Salvador Curiel

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.

Compact protoplanetary disks around the stars of a young binary system

Planet formation is believed to occur in the disks of gas and dust that surround young solar-type stars. Most stars, however, form in multiple systems, where the presence of a close companion could affect the structure of the disk and perhaps interfere with planet formation. It has been difficult to investigate this because of the resolution needed. Here we report interferometric observations (at a wavelength of 7 mm) of the core of the star-forming region L1551. We have achieved a linear resolution of seven astronomical units (less than the diameter of Jupiters orbit). The core of L1551 contains two distinct disks, with a separation of 45 AU; these appear to be associated with a binary system. Both disks are spatially resolved, with semi-major axes of about 10 AU, which is about a factor of ten smaller than disks around isolated stars. The disk masses are of order 0.05 solar masses, which could be enough to form planetary systems like our own.

Spectral Indices of Centimeter Continuum Sources in Star-forming Regions: Implications on the Nature of the Outflow Exciting Sources

We present 6 cm VLA observations of nine regions with molecular or HH outflows that appear to be driven by young stellar objects of low bolometric luminosity. Radio continuum emission at 3.6 cm had been detected previously toward the center of symmetry of 13 outflows in these regions. With the new 6 cm observations we have been able to estimate the spectral index in the 6 to 3.6 cm wavelength range for 11 of these outflow central (Class 0 and Class I) sources. All the spectral indices obtained for these central sources are positive and consistent with partially thick thermal free-free emission. We discuss this result in relation to the current models for the origin of centimeter radio continuum emission in outflow sources. In particular, we find that the observed flux densities of these sources fit very well in the observational correlation between the radio continuum luminosity and the momentum rate of the outflow, and that the results are consistent with the central sources being thermal radio jets. Our data suggest that the ionized part of the jets begins at a distance 10 AU from the star. In addition, we have derived a positive value for the spectral index of HH 32A, indicative of partially thick thermal emission. For the remaining sources detected in the nine fields observed, our results indicate that the emission is, in general, nonthermal, as expected for background extragalactic sources. However, a few of these sources could be related to the star-forming regions studied.

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.

The triple radio continuum source in serpens : the birth of a Herbig-Haro system ?

VLA observations of the triple radio continuum source in Serpens have revealed its extraordinary characteristics. While it is associated with a star-forming region, its outer components exhibit nonthermal spectra and large proper motions. Here, we present the results of high-sensitivity, multifrequency VLA radio continuum observations of this source

Radio Continuum-H2O Maser Systems in NGC 2071: H2O Masers Tracing a Jet (IRS 1) and a Rotating Proto-Planetary Disk of Radius 20 AU (IRS 3)

We have observed simultaneously the 1.3 cm continuum and H2O maser emission toward the core of the star-forming region NGC 2071 using the Very Large Array in its A configuration. Two 1.3 cm continuum sources have been detected in the region, coinciding with the infrared sources IRS 1 and IRS 3, respectively. The radio emission in IRS 3 is consistent with an ionized thermal bipolar radio jet. Two clusters of H2O maser spots are detected, one associated with IRS 1 (22 spots) and the other one associated with IRS 3 (13 spots). The H2O maser distribution in IRS 1 seems to be tracing at scales of 300 AU the larger scale H2 outflow observed at a few thousands of AU from the exciting source. On the other hand, the H2O masers in IRS 3 are distributed as an apparent disk of 005 (20 AU) radius, oriented almost perpendicular to the major axis of the radio jet. There is a clear velocity gradient (0.35 km s-1 AU-1) along the major axis of the H2O maser distribution, which can be gravitationally bound by a central mass of 1 M☉. These results, together with the low-mass and early evolutionary stage of IRS 3, suggest that masers around this source are tracing a rotating proto-planetary disk within a proto-solar-like system. This represents direct kinematic evidence of the smallest rotating circumstellar disk ever observed around a young stellar object (YSO). We discuss the dichotomy of H2O masers tracing either outflows or disks around YSOs, based on the evolutionary scheme proposed in our earlier work. We suggested that systems in which H2O masers trace disks are less evolved than those in which masers trace outflows. In this scheme, IRS 3 would then represent a relatively less evolved object than IRS 1. This prediction is consistent with independent infrared observations showing that IRS 3 is at an earlier phase of evolution.

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.

The Thermal Radio Jet of Cepheus A HW2 and the Water Maser Distribution at 0.″08 Scale (60 AU)

We report observations of the thermal radio jet in Cepheus A HW2, and the associated water masers, carried out with the highest (008) angular resolution available to date at the VLA (A configuration at λ = 1.3 cm). To calibrate the 1.3 cm continuum emission, we used the strong (~1000 Jy) H2O maser source as the reference, thus correcting the amplitude and phase instabilities introduced by the atmosphere. This powerful technique, first applied here to a star-forming region, allowed us to achieve a dynamic range of 15,000:1 for the strongest maser feature, a signal-to-noise ratio (S/N) of 70:1 for the radio jet and an accuracy of the order of 1 mas in the relative positions between the radio-continuum jet and the H2O masers in the region. We resolved the 1.3 cm jet into two maxima plus a fainter tail to the southwest. The separation between these two maxima (014) and the total size of the jet (039) are both consistent with models for a biconical ionized jet. The observed flux density (39 mJy) is, however, higher than expected. We detected 39 H2O maser spots toward the Cepheus A region, 28 of which are associated with the HW2 object, most of them distributed on either sides of the radio jet. We suggest that these latter maser features might be tracing a circumstellar molecular disk of radius ~300 AU, nearly perpendicular to the radio jet. The velocity gradient of 30 ± 10 km s-1 observed in the H2O spots over 600 AU along the axis perpendicular to the radio jet could be gravitationally bound by a central mass of 70 ± 40 M☉.

Large proper motions in the remarkable triple radio source in Serpens

The triple radio source in the Serpens star formation region consists of a central object and two lobes with spectral indices characteristic of nonthermal emission. Measurements made at the Arecibo Observatory and the VLA indicate that this triple source is probably associated with the Serpens star-forming region. A weak, circularly polarized 1667 MHz OH maser detected at Arecibo has been observed with the VLA and found to coincide in position to within 1 arcsec with the central source. The position of this central source also falls within the position error ellipsoid of IRAS 18273+0113. VLA observations at 6 cm clearly show that the outer components are moving away from the central source. The angular motion of each component with respect to the central one is 1.3 + or - 0.2 arcsec, implying velocities of 300 + or - 100 km/s at a distance of 500 + or - 200 pc. 36 refs.

Detection of radio continuum emission from Herbig-Haro objects 1 and 2 and from their central exciting source

The region in Orion containing HH 1 and HH 2 was observed with the VLA at 20, 6, and 2 cm on several occasions from 1981 to 1984. At lower resolution, four continuum sources were detected. Two of these sources coincide positionally with HH 1 and HH 2. At 6 cm and higher resolution, HH 1 is resolved into at least two components. The emission is probably bremsstrahlung originating in the same region where the visible line emission is produced. This is the first detection of radio continuum from classic Herbig-Haro objects. At a position closely centered between HH 1 and HH 2, an object that can be interpreted as the energy source of the system was detected. The central source spectrum is S(nu) of about nu to the alpha power, where alpha = 0.4 + or - 0.2, suggesting a stellar wind. Finally, the fourth radio continuum source coincides positionally with an H2O maser and is probably excited by an independent star. There is evidence of time variability in its radio flux. No emission was detected from the Cohen-Schwartz (1979) star at the 0.1 mJy level.