabulate cross-identifications for 449 previously observed NIR sources. Using the cross-identifications, we derive linear transformations between the differing photometric systems and use rather strict criteria to identify NIR variables.

Compact Outflows Associated with TMC-1 and TMC-1A

High spatial resolution observations are presented of the compact outflows associated with the young protostars TMC-1 (IRAS 04381+2540) and TMC-1A (IRAS 04365+2535) in Taurus. Emission in CO(1–0) imaged with the Owens Valley Millimeter Array shows the outflow lobes to be conical close to the star. Analysis of the outflow dynamics indicates that these objects are low-luminosity versions of the energetic outflows more commonly observed. Near-infrared images at H and K bands show a close correspondence between reflection nebulosity and the location of high-velocity gas and suggest the outflow cavity is evacuated, as do position-velocity diagrams of the CO(2-1). Comparison of the J = 1–0 transition with emission in the 2–1 line indicates that the excitation temperature in the high-velocity gas is higher than the surrounding Taurus cloud. We place limits on the inclination of both objects by comparing the data with theoretical outflow models and conclude that i ~ 40°–70° for both objects. The depro-jected opening angles of the outflow cones are then in the range 30°–40°. None of the current outflow models satisfactorily explains the results for TMC-1 and TMC-1A, which are among the youngest class I sources in Taurus. We find their outflow structure shares many similarities with the more obscured and possibly younger class 0 objects, B335 and L1448-C. The main difference is the lower mechanical luminosities of the TMC-1 and TMC-1A outflows, reflecting a factor of 3–4 smaller linear extent and velocity and a factor of 10–20 lower mass than the L1448-C molecular jet source. Taken together, the four protostars share the common properties of (1) conical outflow lobes close to the star, (2) evacuated outflow cavities, and (3) relatively wide 30°-40° opening angles. A successful theory of young stellar outflows must be able to explain these characteristics.

The Near-Infrared Extinction Law and Limits on the Pre-Main-Sequence Population of the ρ Ophiuchi Dark Cloud

We describe new techniques to measure the near-infrared extinction law and to place limits on the pre–main-sequence stellar population of a dark cloud. We analyze JHK imaging data for the central 1 deg2 of the ρ Ophiuchi cloud core and show that nearly all stars projected onto regions of low CS intensity, ICS ≤ 10 K km s-1, are background stars. Most sources at larger CS intensities lie within cloud material. We use the background stars to derive the slope of the near-IR extinction law, EJ - H/EH - K = 1.57 ± 0.03. This result is consistent with previous extinction laws but has a factor of 2–3 smaller uncertainty. The new ρ Oph extinction law yields strong constraints on the number of previously undiscovered pre–main-sequence stars in the cloud, 46 ± 11, and the number of previously undiscovered young stars with near-IR excesses, 15 ± 4. Neither limit exceeds the number of known pre–main-sequence stars in the cloud, ~100. Thus, current samples of pre–main-sequence stars are reasonably complete for K ≤ 14.

A mid-infrared imaging survey of embedded young stellar objects in the ρ Ophiuchi Cloud Core

Results of a comprehensive, new, ground-based mid-infrared imaging survey of the young stellar population of the ρ Ophiuchi cloud are presented. Data were acquired at the Palomar 5 m and at the Keck 10 m telescopes with the MIRLIN and LWS instruments, at 05 and 025 resolutions, respectively. Of 172 survey objects, 85 were detected. Among the 22 multiple systems observed, 15 were resolved and their individual component fluxes determined. A plot of the frequency distribution of the detected objects with SED spectral slope shows that YSOs spend ~4 × 105 yr in the flat-spectrum phase, clearing out their remnant infall envelopes. Mid-infrared variability is found among a significant fraction of the surveyed objects and is found to occur for all SED classes with optically thick disks. Large-amplitude near-infrared variability, also found for all SED classes with optically thick disks, seems to occur with somewhat higher frequency at the earlier evolutionary stages. Although a general trend of mid-infrared excess and near-infrared veiling exists progressing through SED classes, with Class I objects generally exhibiting rK ≥ 1, flat-spectrum objects with rK ≥ 0.58, and Class III objects with rK = 0, Class II objects exhibit the widest range of rK values, ranging from 0 ≤ rK ≤ 4.5. However, the highly variable value of veiling that a single source can exhibit in any of the SED classes in which active disk accretion can take place is striking and is direct observational evidence for highly time-variable accretion activity in disks. Finally, by comparing mid-infrared versus near-infrared excesses in a subsample with well-determined effective temperatures and extinction values, disk-clearing mechanisms are explored. The results are consistent with disk clearing proceeding from the inside out.

X-Ray Properties of Pre-Main-Sequence Stars in the Orion Nebula Cluster with Known Rotation Periods

We reanalyze all archival Chandra/ACIS observations of the Orion Nebula Cluster (ONC) to study the X-ray properties of a large sample of pre–main-sequence (PMS) stars with optically determined rotation periods. Our goal is to elucidate the origins of X-rays in PMS stars by seeking out connections between the X-rays and the mechanisms most likely driving their production—rotation and accretion. Stars in our sample have LX/Lbol near, but below, the ‘‘saturation’’ value of 10 � 3 . In addition, in this sample X-ray luminosity is significantly correlated with stellar rotation, in the sense of decreasing LX/Lbol with more rapid rotation. These findings suggest that stars with optical rotation periods are in the ‘‘super-saturated’’ regime of the rotation-activity relationship, consistent with their Rossby numbers. However, we also find that stars with optical rotation periods are significantly biased to high LX. This is not the result of magnitude bias in the optical rotation–period sample, but rather of the diminishingly small amplitude of optical variations in stars with low LX. Evidently, there exists in the ONC a population of stars whose rotation periods are unknown and that possess lower average X-ray luminosities than those of stars with known rotation periods. These stars may sample the linear regime of the rotation-activity relationship. Accretion also manifests itself in X-rays, though in a somewhat counterintuitive fashion: while stars with spectroscopic signatures of accretion show harder X-ray spectra than nonaccretors, they show lower X-ray luminosities and no enhancement of X-ray variability. We interpret these findings in terms of a common origin for the X-ray emission observed from both accreting and nonaccreting stars, with the X-rays from accreting stars simply being attenuated by magnetospheric accretion columns. This suggests that X-rays from PMS stars have their origins primarily in chromospheres, not accretion.

A Near-Infrared Multiplicity Survey of Class I/Flat-Spectrum Systems in Six Nearby Molecular Clouds

We present new near-IR observations of 76 Class I/flat-spectrum objects in the nearby (d 320 pc) Perseus, Taurus, Chamaeleon I and II, ρ Ophiuchi, and Serpens dark clouds. These observations are part of a larger systematic infrared multiplicity survey of self-embedded objects in the nearest dark clouds. When combined with the results of our previously published near-infrared multiplicity survey, we find a restricted companion star fraction of 14/79 (18% ± 4%) of the sources surveyed to be binary or higher order multiple systems over a separation range of ~300–2000 AU with a magnitude difference ΔK ≤ 4 and with no correction for background contamination or completeness. This is consistent with the fraction of binary/multiple systems found among older pre–main-sequence T Tauri stars in each of the Taurus, ρ Oph, and Chamaeleon star-forming regions over a similar separation range, as well as the combined companion star fraction for these regions. However, the companion star fraction for solar-type, and lower mass M dwarf, main-sequence stars in the solar neighborhood in this separation range (11% ± 3%) is approximately one-half that of our sample. Together with multiplicity statistics derived for previously published samples of Class 0 and Class I sources, our study suggests that a significant number of binary/multiple objects may remain to be discovered at smaller separations among our Class I/flat-spectrum sample and/or most of the evolution of binary/multiple systems occurs during the Class 0 phase of early stellar evolution.

Monte Carlo study of invasion percolation clusters in two and three dimensions

The authors consider the process of growing invasion percolation clusters from a point into an infinite medium. The form considered is the simplest one in which the cluster is permitted to grow into regions it has previously surrounded. It is shown that this process can yield extremely good Monte Carlo estimates of the percolation threshold pc. For the square, triangular and match-square lattices they obtain pc values of 0.5925+or-0.0003, 0.5000+or-0.0003 and 0.4072+or-0.0002, and for the simple cubic lattice 0.31158+or-0.00006. The errors quoted are purely statistical, and represent one standard deviation. Two critical exponents are obtained which they suggest should be identified in terms of the fractal dimension D and gap exponent Delta of ordinary percolation. Based on these identifications they obtain values for 1/D and 1/ Delta of 0.527+or-0.002 and 0.393+or-0.004 in two dimensions and 0.402+or-0.003 and 0.454+or-0.005 in three dimensions. These results are consistent with known exact results and best series and Monte Carlo estimates, suggesting that the form of invasion percolation considered is probably in the same universality class as ordinary percolation.

Star formation in massive protoclusters in the Monoceros OB1 dark cloud

We present far-infrared, submillimetre, and millimetre observations of bright IRAS sources and outflows that are associated with massive CS clumps in the Monoceros OB1 Dark Cloud. Individual star-forming cores are identified within each clump. We show that combining submillimetre maps, obtained with SCUBA on the JCMT, with HIRES-processed and modelled IRAS data is a powerful technique that can be used to place better limits on individual source contributions to the far-infrared flux in clustered regions. Three previously categorized “Class I objects” are shown to consist of multiple sources in different evolutionary stages. In each case, the IRAS point source dominates the flux at 12 & 25 µm. In two cases, the IRAS point source is not evident at submillimetre wavelengths. The submillimetre sources contribute significantly to the 60 & 100 µm fluxes, dominating the flux in the 100 µm waveband. Using fluxes derived from our technique, we present the spectral energy distribution and physical parameters for an intermediate-mass Class 0 object in one of the regions. Our new CO J=2!1 outflow maps of the three regions studied indicate complex morphology suggestive of multiple driving sources. We discuss the possible implications of our results for published correlations between outflow momentum deposition rates and “source” luminosities, and for using these derived properties to estimate the ratio of mass ejection rates to mass accretion rates onto protostars.