Relative Evolutionary Time Scale of Hot Molecular Cores with Respect to Ultra Compact HII Regions

Using the Owens Valley and Nobeyama Radio Observatory interferometers, we carried out an unbiased search for hot molecular cores and ultracompact UC HII regions toward the high-mass star forming region G19.61--0.23. In addition, we performed 1.2 mm imaging with SIMBA, and retrieved 3.5 and 2 cm images from the VLA archive data base. The newly obtained 3 mm image brings information on a cluster of high-mass (proto)stars located in the innermost and densest part of the parsec scale clump detected in the 1.2 mm continuum. We identify a total of 10 high-mass young stellar objects: one hot core (HC) and 9 UC HII regions, whose physical parameters are obtained from model fits to their continuum spectra. The ratio between the current and expected final radii of the UC \HII regions ranges from 0.3 to 0.9, which leaves the possibility that all O-B stars formed simultaneously. Under the opposite assumption -- namely that star formation occurred randomly -- we estimate that HC lifetime is less than $\sim$1/3 of that of UCHII regions on the basis of the source number ratio between them.Comment: 13 pages, 2 figs, including a color fi

New Results on the Submillimeter Polarization Spectrum of the Orion Molecular Cloud

We have used the SHARP polarimeter at the Caltech Submillimeter Observatory to map the polarization at wavelengths of 350 and 450 micron in a ~2 x 3 arcmin region of the Orion Molecular Cloud. The map covers the brightest region of the OMC-1 ridge including the Kleinmann-Low (KL) nebula and the submillimeter source Orion-south. The ratio of 450-to-350 micron polarization is ~ 1.3 +/- 0.3 in the outer parts of the cloud and drops by a factor of 2 towards KL. The outer cloud ratio is consistent with measurements in other clouds at similar wavelengths and confirms previous measurements placing the minimum of the polarization ratio in dusty molecular clouds at a wavelength ~ 350 micron.Comment: 7 pages, 6 figures, submitted to ApJ Let

IRC+10216's Innermost Envelope -- The eSMA's View

We used the Extended Submillimeter Array (eSMA) in its most extended configuration to investigate the innermost (within a radius of 290 R* from the star) circumstellar envelope (CSE) of IRC+10216. We imaged the CSE using HCN and other molecular lines with a beam size of 0."22 x 0."46, deeply into the very inner edge (15 R*) of the envelope where the expansion velocity is only 3 km/s. The excitation mechanism of hot HCN and KCl maser lines is discussed. HCN maser components are spatially resolved for the first time on an astronomical object. We identified two discrete regions in the envelope: a region with a radius of . 15 R*, where molecular species have just formed and the gas has begun to be accelerated (region I) and a shell region (region II) with a radius of 23 R* and a thickness of 15 R*, whose expansion velocity has reached up to 13 km/s, nearly the terminal velocity of 15 km/s. The Si$^{34}$S line detected in region I shows a large expansion velocity of 16 km/s due to strong wing components, indicating that the emission may arise from a shock region in the innermost envelope. In region II, the P.A. of the most copious mass loss direction was found to be 120 +/- 10 degrees, which may correspond to the equatorial direction of the star. Region II contains a torus-like feature. These two regions may have emerged due to significant differences in the size distributions of the dust particles in the two regions.Comment: 26 pages, 8 figures, accepted for publication in The Astrophysical Journal. Please find the pdf at http://www.submm.caltech.edu/~hs/astroph/0904.0280.pdf and the ps file at http://www.submm.caltech.edu/~hs/astroph/0904.0280.p

Submillimeter Observations of The Isolated Massive Dense Clump IRAS 20126+4104

We used the CSO 10.4 meter telescope to image the 350 micron and 450 micron continuum and CO J=6-5 line emission of the IRAS 20126+4104 clump. The continuum and line observations show that the clump is isolated over a 4 pc region and has a radius of ~ 0.5 pc. Our analysis shows that the clump has a radial density profile propto r ^{-1.2} for r <~ 0.1 pc and has propto r^{-2.3} for r >~ 0.1 pc which suggests the inner region is infalling, while the infall wave has not yet reached the outer region. Assuming temperature gradient of r^{-0.35}, the power law indices become propto r ^{-0.9} for r < ~0.1 pc and propto r^{-2.0} for r >~ 0.1 pc. Based on a map of the flux ratio of 350micron/450micron, we identify three distinct regions: a bipolar feature that coincides with the large scale CO bipolar outflow; a cocoon-like region that encases the bipolar feature and has a warm surface; and a cold layer outside of the cocoon region. The complex patterns of the flux ratio map indicates that the clump is no longer uniform in terms of temperature as well as dust properties. The CO emission near the systemic velocity traces the dense clump and the outer layer of the clump shows narrow line widths (< ~3 km/s). The clump has a velocity gradient of ~ 2 km/s pc^{-1}, which we interpret as due to rotation of the clump, as the equilibrium mass (~ 200 Msun) is comparable to the LTE mass obtained from the CO line. Over a scale of ~ 1 pc, the clump rotates in the opposite sense with respect to the >~ 0.03 pc disk associated with the (proto)star. This is one of four objects in high-mass and low-mass star forming regions for which a discrepancy between the rotation sense of the envelope and the core has been found, suggesting that such a complex kinematics may not be unusual in star forming regions.Comment: 34 pages, 13 figures, Accepted for publication in the Ap

ALMA CN Zeeman Observations of AS 209: Limits on Magnetic Field Strength and Magnetically Driven Accretion Rate

While magnetic fields likely play an important role in driving the evolution of protoplanetary disks through angular momentum transport, observational evidence of magnetic fields has only been found in a small number of disks. Although dust continuum linear polarization has been detected in an increasing number of disks, its pattern is more consistent with that from dust scattering than from magnetically aligned grains in the vast majority of cases. Continuum linear polarization from dust grains aligned to a magnetic field can reveal information about the magnetic field´s direction, but not its strength. On the other hand, observations of circular polarization in molecular lines produced by Zeeman splitting offer a direct measure of the line-of-sight magnetic field strength in disks. We present upper limits on the net toroidal and vertical magnetic field strengths in the protoplanetary disk AS 209 derived from Zeeman splitting observations of the CN 2-1 line. The 3σ upper limit on the net line-of-sight magnetic field strength in AS 209 is 5.0 mG on the redshifted side of the disk and 4.2 mG on the blueshifted side of the disk. Given the disk´s inclination angle, we set a 3σ upper limit on the net toroidal magnetic field strength of 8.7 and 7.3 mG for the red and blue sides of the disk, respectively, and 6.2 and 5.2 mG on the net vertical magnetic field on the red and blue sides of the disk. If magnetic disk winds are a significant mechanism of angular momentum transport in the disk, magnetic fields of a strength close to the upper limits would be sufficient to drive accretion at the rate previously inferred for regions near the protostar.Fil: Harrison, Rachel. University of Illinois at Urbana; Estados UnidosFil: Looney, L. W.. University of Illinois at Urbana; Estados UnidosFil: Stephens, I. W.. Harvard-Smithsonian Center for Astrophysics; Estados UnidosFil: Li, Z.-Y.. University of Virginia; Estados UnidosFil: Teague, Richard. Department Of Astronomy; Estados UnidosFil: Crutcher, Richard. University of Illinois at Urbana; Estados UnidosFil: Yang, H.. Tsinghua University; ChinaFil: Cox, E. G.. Northwestern University; Estados UnidosFil: Fernandez Lopez, Manuel. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Instituto Argentino de Radioastronomía. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto Argentino de Radioastronomía; ArgentinaFil: Shinnaga, H.. Kagoshima University; Japó

Magnetic Fields in Evolved Stars: Imaging the Polarized Emission of High-Frequency SiO Masers

We present Submillimeter Array observations of high frequency SiO masers around the supergiant VX Sgr and the semi-regular variable star W Hya. The J=5-4, v=1 28SiO and v=0 29SiO masers of VX Sgr are shown to be highly linearly polarized with a polarization from ~5-60%. Assuming the continuum emission peaks at the stellar position, the masers are found within ~60 mas of the star, corresponding to ~100 AU at a distance of 1.57 kpc. The linear polarization vectors are consistent with a large scale magnetic field, with position and inclination angles similar to that of the dipole magnetic field inferred in the H2O and OH maser regions at much larger distances from the star. We thus show for the first time that the magnetic field structure in a circumstellar envelope can remain stable from a few stellar radii out to ~1400 AU. This provides further evidence supporting the existence of large scale and dynamically important magnetic fields around evolved stars. Due to a lack of parallactic angle coverage, the linear polarization of masers around W Hya could not be determined. For both stars we observed the 28SiO and 29SiO isotopologues and find that they have a markedly different distribution and that they appear to avoid each other. Additionally, emission from the SO 5_5-4_4 line was imaged for both sources. Around W Hya we find a clear offset between the red- and blue-shifted SO emission. This indicates that W Hya is likely host to a slow bipolar outflow or a rotating disk-like structure.Comment: 8 pages, 7 figures, accepted for publication in ApJ. Online table will be available with published versio

Infall, outflow, and rotation in the G19.61-0.23 hot molecular core

Aims: The main goal of this study is to perform a sub-arcsecond resolution analysis of the high-mass star formation region G19.61-0.23, both in the continuum and molecular line emission. While the centimeter continuum images will be discussed in detail in a forthcoming paper, here we focus on the (sub)mm emission, devoting special attention to the hot molecular core. Results: Our observations resolve the HMC into three cores whose masses are on the order of 10^1-10^3 Msun. No submm core presents detectable free-free emission in the centimeter regime, but they appear to be associated with masers and thermal line emission from complex organic molecules. Towards the most massive core, SMA1, the CH3CN (18_K-17_K) lines reveal hints of rotation about the axis of a jet/outflow traced by H2O maser and H13CO+ (1--0) line emission. Inverse P-Cygni profiles of the 13CO (3--2) and C18O (3--2) lines seen towards SMA1 indicate that the central high-mass (proto)star(s) is (are) still gaining mass with an accretion rate $ge 3 ~10^{-3}$ Msun/yr. Due to the linear scales and the large values of the accretion rate, we hypothesize that we are observing an accretion flow towards a cluster in the making, rather than towards a single massive star.Comment: A&A accepted; 18 pages; Preprint with full-resolution figures is available at http://subarutelescope.org/staff/rsf/publication.htm