arXiv subjects

Claire J. Chandler

Publications and source records attributed to Claire J. Chandler.

50 records · Page 3Linked to original sources

arXiv2013

VLA and CARMA Observations of Protostars in the Cepheus Clouds: Sub-arcsecond Proto-Binaries Formed via Disk Fragmentation

We present observations of three Class 0/I protostars (L1157-mm, CB230 IRS1, and L1165-SMM1) using the Karl G. Jansky Very Large Array (VLA) and observations of two (L1165-SMM1 and CB230 IRS1) with the Combined Array for Research in Millimeter-wave Astronomy (CARMA). The VLA observations were taken at wavelengths of $λ= 7.3$ mm, 1.4 cm, 3.3 cm, 4.0 cm, and 6.5 cm with a best resolution of $\sim$0\farcs06 (18 AU) at 7.3 mm. The L1165-SMM1 CARMA observations were taken at $λ= 1.3$ mm with a best resolution of $\sim0\farcs3$ (100 AU), and the CB230 IRS1 observations were taken at $λ= 3.4$ mm with a best resolution of $\sim$3\arcsec\ (900 AU). We find that L1165-SMM1 and CB230 IRS1 have probable binary companions at separations of $\sim$0\farcs3 (100 AU) from detections of secondary peaks at multiple wavelengths. The position angles of these companions are nearly orthogonal to the direction of the observed bipolar outflows, consistent with the expected protostellar disk orientations. We suggest that these companions may have formed from disk fragmentation; turbulent fragmentation would not preferentially arrange the binary companions to be orthogonal to the outflow direction. For L1165-SMM1, both the 7.3 mm and 1.3 mm emission show evidence of a large (R $>$ 100 AU) disk. For the L1165-SMM1 primary protostar and the CB230 IRS1 secondary protostar, the 7.3 mm emission is resolved into structures consistent with $\sim20$ AU radius disks. For the other protostars, including L1157-mm, the emission is unresolved, suggesting disks with radii $< 20$ AU.

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arXiv2012

ALMA and VLA observations of the outflows in IRAS 16293-2422

We present ALMA and VLA observations of the molecular and ionized gas at 0.1-0.3 arcsec resolution in the Class 0 protostellar system IRAS 16293-2422. These data clarify the origins of the protostellar outflows from the deeply embedded sources in this complex region. Source A2 is confirmed to be at the origin of the well known large scale north-east--south-west flow. The most recent VLA observations reveal a new ejection from that protostar, demonstrating that it drives an episodic jet. The central compact part of the other known large scale flow in the system, oriented roughly east-west, is well delineated by the CO(6-5) emission imaged with ALMA and is confirmed to be driven from within component A. Finally, a one-sided blueshifted bubble-like outflow structure is detected here for the first time from source B to the north-west of the system. Its very short dynamical timescale (~ 200 yr), low velocity, and moderate collimation support the idea that source B is the youngest object in the system, and possibly one of the youngest protostars known.

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arXiv2012

Constraints on the Radial Variation of Grain Growth in the AS 209 Circumstellar Disk

We present dust continuum observations of the protoplanetary disk surrounding the pre-main sequence star AS 209, spanning more than an order of magnitude in wavelength from 0.88 to 9.8 mm. The disk was observed with sub-arcsecond angular resolution (0.2"-0.5") to investigate radial variations in its dust properties. At longer wavelengths, the disk emission structure is notably more compact, providing model-independent evidence for changes in the grain properties across the disk. We find that physical models which reproduce the disk emission require a radial dependence of the dust opacity κ_ν. Assuming that the observed wavelength-dependent structure can be attributed to radial variations in the dust opacity spectral index (β), we find that β(R) increases from β<0.5 at \sim20 AU to β>1.5 for R>80 AU, inconsistent with a constant value of β across the disk (at the 10σ level). Furthermore, if radial variations of κ_ν are caused by particle growth, we find that the maximum size of the particle-size distribution (a_{max}) increases from sub-millimeter-sized grains in the outer disk (R>70 AU) to millimeter and centimeter-sized grains in the inner disk regions (R< 70 AU). We compare our observational constraint on a_{max}(R) with predictions from physical models of dust evolution in proto-planetary disks. For the dust composition and particle-size distribution investigated here, our observational constraints on a_{max}(R) are consistent with models where the maximum grain size is limited by radial drift.

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arXiv2012

Complex Structure in Class 0 Protostellar Envelopes III: Velocity Gradients in Non-Axisymmetric Envelopes, Infall or Rotation?

We present an interferometric kinematic study of morphologically complex protostellar envelopes based on observations of the dense gas tracers N2H+ and NH3. The strong asymmetric nature of most envelopes in our sample leads us to question the common interpretation of velocity gradients as rotation, given the possibility of projection effects in the observed velocities. Several "idealized" sources with well-ordered velocity fields and envelope structures are now analyzed in more detail. We compare the interferometric data to position-velocity diagrams of kinematic models for spherical rotating collapse and filamentary rotating collapse. For this purpose, we developed a filamentary parametrization of the rotating collapse model to explore the effects of geometric projection on the observed velocity structures. We find that most envelopes in our sample have PV structures that can be reproduced by an infalling filamentary envelope projected at different angles within the plane of the sky. The infalling filament produces velocity shifts across the envelope that can mimic rotation, especially when viewed at single-dish resolutions and the axisymmetric rotating collapse model does not uniquely describe any dataset. Furthermore, if the velocities are assumed to reflect rotation, then the inferred centrifugal radii are quite large in most cases, indicating significant fragmentation potential or more likely another component to the line-center velocity. We conclude that ordered velocity gradients cannot be interpreted as rotation alone when envelopes are non-axisymmetric and that projected infall velocities likely dominate the velocity field on scales larger than 1000 AU.

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arXiv2011

Complex Structure in Class 0 Protostellar Envelopes II: Kinematic Structure from Single-Dish and Interferometric Molecular Line Mapping

We present a study of dense molecular gas kinematics in seventeen nearby protostellar systems using single-dish and interferometric molecular line observations. The non-axisymmetric envelopes around a sample of Class 0/I protostars were mapped in the N2H+ (J=1-0) tracer with the IRAM 30m, CARMA and PdBI as well as NH3 (1,1) with the VLA. The molecular line emission is used to construct line-center velocity and linewidth maps for all sources to examine the kinematic structure in the envelopes on spatial scales from 0.1 pc to ~1000 AU. The direction of the large-scale velocity gradients from single-dish mapping is within 45 degrees of normal to the outflow axis in more than half the sample. Furthermore, the velocity gradients are often quite substantial, the average being ~2.3 km\s\pc. The interferometric data often reveal small-scale velocity structure, departing from the more gradual large-scale velocity gradients. In some cases, this likely indicates accelerating infall and/or rotational spin-up in the inner envelope; the median velocity gradient from the interferometric data is ~10.7 km/s/pc. In two systems, we detect high-velocity HCO+ (J=1-0) emission inside the highest-velocity \nthp\ emission. This enables us to study the infall and rotation close to the disk and estimate the central object masses. The velocity fields observed on large and small-scales are more complex than would be expected from rotation alone, suggesting that complex envelope structure enables other dynamical processes (i.e. infall) to affect the velocity field.

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arXiv2008

Disks, young stars, and radio waves: the quest for forming planetary systems

Kant and Laplace suggested the Solar System formed from a rotating gaseous disk in the 18th century, but convincing evidence that young stars are indeed surrounded by such disks was not presented for another 200 years. As we move into the 21st century the emphasis is now on disk formation, the role of disks in star formation, and on how planets form in those disks. Radio wavelengths play a key role in these studies, currently providing some of the highest spatial resolution images of disks, along with evidence of the growth of dust grains into planetesimals. The future capabilities of EVLA and ALMA provide extremely exciting prospects for resolving disk structure and kinematics, studying disk chemistry, directly detecting proto-planets, and imaging disks in formation.

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arXiv2007

New Radio Sources and the Composite Structure of Component B in the Very Young Protostellar System IRAS 16293-2422

In this article, we report high-resolution (~ 0.1" -- 0.3"), high-sensitivity (~ 50 -100 uJy beam-1) Very Large Array 0.7 and 1.3 cm observations of the young stellar system IRAS 16293-2422 in rho-Ophiuchus. In the 0.7 cm image, component A to the south-east of the system looks like its usual binary self. In the new 1.3 cm image, however, component A2 appears to have split into two sub-components located roughly symmetrically around the original position of A2. This change of morphology is likely the result of a recent bipolar ejection, one of the very first such events observed in a low-mass source. Also in component A, a marginal detection of 0.7 cm emission associated with the submillimeter component Ab is reported. If confirmed, this detection would imply that Ab is a relatively extended dusty structure, where grain coagulation may already have taken place. With an angular size increasing with frequency, and an overall spectra index of 2, the emission from component B to the north-west of the system is confirmed to be dominated by optically thick thermal dust emission associated with a fairly massive, nearly face-on, circumstellar disk. In the central region, however, we find evidence for a modest free-free contribution that originates in a structure elongated roughly in the east-west direction. We argue that this free-free component traces the base of the jet driving the large-scale bipolar flow at a position angle of about 110 degrees that has long been known to be powered by IRAS 16293-2422.

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arXiv2005

The radio properties of the cD galaxy of Abell 2390

We present multi-frequency, multi-epoch radio imaging of the complex radio source B2151+174 in the core of the cluster, Abell 2390 (z~0.23). From new and literature data we conclude that the FRII-powerful radio source is the combination of a compact, core-dominated `medium-symmetric object' (MSO) with a more extended, steeper spectrum mini-halo. B2151+174 is unusual in a number of important aspects: i) it is one of the most compact and flat spectrum sources in a cluster core known; ii) it shows a complex, compact twin-jet structure in a north-south orientation; iii) the orientation of the jets is 45deg misaligned with apparent structure (ionization cones and dust disk) of the host galaxy on larger scales. Since the twin-jet of the MSO has its northern half with an apparent `twist', it might be that precession of the central supermassive black hole explains this misalignement. B2151+174 may be an example of the early stage (10^3-10^4 yrs duration) of a `bubble' being blown into the ICM where the plasma has yet to expand.

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arXiv2005

IRAS 16293-2422: proper motions, jet precession, the hot core, and the unambiguous detection of infall

We present observations of IRAS 16293-2422 using the Submillimeter Array (SMA) at 300 GHz, and the Very Large Array (VLA) at frequencies from 1.5 to 43 GHz. This source was already known to be a binary system with components A and B separated by ~5". The new SMA data now separate source A into two submm continuum components, denoted Aa and Ab. The strongest of these, Aa, peaks between the cm radio sources A1 and A2. Archival VLA data show proper motion of sources A and B of 17 mas/yr, associated with motion of the rho Ophiuchi cloud. We also find relative motion between A1 and A2 which excludes the possibility that they are gravitationally bound. A2 remains stationary relative to source B, and we identify it as the protostar driving the NE-SW CO outflow. A1 is shock-ionized gas where a precessing jet interacts with dense gas. This jet probably drives the E-W CO outflow. The origin of this jet must be located close to A2, and may be the submm source Aa. Thus source A is now shown to comprise three components within 1". Source B, however, is single, exhibits optically-thick dust emission even at 8 GHz, has a high luminosity, and yet shows no sign of outflow, indicating it is probably very young. The SMA spectrum of IRAS 16293-2422 reports the first astronomical identification of many lines at 300 and 310 GHz. The species detected are typical of hot cores, and are mainly associated with source A. We suggest the shock at A1 may explain the enhanced abundances measured, especially of sulphur-bearing molecules. We use the high brightness temperature of the submm dust emission from source B to demonstrate the unambiguous detection of infall by observing redshifted SO (7_7-6_6) absorption against the dust disk.

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arXiv2003

Dense molecular gas in quasar host galaxies: a search for HCN emission from BR B1202-0725 at z=4.695

We report on the results of a search using the VLA for redshifted HCN(1-0) emission from the host galaxy of BR B1202-0725, an optically luminous quasar at z=4.695. The host galaxy emits strongly in the rest-frame far-infrared, and shows characteristics very similar to that of more local, ultraluminous infrared galaxies, in which a significant fraction of the far-infrared emission is powered by star formation. We find a 3-sigma upper limit to the HCN(1-0) emission of 4.9 x 10^10 K kms^-1 pc^2, assuming a lambda-cosmology. This limit is consistent with correlations derived from measurements of the HCN, CO, and far-infrared emission for a sample of more local galaxies, including starbursts (Solomon et al, 1992a).

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arXiv2003

A survey of SiO 5-4 emission towards outflows from low-luminosity protostellar candidates

We have observed the SiO J=5--4 line towards a sample of 25 low-luminosity (L* < 10^3 Lsun) protostellar outflow systems. The line was detected towards 7 of the 25 sources, a detection rate of 28 per cent. The majority (5 out of 7) of sources detected were of class 0 status. We detected a higher fraction of class 0 sources compared with the class I and II sources, although given the small numbers involved the significance of this result should be regarded as tentative. Most of the detected sources showed emission either at or close to the central position, coincident with the protostar. In four cases (HH211, HH25MMS, V-380 OriNE and HH212) emission was also detected at positions away from the center, and was stronger than that observed at the centre position. SiO abundances of 10^(-8) to 8x10^(-7) are derived from LTE analysis. For 2 sources we have additional transitions which we use to conduct statistical equilibrium modeling to estimate the gas density in the SiO-emitting regions. For HH25MMS these results suggest that the SiO emission arises in a higher-density region than the methanol previously observed. We find that the most likely explanation for the preferential detection of SiO emission towards class 0 sources is the greater density of those environments, reinforced by higher shock velocities. We conclude that while not all class 0 sources exhibit SiO emission, SiO emission is a good signpost for the presence of class 0 sources.

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arXiv2002

Completion of a SCUBA survey of Lynds dark clouds and implications for low-mass star formation

We have carried out a survey of optically-selected dark clouds using the bolometer array SCUBA on the James Clerk Maxwell Telescope, at 850 microns wavelength. The survey covers a total of 0.5 square degrees and is unbiased with reference to cloud size, star formation activity, or the presence of infrared emission. Several new protostars and starless cores have been discovered; the protostars are confirmed through the detection of their accompanying outflows in CO(2-1) emission. The survey is believed to be complete for Class 0 and Class I protostars, and yields two important results regarding the lifetimes of these phases. First, the ratio of Class 0 to Class protostars in the sample is roughly unity, very different from the 1:10 ratio that has previously been observed for the rho Ophiuchi star-forming region. Assuming star formation to be a homogeneous process in the dark clouds, this implies that the Class 0 lifetime is similar to the Class I phase, which from infrared surveys has been established to be approximately 200,000 yr. It also suggests there is no rapid initial accretion phase in Class 0 objects. A burst of triggered star formation some 100,000 yr ago can explain the earlier results for rho Ophiuchus. Second, the number of starless cores is approximately twice that of the total number of protostars, indicating a starless core lifetime of approximately 800,000 yr. These starless cores are therefore very short-lived, surviving only two or three free-fall times. This result suggests that, on size scales of 10,000 AU at least, the dynamical evolution of starless cores is probably not controlled by magnetic processes.

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arXiv2001

Sub-arcsecond imaging of SiO in the HH 211 protostellar jet

We present images of the HH 211 molecular jet in the SiO v=0, J=1-0 line at 43 GHz made with the Very Large Array at approximately 0.5 arcsec resolution. The SiO emission appears to trace primarily internal bowshocks in the outflow, suggesting that the dust and molecular gas are accelerated via prompt entrainment at internal working surfaces in the jet. There is also some evidence for limb-brightening of the SiO emission, indicating that SiO emission may also arise from entrainment in the jet's boundary layer. Excitation temperatures of >~150-200 K are inferred from the SiO emission. Enhancements in the SiO abundance of ~10^6 over interstellar values are observed, and the possible origin of the SiO is discussed.

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arXiv1999

The structure of protostellar envelopes derived from submillimeter continuum images

High dynamic range imaging of submillimeter dust emission from the envelopes of eight young protostars in the Taurus and Perseus star-forming regions has been carried out using the SCUBA submillimeter camera on the James Clerk Maxwell Telescope. Good correspondence between the spectral classifications of the protostars and the spatial distributions of their dust emission is observed, in the sense that those with cooler spectral energy distributions also have a larger fraction of the submillimeter flux originating in an extended envelope compared with a disk. This results from the cool sources having more massive envelopes rather than warm sources having larger disks. Azimuthally-averaged radial profiles of the dust emission are used to derive the power-law index of the envelope density distributions, p (defined by rho proportional to r^-p), and most of the sources are found to have values of p consistent with those predicted by models of cloud collapse. However, the youngest protostars in our sample, L1527 and HH211-mm, deviate significantly from the theoretical predictions, exhibiting values of p somewhat lower than can be accounted for by existing models. For L1527 heating of the envelope by shocks where the outflow impinges on the surrounding medium may explain our result. For HH211-mm another explanation is needed, and one possibility is that a shallow density profile is being maintained in the outer envelope by magnetic fields and/or turbulence. If this is the case star formation must be determined by the rate at which the support is lost from the cloud, rather than the hydrodynamical properties of the envelope, such as the sound speed.

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