arXiv subjects

Jonathan P. Williams

Publications and source records attributed to Jonathan P. Williams.

At least 181 records · Page 10Linked to original sources

arXiv2006

High Resolution Submillimeter Constraints on Circumstellar Disk Structure

We present a high spatial resolution submillimeter continuum survey of 24 circumstellar disks in the Tau-Aur and Oph-Sco star formation regions using the SMA. In the context of a simple model, we use broadband SEDs and submillimeter visibilities to derive constraints on some basic parameters that describe the structure of these disks. For the typical disk in the sample we infer a radial surface density distribution Σ~ r^-p with a median p ~ 0.5, although consideration of the systematic effects of some of our assumptions suggest that steeper distributions with p ~ 0.7-1.0 are more reasonable. The distribution of the outer radii of these disks shows a distinct peak at R_d = 200 AU, with only a few cases where the disk emission is completely unresolved. Based on these disk structure measurements, the mass accretion rates, and the typical spectral and spatial distributions of submillimeter emission, we show that the observations are in good agreement with similarity solutions for steady accretion disks that have a viscosity parameter alpha ~ 0.01. We provide new estimates of the spectral dependence of the disk opacity with a median spectral index of ~0.7, corrected for optically thick emission. This typical value is consistent with model predictions for the collisional growth of solids to millimeter size scales in the outer disk. Although direct constraints on planet formation in these disks are not currently available, the extrapolated density distributions inferred here are substantially shallower than those calculated based on the solar system or extrasolar planets and typically used in planet formation models. It is possible that we are substantially underestimating disk densities due to an incomplete submillimeter opacity prescription.

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arXiv2006

The Dust Properties of Eight Debris Disk Candidates as Determined by Submillimeter Photometry

The nature of far-infrared dust emission toward main sequence stars, whether interstellar or circumstellar, can be deduced from submillimeter photometry. We present JCMT/SCUBA flux measurements at 850 microns toward 8 stars with large photospheric excesses at 60-100 microns. 5 sources were detected at 3-sigma or greater significance and one was marginally detected at 2.5-sigma. The inferred dust masses and temperatures range from 0.033 to 0.24 Earth masses and 43-65 K respectively. The frequency behavior of the opacity, tau_nu ~ nu^beta, is relatively shallow, beta < 1. These dust properties are characteristic of circumstellar material, most likely the debris from planetesimal collisions. The 2 non-detections have lower temperatures, 35-38 K and steeper opacity indices, beta > 1.5, that are more typical of interstellar cirrus. The confirmed disks all have inferred diameters > 2'', most lie near the upper envelope of the debris disk mass distribution, and 4 are bright enough to be feasible for high resolution imaging.

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arXiv2005

Submillimeter studies of circumstellar disks in Taurus and Orion

We highlight two recent studies of circumstellar disks in the Taurus and Orion star forming regions. Using the JCMT and CSO, we measure disk fluxes in Taurus over a wide range of submillimeter wavelengths and determine the frequency dependence of the dust opacity. We find clear evidence for a systematic change in its behavior with time, most readily explained by grain growth. Using the SMA, we observed the protoplanetary disks (proplyds) in the Orion Trapezium cluster. The combination of high resolution, high frequency, and high sensitivity that this instrument provides made it possible to resolve disks from one another, distinguish their emission from background cloud material and surrounding ionized gas, and to detect thermal dust emission. This allowed us to make the first mass measurements of the proplyds and to assess their viability for planet formation.

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arXiv2005

An 850 micron survey for dust around solar mass stars

We present the results of an 850 micron JCMT/SCUBA survey for dust around 13 nearby solar mass stars. The dust mass sensitivity ranged from 0.005 to 0.16 Earth masses. Three sources were detected in the survey, one of which (HD 107146) has been previously reported. One of the other two submillimeter sources, HD 104860, was not detected by IRAS and is surrounded by a cold, massive dust disk with a dust temperature and mass of Tdust = 33 K and Mdust = 0.16 Mearth. The third source, HD 8907, was detected by IRAS and ISO at 60-87 microns, and has a dust temperature and mass Tdust = 48 K and Mdust = 0.036 Mearth. We find that the deduced masses and radii of the dust disks in our sample are roughly consistent with models for the collisional evolution of planetesimal disks with embedded planets. We also searched for residual gas in two of the three systems with detected submillimeter excesses and place limits on the mass of gas residing in these systems. When the properties measured for the detected excess sources are combined with the larger population of submillimeter excess sources from the literature, we find strong evidence that the mass in small grains declines significantly on a ~200 Myr timescale, approximately inversely with age. However, we also find that the characteristic dust radii of the population, obtained from the dust temperature of the excess and assuming blackbody grains, is uncorrelated with age. This is in contrast to self-stirred collisional models for debris disk evolution which predict a trend of radius increasing with age, t ~ R^3. The lack of agreement suggests that processes beyond self-stirring, such as giant planet formation, play a role in the evolutionary histories of planetesimal disks.

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arXiv2005

The Masses of the Orion Proplyds from Submillimeter Dust Emission

We have imaged the 880 micron continuum emission from the "proplyds" in the center of the Trapezium Cluster in Orion using the Submillimeter Array with a beam size 1.5'' FWHM and an rms of 2.7 mJy. Five sources are detected with fluxes in the range 18 to 38 mJy, which includes dust emission from four proplyds and ionized gas from theta1 Ori G. The total masses of the detected proplyds derived from their dust emission range from 0.013 to 0.024 Msun assuming a dust temperature of 20 K and mass opacity of 0.03 cm^2/g. The eighteen other proplyds within the field-of-view were not formally detected individually, but by combining the fluxes measured at their locations, an average flux of 1.1mJy was determined for them on a statistical basis, corresponding to a mass of 8e-4 Msun. The four detected proplyds have sufficient disk mass bound to their central stars to form planetary systems on the scale of our Solar System.

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arXiv2005

Circumstellar Dust Disks in Taurus-Auriga: The Submillimeter Perspective

We present a sensitive, multiwavelength submillimeter continuum survey of 153 young stellar objects in the Taurus-Auriga star formation region. The submillimeter detection rate is 61% to a completeness limit of ~10 mJy (3-sigma) at 850 microns. The inferred circumstellar disk masses are log-normally distributed with a mean mass of ~0.005 solar masses and a large dispersion (0.5 dex). Roughly one third of the submillimeter sources have disk masses larger than the minimal nebula from which the solar system formed. The median disk to star mass ratio is 0.5%. The empirical behavior of the submillimeter continuum is best described as F_nu ~ nu^(2.0 +/- 0.5) between 350 microns and 1.3 mm, which we argue is due to the combined effects of the fraction of optically thick emission and a flatter frequency behavior of the opacity compared to the ISM. This latter effect could be due to a substantial population of large dust grains, which presumably would have grown through collisional agglomeration. In this sample, the only stellar property that is correlated with the outer disk is the presence of a companion. We find evidence for significant decreases in submillimeter flux densities, disk masses, and submillimeter continuum slopes along the canonical infrared spectral energy distribution evolution sequence for young stellar objects. The fraction of objects detected in the submillimeter is essentially identical to the fraction with excess near-infrared emission, suggesting that dust in the inner and outer disk are removed nearly simultaneously.

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arXiv2004

Submillimeter Array Observations of Disks in the SR 24 Multiple Star System

We present high-resolution aperture synthesis images from the Submillimeter Array of the 225 GHz (1.3 mm) continuum and CO J=2-1 line emission from the disks around components of the hierarchical triple system SR 24, located in the Ophiuchus star-forming region. The most widely separated component, SR 24 S (with a projected semimajor axis of 832 AU), has a circumstellar disk with properties typical of those around single T Tauri stars. The binary SR 24 N (with a projected semimajor axis of 32 AU) is undetected in the continuum, but has strong, resolved CO emission which likely originates in a circumbinary disk with a central gap. The data constrain the total disk mass in the SR 24 N system to be < 10^-3 M_sun and indicate that the depletion of CO onto dust grains is not more than 100 times larger than the mean value in the interstellar medium. The SR 24 N disk is unusual in that it is only detected in line emission. It is possible that other low mass disks around binaries and single stars may have been missed in single-dish continuum surveys.

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arXiv2004

Near-arcsecond resolution observations of the hot corino of the solar type protostar IRAS 16293-2422

Complex organic molecules have previously been discovered in solar type protostars, raising the questions of where and how they form in the envelope. Possible formation mechanisms include grain mantle evaporation, interaction of the outflow with its surroundings or the impact of UV/X-rays inside the cavities. In this Letter we present the first interferometric observations of two complex molecules, CH3CN and HCOOCH3, towards the solar type protostar IRAS16293-2422. The images show that the emission originates from two compact regions centered on the two components of the binary system. We discuss how these results favor the grain mantle evaporation scenario and we investigate the implications of these observations for the chemical composition and physical and dynamical state of the two components.

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arXiv2004

Complex molecules in the hot core of the low mass protostar NGC1333-IRAS4A

We report the detection of complex molecules (HCOOCH_3, HCOOH and CH_3CN), signposts of a "hot core" like region, toward the low mass, Class 0 source NGC1333-IRAS4A. This is the second low mass protostar where such complex molecules have been searched for and reported, the other source being IRAS16293-2422. It is therefore likely that compact (few tens of AUs) regions of dense and warm gas, where the chemistry is dominated by the evaporation of grain mantles, and where complex molecules are found, are common in low mass Class 0 sources.Given that the chemical formation timescale is much shorter than the gas hot core crossing time, it is not clear whether the reported complex molecules are formed on the grain surfaces (first generation molecules) or in the warm gas by reactions involving the evaporated mantle constituents (second generation molecules). We do not find evidence for large differences in the molecular abundances, normalized to the formaldehyde abundance, between the two solar type protostars, suggesting perhaps a common origin.

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arXiv2004

Modeling the submillimeter emission from the Cepheus A young stellar cluster: Evidence for large scale collapse

Evidence for a large scale flow of low density gas onto the Cepheus A young stellar cluster is presented. Observations of K-band near-infrared and multi-transition CS and N2H+ millimeter line emission are shown in relation to a sub-millimeter map of the cool dust around the most embedded stars. The near-infrared emission is offset from the dust peak suggesting a shift in the location of star formation over the history of the core. The CS emission is concentrated toward the core center but N2H+ peaks in two main cores offset from the center, opposite to the chemistry observed in low mass cores. A starless core with strong CS but weak N2H+ emission is found toward the western edge of the region. The average CS(2-1) spectrum over the cluster forming core is asymmetrically self-absorbed suggesting infall. We analyze the large scale dynamics by applying a one-dimensional radiative transfer code to a model spherical core with constant temperature and linewidth, and a density profile measured from an archival 850 micron map of the region. The best fit model that matches the three CS profiles requires a low CS abundance in the core and an outer, infalling envelope with a low density and undepleted CS abundance. The integrated intensities of the two N2H+ lines is well matched with a constant N2H+ abundance. The envelope infall velocity is tightly constrained by the CS(2-1) asymmetry and is sub-sonic but the size of the infalling region is poorly determined. The picture of a high density center with depleted CS slowly accreting a low density outer envelope with normal CS abundance suggests that core growth occurs at least partially by the dissipation of turbulent support on large scales.

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arXiv2004

A Sub-Millimeter Search of Nearby Young Stars for Cold Dust: Discovery of Debris Disks around Two Low-Mass Stars

(Abridged) We present results from a JCMT/SCUBA 850 um search for cold dust around nearby young stars belonging to the beta Pic (t~12 Myr) and the Local Association (t~50 Myr) moving groups. Unlike most past sub-mm studies, our sample was chosen on the basis of stellar age. Our observations achieve about an order of magnitude greater sensitivity in dust mass compared to previous work in this age range. We detected two of the three M dwarfs in our sample at 850 um, GJ 182 and GJ 803. GJ 182 may also possess a 25 um excess, indicative of warm dust in the inner few AU of its disk. For GJ 803 (AU Mic), sub-mm mapping finds that the 850 um emission is unresolved. A non-detection of the CO 3-2 line indicates the system is gas-poor, and the SED suggests the presence of a large inner disk hole (~17 AU = 1.7 arcsec in radius). These are possible indications that planets at large separations can form around M dwarfs within \~10 Myr. In a companion paper (Kalas, Liu & Matthews 2004), we confirm the existence of a dust disk around GJ 803 using optical coronagraphic imaging. Given its youthfulness, proximity, and detectability, the GJ 803 disk will be a valuable system for studying disk, and perhaps planet, formation in great detail. Overall, sub-mm measurements of debris disks point to a drop in dust mass by a factor of about 10^3 within the first ~10 Myr, with the subsequent decline in the masses of sub-mm detected disks consistent with t^{-0.5} to t^{-1}.

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arXiv2003

Detection of cool dust around the G2V star HD 107146

We report the detection of dust emission at sub-millimeter wavelengths from HD 107146, a G2V star with an age estimated to lie between 80 and 200 Myr. The emission is resolved at 450um with a size 300 AU x 210 AU. A fit to the spectral energy distribution gives a dust temperature of 51 K and dust mass of 0.10 Earth masses. No excess emission above the photosphere was detected at 18um showing that there is very little warm dust and implying the presence of a large inner hole, at least 31 AU (~ 1'') in radius, around the star. The properties of this star-disk system are compared with similar observations of other systems. We also discuss prospects for future observations that may be able to determine whether the inner hole is maintained by the dynamical effect of an unseen orbiting companion.

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arXiv2003

High resolution imaging of CO outflows in OMC-2 and OMC-3

A large scale, high resolution map of CO(1-0) emission toward the OMC-2 and OMC-3 star forming regions is presented. The map is a mosaic of 46 fields using the Berkeley-Illinois-Maryland Array (BIMA) and covers $\sim 10'\times 15'$ at $\sim 10" $ resolution. These data are combined with singledish FCRAO observations and analyzed to identify and determine the properties of nine protostellar outflows. The BIMA data alone almost completely resolve out the cloud emission at central velocities and only recover 1/20 of the flux in the high velocity gas showing that outflows are generally broadly dispersed over $\sim 1'$ angular scales. All nine identified outflows emanate from known Class 0 or borderline Class 0/I sources, are associated with knots of shocked \h2\ emission, and have short dynamical times. It is suggested that only the youngest, most spatially compact, and energetic outflows have been found and that more distributed high velocity gas undetected by BIMA is due to older outflows continuing through the Class I phase of protostellar evolution. The mechanical energy injection rate into the cloud is estimated to be $\sim 1.5 L_\odot$ which is comparable to the turbulent energy dissipation rate. Outf`lows appear capable, therefore, of sustaining cloud turbulence but a high starformation rate is required implying a short cloud lifetime $\leq 5$ `Myr.

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arXiv2002

Mid-Infrared Imaging of the Protostellar Binary L1448N--IRS3(A,B)

Mid-infrared (10-25 \micron) imaging of the protostellar binary system L1448N-IRS3(A,B) is presented. Only one source, IRS3(A), was detected at mid-infrared wavelengths -- all of the mid-infrared emission from IRS3(A,B) emanates from IRS3(A). The mid-infrared luminosity of IRS3(A) is $L_{midir} = 1.3(\frac{d}{300\rm{pc}})^2 L_\sun$, which yields a central source mass, depending on the mass infall rate, of $M_* = 0.2 M_\sun \frac{10^{-6}M_\sun yr^{-1}}{\dot M}$. The envelope mass surrounding IRS3(A) is $\sim 0.15 M_\sun$, suggesting that the central source and the envelope are of comparable mass. The locations of IRS3(A) and IRS3(B) on an $M_{env} - L_{bol}$ diagram indicate that IRS3(A) and IRS3(B) appear to be class I and class 0 protostars, respectively.

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arXiv2001

Gas flows around 2 young stellar clusters in NGC2264

Observations of the dust and gas toward two young stellar clusters, IRS1 and IRS2, in the NGC2264 star forming region are presented. Continuum emission is used to locate the dusty envelopes around the clusters and individual protostars within and line emission from the J=3-2 transitions of HCO+ and H13CO+ is used to diagnose the gas flows around them. The molecular abundance, velocity centroid and dispersion are approximately constant across the IRS1 clump. With these constraints, the self-absorbed HCO+ lines are modeled as a large scale collapse, with speed v_in=0.3 km/s and mass infall rate dM/dt=4e-4 Msun/yr, falling onto an expanding central core. The signature of large scale collapse, with a similar speed and mass infall rate, is also found toward IRS2 but again appears disrupted at small scales. Individual protostars are resolved in this cluster and their size and velocity dispersion show that the stellar system is currently bound and no older than 0.5 Myr, but is destined to become unbound and disperse as the surrounding cloud material is lost.

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arXiv2000

Evidence for Pressure Driven Flows and Turbulent Dissipation in the Serpens NW Protocluster

We have mapped the dense gas distribution and dynamics in the NW region of the Serpens molecular cloud in the CS(2-1) and N2H+(1-0) lines and 3 mm continuum using the FCRAO telescope and BIMA interferometer. 7 continuum sources are found. The N2H+ spectra are optically thin and fits to the 7 hyperfine components are used to determine the distribution of velocity dispersion. 8 cores, 2 with continuum sources, 6 without, lie at a local linewidth minimum and optical depth maximum. The CS spectra are optically thick and generally self-absorbed over the full 0.2 pc extent of the map. We use the line wings to trace outflows around at least 3, and possibly 4, of the continuum sources, and the asymmetry in the self-absorption as a diagnostic of relative motions between core centers and envelopes. The quiescent regions with low N2H+ linewidth tend to have more asymmetric CS spectra than the spectra around the continuum sources indicating higher infall speeds. These regions have typical sizes ~5000 AU, linewidths ~0.5 km/s, and infall speeds ~0.05 km/s. The correlation of CS asymmetry with N2H+ velocity dispersion suggests that the inward flows of material that build up pre-protostellar cores are driven at least partly by a pressure gradient rather than by gravity alone. We discuss a scenario for core formation and eventual star forming collapse through the dissipation of turbulence.

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arXiv1999

The Spectral Correlation Function -- A New Tool for Analyzing Spectral-Line Maps

The "spectral correlation function" analysis we introduce in this paper is a new tool for analyzing spectral-line data cubes. Our initial tests, carried out on a suite of observed and simulated data cubes, indicate that the spectral correlation function [SCF] is likely to be a more discriminating statistic than other statistical methods normally applied. The SCF is a measure of similarity between neighboring spectra in the data cube. When the SCF is used to compare a data cube consisting of spectral-line observations of the ISM with a data cube derived from MHD simulations of molecular clouds, it can find differences that are not found by other analyses. The initial results presented here suggest that the inclusion of self-gravity in numerical simulations is critical for reproducing the correlation behavior of spectra in star-forming molecular clouds.

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arXiv1999

A Contracting, Turbulent, Starless Core in the Serpens Cluster

We present combined single-dish and interferometric CS(2--1) and N2H+(1--0) observations of a compact core in the NW region of the Serpens molecular cloud. The core is starless according to observations from optical to millimeter wavelengths and its lines have turbulent widths and ``infall asymmetry''. Line profile modeling indicates supersonic inward motions v_in>0.34 km/s over an extended region L>12000AU. The high infall speed and large extent exceeds the predictions of most thermal ambipolar diffusion models and points to a more dynamical process for core formation. A short (dynamic) timescale, ~1e5 yr=L/v_in, is also suggested by the low N2H+ abundance ~1e-10.

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