Bruce A. Wilking

Spectroscopy of Brown Dwarf Candidates in the ρ Ophiuchi Molecular Core

We present an analysis of low-resolution infrared spectra for 20 brown dwarf candidates in the core of the ρ Ophiuchi molecular cloud. Fifteen of the sources display absorption-line spectra characteristic of late-type stars. By comparing the depths of water vapor absorption bands in our candidate objects with a grid of M dwarf standards, we derive spectral types that are independent of reddening. Optical spectroscopy of one brown dwarf candidate confirms the spectral type derived from the water bands. Combining their spectral types with published near-infrared photometry, effective temperatures and bolometric stellar luminosities are derived, enabling us to place our sample on the Hertzsprung-Russell diagram. We compare the positions of the brown dwarf candidates in this diagram with two sets of theoretical models in order to estimate their masses and ages. Considering uncertainties in placing the candidates in the H-R diagram, six objects consistently lie in the brown dwarf regime and another five objects lie in the transition region between stellar and substellar objects. The ages inferred for the sample are consistent with those derived for higher mass association members. Three of the newly identified brown dwarfs display infrared excesses at λ = 2.2 μm, suggesting that young brown dwarfs can have active accretion disks. Comparing our mass estimates of the brown dwarf candidates with those derived from photometric data alone suggests that spectroscopy is an essential component of investigations of the mass functions of young clusters.

Optical Spectroscopy of the Surface Population of the ρ Ophiuchi Molecular Cloud: The First Wave of Star Formation

We present the results of optical spectroscopy of 139 stars obtained with the Hydra multiobject spectrograph. The objects extend over a 1.3 deg2 area surrounding the main cloud of the ρ Oph complex. The objects were selected from narrowband images to have Hα in emission. Using the presence of strong Hα emission, lithium absorption, location in the Hertzsprung-Russell diagram, or previously reported X-ray emission, we were able to identify 88 objects as young stars associated with the cloud. Strong Hα emission was confirmed in 39 objects with line widths consistent with their origin in magnetospheric accretion columns. Two of the strongest emission-line objects are young, X-ray-emitting brown dwarf candidates with M8 spectral types. Comparisons of the bolometric luminosities and effective temperatures with theoretical models suggest a median age for this population of 2.1 Myr, which is significantly older than the ages derived for objects in the cloud core. It appears that these stars formed contemporaneously with low-mass stars in the Upper Scorpius subgroup, likely triggered by massive stars in the Upper Centaurus subgroup.

Distribution and Motion of the Water Masers near IRAS 05413–0104

We have used the NRAO Very Long Baseline Array to image water masers associated with the low-luminosity, young stellar object (YSO) IRAS 05413-0104 at four epochs over a period of 10 weeks. The maser images show the detail of a symmetric, jetlike structure about 300 mas in extent. The 23° ± 2° position angle of the maser spot distribution is in excellent agreement with measured position angles for the observed larger scale H2 and SiO emission distributions; radial velocities are in agreement with SiO measurements showing redshifted gas to the southwest and blueshifted gas to the northeast. We have detected proper motions of numerous maser spots averaging 30 ± 12 mas yr−1, implying space velocities of 64 ± 27 km s−1 for a source distance of 450 pc. Some masers are located within a projected distance of 40 AU of the origin of expansion, the assumed position of the central source, suggesting that jet formation and acceleration takes place within this radius of the YSO. We compute an inclination of the outflow system to the plane of the sky of 4°, based on the relative magnitude of the proper motions and radial velocities of the masers. VLBI observations of water masers in YSOs are clearly demonstrated to be interesting and competitive probes of the kinematics of the gas in YSO jets.

Spectroscopic evidence for infall around an extraordinary IRAS source in Ophiuchus

Millimeter-wave continuum observations and spectral line observations of the J = 5-4 and J = 2-1 transitions of CS toward IRAS 16293-2422, an extremely cold infrared source associated with a high-velocity molecular outflow in the Rho Ophiuchi molecular cloud, are reported. The observations indicate that this source is a dust-enshrouded object embedded in a dense, elongated configuration of molecular gas whose major axis is roughly orthogonal to the direction of the double bipolar outflow observed in it. The observations appear to provide spectroscopic evidence for significant mass infall motions associated with a young stellar object, and suggest that IRAS 16293-2422 is a true protostar. 13 references.

Deep Infrared Imaging of the R Coronae Australis Cloud Core

Preliminary analysis is presented for J, H, and K 0 images of the Corona Australis cloud core. Color-color and color-magnitude diagrams reveal relatively few new cluster members. We conclude that the R Cr A infrared cluster consists of only about 30 members and its size reeects the available reservoir of molecular gas.

Infrared Properties of Weak Radio Sources in the ρ Ophiuchi Molecular Cloud

We report mid-infrared ISOCAM observations for a sample of radio continuum sources in the ρ Ophiuchi molecular cloud core with unknown or poorly studied infrared counterparts. These data are combined with existing infrared photometry, including recently published ISOCAM data, for previously studied radio sources to investigate the evolutionary states of 35 radio-emitting young stars in the ρ Oph cloud core. About 50% of the radio stars are found to have class I, flat, or class II spectral energy distributions with near-infrared excesses arising from circumstellar disks. Their radio emission is most likely thermal emission from gas ionized by stellar winds. The remaining radio emitters are young stars with class III spectral energy distributions that lack infrared excesses and circumstellar disks. Their radio emission likely is nonthermal emission from magnetic surface activity. The lack of young stars with weak infrared excesses supports earlier suggestions that disk dissipation is rapid. Class III sources are twice as common as class II sources in this radio-selected sample in comparison with extinction-limited samples, which underscores the importance of radio surveys in obtaining a complete census of young stellar objects. The concentration of diskless class III objects in the high column-density molecular core, and their median age of 0.3 Myr derived from their positions in a Hertzsprung-Russell diagram, indicate they are contemporaries of class II objects which include the classical T Tauri stars. It appears that these class III objects have shorter disk survival times than class II objects in the cloud.

Infrared observations of dust cloud structure in young R associations - NGC 1333, S68, and NGC 7129

Infrared photometry is presented from 1 to 100 microns of the major sources of luminosity in three regions of star formation which contain young B and A, but not O, stars. The reflection nebulae in all three regions are sources of 100-micron emission similar to previously observed optical reflection nebulae. All three regions also contain highly obscured sources which are often associated with radio maser activity, high-velocity gas outflows, and unusual radio continuum emission. There is a strong correlation between the occurrence of these radio phenomena and the presence of dust close enough to the central objects to radiate strongly in the 3-20 micron spectral region. These obscured sources are likely to be the pre-main-sequence counterparts to the nearby visible main-sequence stars; the obscured objects may still be undergoing mass accretion.

Coordinated Multiwavelength Observation of 3C 66A during the WEBT Campaign of 2003-2004*

The BL Lac object 3C 66A was the target of an extensive multiwavelength monitoring campaign from 2003 July through 2004 April (with a core campaign from 2003 September to 2003 December) involving observations throughout the electromagnetic spectrum. Radio, infrared, and optical observations were carried out by the WEBT-ENIGMA collaboration. At higher energies, 3C 66A was observed in X-rays (RXTE), and at very high energy (VHE) in γ-rays (STACEE, VERITAS). In addition, the source has been observed with the VLBA at nine epochs throughout the period 2003 September to 2004 December, including three epochs contemporaneous with the core campaign. A gradual brightening of the source over the course of the campaign was observed at all optical frequencies, culminating in a very bright maximum around 2004 February 18. The WEBT campaign revealed microvariability with flux changes of ~5% on timescales as short as ~2 hr. The source was in a relatively bright state, with several bright flares on timescales of several days. The spectral energy distribution (SED) indicates a νFν peak in the optical regime. A weak trend of optical spectral hysteresis with a trend of spectral softening throughout both the rising and decaying phases has been found. On longer timescales, there appears to be a weak indication of a positive hardness-intensity correlation for low optical fluxes, which does not persist at higher flux levels. The 3-10 keV X-ray flux of 3C 66A during the core campaign was historically high and its spectrum very soft, indicating that the low-frequency component of the broadband SED extends beyond ~10 keV. No significant X-ray flux and/or spectral variability was detected. STACEE and Whipple observations provided upper flux limits at >150 and >390 GeV, respectively. The 22 and 43 GHz data from the three VLBA epochs made between 2003 September and 2004 January indicate a rather smooth jet with only very moderate internal structure. Evidence for superluminal motion (8.5 ± 5.6 h-1 c) was found in only one of six components, while the apparent velocities of all other components are consistent with 0. The radial radio brightness profile suggests a magnetic field decay ∝r-1 and, thus, a predominantly perpendicular magnetic field orientation.

Structure and chemistry in the northwestern condensation of the Serpens molecular cloud core

We present single-dish and interferometric observations of gas and dust in the core of the Serpens molecular cloud, focusing on the northwestern condensation. Single-dish molecular line observations are used to probe the structure and chemistry of the condensation while high-resolution images of CS and CH_(3)0H are combined with continuum observations from λ = 1.3 mm to λ = 3.5 cm to study the subcondensations and overall distribution of dust. For the northwestern condensation, we derive a characteristic density of 3 x 10^5 cm^(-3) and an estimated total mass of approximately 70 M_⊙. We find compact molecular emission associated with the far-infrared source S68 FIRS 1, and with a newly detected subcondensation named S68 N. Comparison of the large-and small-scale emission reveals that most of the material in the northwest condensation is not directly associated with these compact sources, suggesting a youthful age for this region. CO J = 1 approaches 0 observations indicate widespread outflow activity. However, no unique association of embedded objects with outflows is possible with our observations. The SiO emission is found to be extended with the overall emission centered about S68 FIRS 1; the offset of the peak emission from all of the known continuum sources and the coincidence between the blueshifted SiO emission and blueshifted high-velocity gas traced by CO and CS is consistent with formation of SiO in shocks. Derived abundances of CO and HCO^(+) are consistent with quiescent and other star-forming regions while CS, HCN, and H2CO abundances indicate mild depletions within the condensation. Spectral energy distribution fits to S68 FIRS 1 indicate a modest luminosity (50-60 L_⊙), implying that it is a low-mass (0.5-3 M_⊙) young stellar object. Radio continuum observations of the triple source toward S68 FIRS 1 indicate that the lobe emission is varying on timescales ≤ 1 yr while the central component is relatively constant over ~14 yr. The nature of a newly detected compact emission region, S68 N, is less certain due to the absence of firm continuum detections; based on its low luminosity (<5 L_⊙) and strong molecular emission, S68 N may be prestellar subcondensation of gas and dust.

Circularly Polarized Radio Emission from an X-Ray Protostar

IRS 5, an embedded young stellar object in the nearby Corona Australis molecular cloud, was recently detected as an X-ray source with the ASCA and ROSAT satellites. We report here the detection of circularly polarized continuum emission from IRS 5 at centimeter wavelengths. Already known to be a highly variable radio source, the polarization fraction is seen to range from V/I10% to 37% on a day timescale. This demonstrates that radio emission from protostars, previously attributed in most cases to ionized thermal outflows, can sometimes arise instead from nonthermal processes, i.e., gyrosynchrotron emission from particles accelerated in situ by magnetic reconnection flares. Together with the X-ray data and indications of MeV particles in the solar nebula obtained from meteoritic materials, it contributes to the growing evidence for high-energy processes during the earliest stages of low-mass star formation.