Nimesh A. Patel

A disk of dust and molecular gas around a high-mass protostar

The processes leading to the birth of low-mass stars such as our Sun have been well studied, but the formation of high-mass (over eight times the Suns mass, M[circdot]) stars remains poorly understood. Recent studies suggest that high-mass stars may form through accretion of material from a circumstellar disk, in essentially the same way as low-mass stars form, rather than through the merging of several low-mass stars. There is as yet, however, no conclusive evidence. Here we report the presence of a flattened disk-like structure around a massive 15M[circdot] protostar in the Cepheus A region, based on observations of continuum emission from the dust and line emission from the molecular gas. The disk has a radius of about 330 astronomical units (au) and a mass of 1 to 8 M[circdot]. It is oriented perpendicular to, and spatially coincident with, the central embedded powerful bipolar radio jet, just as is the case with low-mass stars, from which we conclude that high-mass stars can form through accretion.

Large Proper Motions in the Jet of the High-Mass YSO Cepheus A HW2

Using high angular resolution (~025-005) Very Large Array (VLA) observations made at 3.6 cm, 1.3 cm, and 7 mm during the period 1991-2004, we report the detection of large proper motions in the components of the radio continuum jet associated with the high-mass young stellar object (YSO) HW2 in the star-forming region Cepheus A. The relative proper motions observed for the two main components of the outflow, moving away from the central source in nearly opposite directions, are of the order of 140 mas yr-1, or ~480 km s-1 at a distance of 725 pc. The proper motions observed in the northeast and southwest lobes are not completely antiparallel, and the central elongated source seems to be changing orientation. We discuss possible scenarios to account for these and other observed characteristics. We also report the detection of a 7 mm compact continuum condensation of emission near the center of the thermal radio continuum jet, which we propose as the location of the exciting star.

Evidence for Evolution of the Outflow Collimation in Very Young Stellar Objects

We present Very Long Baseline Array proper-motion measurements of water masers toward two young stellar objects (YSOs) of the W75 N star-forming region. We find that these two objects are remarkable for having a similar spectral type, being separated by 07 (corresponding to 1400 AU), and sharing the same environment, but with a strikingly different outflow ejection geometry. One source has a collimated, jetlike outflow at a 2000 AU scale, while the other has a shell outflow at a 160 AU scale expanding in multiple directions with respect to a central compact radio continuum source. This result reveals that outflow collimation is not only a consequence of ambient conditions but is something intrinsic to the individual evolution of stars and brings to light the possibility of noncollimated outflows in the earliest stages of YSOs.

Origin and Evolution of the Cepheus Bubble

The Cepheus bubble is a giant shell with a diameter of about 120pc, discovered by Kun (1987) from IRAS sky flux maps. We have imaged this 10° × 10° region in the CO J=1-0 line, using the FCRAO Quarry system with sample spacing equal to the 45″ FWHM beam width. We estimate the total molecular gas mass to be ∼105 solar masses and a total kinetic energy ∼1051 ergs. We find 49 O and B0–B2 type stars in the Cepheus bubble region, most belong to the CepOB2 association. 120 IRAS point sources, selected according to color criteria representative of young stellar objects, fall within our map. Many of these sources are associated with the globules and filamentary structures seen in the CO emission, similar to the situation found in IC1396 (Patel et al. 1995). These stars, which are still associated with dense molecular gas plausibly represent the third and youngest generation of stars in this region.

A wide-angle outflow with the simultaneous presence of a high-velocity jet in the high-mass Cepheus A HW2 system

We present five epochs of VLBI water maser observations around the massive protostar Cepheus A HW2 with 0.4 mas (0.3 au) resolution. The main goal of these observations was to follow the evolution of the remarkable water maser linear/arcuate structures found in earlier VLBI observations. Comparing the data of our new epochs of observation with those observed 5 yr before, we find that at ‘large’ scales of 1 arcsec (700 au) the main regions of maser emission persist, implying that both the surrounding medium and the exciting sources of the masers have been relatively stable during that time-span. However, at smaller scales of 0.1 arcsec (70 au) we see large changes in the maser structures, particularly in the expanding arcuate structures R4 and R5. R4 traces a nearly elliptical patchy ring of ∼70 mas size (50 au) with expanding motions of ∼5 mas yr −1 (15 km s −1 ), consistent with previous results of Gallimore and collaborators. This structure is probably driven by the wind of a still unidentified YSO located at the centre of the ring (∼0.18 arcsec south of HW2). On the other hand, the R5 expanding bubble structure (driven by the wind of a previously identified YSO located ∼0.6 arcsec south of HW2) is currently dissipating in the circumstellar medium and losing its previous degree of symmetry, indicating a very short lived event. In addition, our results reveal, at scales of ∼1 arcsec (700 au), the simultaneous presence of a relatively slow (∼10– 70 km s −1 ) wide-angle outflow (opening angle of ∼102 ◦ ), traced by the masers, and the fast (∼500 km s −1 ) highly collimated radio jet associated with HW2 (opening angle of ∼18 ◦ ), previously observed with the VLA. This simultaneous presence of a wide-angle outflow and a highly collimated jet associated with a massive protostar is similar to what is found in some low-mass YSOs. There are indications that the primary wind(s) from HW2 could be rotating. The implications of these results in the study of the formation of high-mass stars are discussed.

Observations of Water Masers and Radio Continuum Emission in AFGL 2591

We report results of continuum (1.3 and 3.6 cm) and H2O maser line high angular resolution observations, made with the Very Large Array (VLA) in the A configuration, toward the star-forming region AFGL 2591. Three radio continuum sources (VLA 1, VLA 2, and VLA 3) were detected in the region at 3.6 cm, and one source (VLA 3) at 1.3 cm. VLA 1 and VLA 2 appear resolved and their spectral indices suggest free-free emission from optically thin H II regions. VLA 3 is elongated in the east-west direction, along the axis of the bipolar molecular outflow observed in the region. Its spectral energy distribution is consistent with it being a ~200 AU optically thick disk plus a photoionized wind. In addition, we detected 85 water maser spots toward the AFGL 2591 region, which are distributed in three main clusters. Two of these clusters are spatially associated with VLA 2 and VLA 3, respectively. The third cluster of masers, including the strongest water maser of the region, does not coincide with any known continuum source. We suggest that this third cluster of masers is excited by an undetected protostar that we predict to be located 05 (500 AU) north from VLA 3. The maser spots associated with VLA 3 are distributed along a shell-like structure of 001 size, showing a peculiar velocity-position helical distribution. We propose that VLA 3 is the powering source of the observed molecular outflow in this region. Finally, we support the notion that the AFGL 2591 region is a cluster of B0-B3 type stars.

Spherical episodic ejection of material from a young star

The exact processes by which interstellar matter condenses to form young stars are of great interest, in part because they bear on the formation of planets like our own from the material that fails to become part of the star. Theoretical models suggest that ejection of gas during early phases of stellar evolution is a key mechanism for removing excess angular momentum, thereby allowing material to drift inwards towards the star through an accretion disk. Such ejections also limit the mass that can be accumulated by the stellar core. To date, these ejections have been observed to be bipolar and highly collimated, in agreement with theory. Here we report observations at very high angular resolution of the proper motions of an arc of water-vapour masers near a very young, massive star in Cepheus. We find that the arc of masers can be fitted to a circle with an accuracy of one part in a thousand, and that the structure is expanding. Only a sphere will always produce a circle in projection, so our observations strongly suggest that the perfectly spherical ejection of material from this star took place about 33 years earlier. The spherical symmetry of the ejecta and its episodic nature are very surprising in the light of present theories.

PROPER MOTION OF WATER MASERS ASSOCIATED WITH IRAS 21391)5802: BIPOLAR OUTFLOW AND AN AU-SCALE DUSTY CIRCUMSTELLAR SHELL

We present Very Long Baseline Array (VLBA) observations of water maser emission associated with the star-forming region IRAS 21391+5802, which is embedded in a bright-rimmed cometary globule in IC 1396. The angular resolution of the maps is ~0.8 mas, corresponding to a spatial resolution of ~0.6 AU at an estimated distance of 750 pc. Proper motions are derived for 10 maser features identified consistently over three epochs, which were separated by intervals of about 1 month. The masers appear in four groups, which are aligned linearly on the sky, roughly along a northeast-southwest direction, with a total separation of ~520 AU (~07). The three-dimensional velocities of the masers have a maximum value of ~42 km s-1 (~9 AU yr-1). The average error on the derived proper motions is ~4 km s-1. The overall pattern of proper motions is indicative of a bipolar outflow. Proper motions of the masers in a central cluster, with a projected extent of ~20 AU, show systematic deviations from a radial outflow. However, we find no evidence of Keplerian rotation, as has been claimed elsewhere. A nearly circular loop of masers lies near the middle of the cluster. The radius of this loop is 1 AU, and the line-of-sight velocities of the masers in the loop are within 2 km s-1 of the systemic velocity of the region. These masers presumably exist at the radial distance where significant dust condensation occurs in the outflow emanating from the star.

Dust Properties and Disk Structure of Evolved Protoplanetary Disks in Cep OB2: Grain Growth, Settling, Gas and Dust Mass, and Inside-out Evolution

We present Spitzer/Infrared Spectrograph spectra of 31 T Tauri stars (TTS) and IRAM/1.3 mm observations for 34 low- and intermediate-mass stars in the Cep OB2 region. Including our previously published data, we analyze 56 TTS and 3 intermediate-mass stars with silicate features in Tr 37 (~4 Myr) and NGC 7160 (~12 Myr). The silicate emission features are well reproduced with a mixture of amorphous (with olivine, forsterite, and silica stoichiometry) and crystalline grains (forsterite, enstatite). We explore grain size and disk structure using radiative transfer disk models, finding that most objects have suffered substantial evolution (grain growth, settling). About half of the disks show inside-out evolution, with either dust-cleared inner holes or a radially dependent dust distribution, typically with larger grains and more settling in the innermost disk. The typical strong silicate features nevertheless require the presence of small dust grains, and could be explained by differential settling according to grain size, anomalous dust distributions, and/or optically thin dust populations within disk gaps. M-type stars tend to have weaker silicate emission and steeper spectral energy distributions than K-type objects. The inferred low dust masses are in a strong contrast with the relatively high gas accretion rates, suggesting global grain growth and/or an anomalous gas-to-dust ratio. Transition disks in the Cep OB2 region display strongly processed grains, suggesting that they are dominated by dust evolution and settling. Finally, the presence of rare but remarkable disks with strong accretion at old ages reveals that some very massive disks may still survive to grain growth, gravitational instabilities, and planet formation.

The Distribution of SiO in the Circumstellar Envelope around IRC +10216

New interferometric observations of SiO J =5→ 4 circumstellar line emission around the carbon star IRC+10216, using the Submillimeter Array, are presented. Complemented by multi-transition single-dish observations, including infrared observations of ro-vibrational transitions, detailed radiative transfer modelling suggests that the fractional abundance of SiO in the inner part of the envelope, between ≈ 3–8 stellar radii, is as high as ≈ 1.5× 10. This is more than an order of magnitude higher than predicted by equilibrium stellar atmosphere chemistry in a carbon-rich environment and indicative of the importance of non-LTE chemical processes. In addition to the compact component, a spatially more extended (re ≈ 2.4× 10 16 cm) low-fractional-abundance (f0 ≈ 1.7× 10 ) region is required to fit the observations. This suggests that the majority of the SiO molecules are effectively accreted onto dust grains in the inner wind while the remaining gas-phase molecules are eventually photodissociated at larger distances. Evidence of departure from a smooth wind is found in the observed visibilities, indicative of density variations of a factor 2 to 5 on an angular scale corresponding to a time scale of about 200 years. Additionally, constraints on the velocity structure of the wind are obtained. Subject headings: stars: abundances – stars: AGB and post-AGB – stars: carbon – stars: circumstellar matter – stars: individual (IRC +10216) – stars: mass loss