Submillimeter Array Observations Research Articles

HIGH-RESOLUTION SUBMILLIMETER MULTILINE OBSERVATIONS OF G19.61 – 0.23: SMALL-SCALE CHEMISTRY

We present the Submillimeter Array (SMA) observations of molecular lines at 330 and 340?GHz toward G19.61 ? 0.23. The SMA observations have a spatial resolution of ~2'' and a bandpass of 2?? 2?GHz bandwidth. With the SMA data, we have detected 131 molecular transitions. Ninety-four molecular transitions from 17 species and their isotopomers are identified, including complex organic molecules and simple linear molecules. Most of the complex molecules (CH3OH, 13CH3OH, C2H5OH, HCOOCH3, HNCO, NH2CHO, CH3CN, and CH3CH2CN) have a sufficient number of transitions in this observation to allow analysis using the rotational temperature diagram method. The results from rotation temperature diagram fitting have shown that the complex nitrogen-bearing molecules have higher rotation temperatures (296-609 K) and lower column densities (6.5?? 1015-6.4?? 1016?cm?2). In contrast, the temperatures and column densities of the complex oxygen-bearing molecules range from 95 to 151 K, and from 1.1?? 1016 to 5.2?? 1017?cm?2, respectively. The H2 column density is estimated from the submillimeter continuum, and the fractional abundances of various species relative to H2 are calculated. The oxygen-bearing molecules have higher fractional abundances than those of the nitrogen-bearing molecules. The different gas temperatures and fractional abundances suggest a chemical differentiation between oxygen- and nitrogen-bearing molecules. The images of the spatial distribution of different species have shown that the oxygen-bearing and nitrogen-bearing molecules peak at different positions. Through comparing the rotation temperatures and fractional abundances with the spatial distributions of the molecules, we discuss possible chemical processes for producing the complex molecules, as well as nitrogen and oxygen differentiation in G19.61 ? 0.23.

TRUNCATED DISKS IN TW Hya ASSOCIATION MULTIPLE STAR SYSTEMS

We present high angular resolution (down to 0.3" = 13 AU in diameter) Submillimeter Array observations of the 880 micron (340 GHz) thermal continuum emission from circumstellar dust disks in the nearby HD 98800 and Hen 3-600 multiple star systems. In both cases, the dust emission is resolved and localized around one stellar component - the HD 98800 B and Hen 3-600 A spectroscopic binaries - with no evidence for circum-system material. Using two-dimensional Monte Carlo radiative transfer calculations, we compare the SMA visibilities and broadband spectral energy distributions with truncated disk models to empirically locate the inner and outer edges of both disks. The HD 98800 B disk appears to be aligned with the spectroscopic binary orbit, is internally truncated at a radius of 3.5 AU, and extends to only 10-15 AU from the central stars. The Hen 3-600 A disk is slightly larger, with an inner edge at 1 AU and an outer radius of 15-25 AU. These inferred disk structures compare favorably with theoretical predictions of their truncation due to tidal interactions with the stellar companions.

MILLIMETER DUST EMISSION IN THE GQ LUP SYSTEM

We present Submillimeter Array observations of the GQ Lup system at 1.3 millimeters wavelength with $0\farcs4$ ($\sim$60 AU) resolution. Emission is detected from the position of the primary star, GQ Lup A, and is marginally resolved. No emission is detected from the substellar companion, GQ Lup B, $0\farcs7$ away. These data, together with models of the spectral energy distribution, suggest a compact disk around GQ Lup A with mass $\sim 3$ M$_{Jup}$, perhaps truncated by tidal forces. There is no evidence for a gap or hole in the disk that might be the signature of an additional inner companion body capable of scattering GQ Lup B out to $\sim100$ AU separation from GQ Lup A. For GQ Lup B to have formed {\it in situ}, the disk would have to have been much more massive and extended.

EVIDENCE FOR DUST CLEARING THROUGH RESOLVED SUBMILLIMETER IMAGING

Mid-infrared spectrophotometric observations have revealed a small sub-class of circumstellar disks with spectral energy distributions (SEDs) suggestive of large inner gaps with low dust content. However, such data provide only an indirect and model-dependent method of finding central holes. Imaging of protoplanetry disks provides an independent check of SED modeling. We present here the direct characterization of three 33-47 AU radii inner gaps, in the disks around LkHa 330, SR 21N and HD 135344B, via 340 GHz (880 micron) dust continuum aperture synthesis observations obtained with the Submillimeter Array (SMA). The large gaps are fully resolved at ~0\farcs3 by the SMA observations and mostly empty of dust, with less than 1 - 7.5 x 10^-6 Msolar of fine grained solids inside the holes. Gas (as traced by atomic accretion markers and CO 4.7 micron rovibrational emission) is still present in the inner regions of all three disks. For each, the inner hole exhibits a relatively steep rise in dust emission to the outer disk, a feature more likely to originate from the gravitational influence of a companion body than from a process expected to show a more shallow gradient like grain growth. Importantly, the good agreement of the spatially resolved data and spectrophotometry-based models lends confidence to current interpretations of SEDs, wherein the significant dust emission deficits arise from disks with inner gaps or holes. Further SED-based searches can therefore be expected to yield numerous additional candidates that can be examined at high spatial resolution.

Rotational structure and outflow in the infrared dark cloud 18223-3

We examine an Infrared Dark Cloud (IRDC) at high spatial resolution as a means to study rotation, outflow, and infall at the onset of massive star formation. Submillimeter Array observations combined with IRAM 30 meter data in 12CO(2--1) reveal the outflow orientation in the IRDC 18223-3 region, and PdBI 3 mm observations confirm this orientation in other molecular species. The implication of the outflow's presence is that an accretion disk is feeding it, so using high density tracers such as C18O, N2H+, and CH3OH, we looked for indications of a velocity gradient perpendicular to the outflow direction. Surprisingly, this gradient turns out to be most apparent in CH3OH. The large size (28,000 AU) of the flattened rotating object detected indicates that this velocity gradient cannot be due solely to a disk, but rather from inward spiraling gas within which a Keplerian disk likely exists. From the outflow parameters, we derive properties of the source such as an outflow dynamical age of ~37,000 years, outflow mass of ~13 M_sun, and outflow energy of ~1.7 x 10^46 erg. While the outflow mass and energy are clearly consistent with a high-mass star forming region, the outflow dynamical age indicates a slightly more evolved evolutionary stage than previous spectral energy distribution (SED) modeling indicates. The calculated outflow properties reveal that this is truly a massive star in the making. We also present a model of the observed methanol velocity gradient. The rotational signatures can be modeled via rotationally infalling gas. These data present evidence for one of the youngest known outflow/infall/disk systems in massive star formation. A tentative evolutionary picture for massive disks is discussed.

MULTIDIMENSIONAL CHEMICAL MODELING OF YOUNG STELLAR OBJECTS. II. IRRADIATED OUTFLOW WALLS IN A HIGH-MASS STAR-FORMING REGION

Observations of the high-mass star-forming region AFGL 2591 reveal a large abundance of CO+, a molecule known to be enhanced by far-ultraviolet (FUV) and X-ray irradiation. In chemical models assuming a spherically symmetric envelope, the volume of gas irradiated by protostellar FUV radiation is very small due to the high extinction by dust. The abundance of CO+ is thus underpredicted by orders of magnitude. In a more realistic model, FUV photons can escape through an outflow region and irradiate gas at the border to the envelope. Thus, we introduce the first two-dimensional axisymmetric chemical model of the envelope of a high-mass star-forming region to explain the CO+ observations as a prototypical FUV tracer. The model assumes an axisymmetric power-law density structure with a cavity due to the outflow. The local FUV flux is calculated by a Monte Carlo radiative transfer code taking scattering on dust into account. A grid of precalculated chemical abundances, introduced in the first part of this series of papers, is used to quickly interpolate chemical abundances. This approach allows us to calculate the temperature structure of the FUV-heated outflow walls self-consistently with the chemistry. Synthetic maps of the line flux are calculated using a raytracer code. Single-dish and interferometric observations are simulated and the model results are compared to published and new JCMT and Submillimeter Array (SMA) observations. The two-dimensional model of AFGL 2591 is able to reproduce the JCMT single-dish observations and also explains the nondetection by the SMA. We conclude that the observed CO+ line flux and its narrow width can be interpreted by emission from the warm and dense outflow walls irradiated by protostellar FUV radiation.

MASSIVE PROTOPLANETARY DISKS IN ORION BEYOND THE TRAPEZIUM CLUSTER

We present Submillimeter Array observations of the 880 micron continuum emission from three circumstellar disks around young stars in Orion that lie several arcminutes (> 1-pc) north of the Trapezium cluster. Two of the three disks are in the binary system 253-1536. Silhouette disks 216-0939 and 253-1536a are found to be more massive than any previously observed Orion disks, with dust masses derived from their submillimeter emission of 0.045 Msun and 0.066 Msun, respectively. The existence of these massive disks reveals the disk mass distribution in Orion does extend to high masses, and that the truncation observed in the central Trapezium cluster is a result of photoevaporation due to the proximity of O-stars. 253-1536b has a disk mass of 0.018 Msun, making the 253-1536 system the first optical binary in which each protoplanetary disk is massive enough to potentially form Solar systems.

SUBARCSECOND IMAGING OF THE HIGH-MASS STAR-FORMING REGION ONSALA 1

We report subarcsecond images of the high-mass star forming region Onsala 1 (ON 1) made with the Submillimeter Array (SMA) at 0.85 mm and the Very Large Array at 1.3 cm and 3.6 cm. ON~1 is one of the smallest ultracompact HII regions in the Galaxy and exhibits various star formation signposts. With our VLA and SMA observations, two new cm-wave sources and five sub-mm dust sources, respectively, within a field of ~3" (corresponding to a linear scale of 0.05 pc) are identified, indicating the multiplicity at the center of the ON 1 region. The dust and gas masses of these sub-mm sources are in the range of 0.8 to 6.4 M_sun. Among the five sub-mm dust sources, SMA2, with a dust and gas mass of 2.6 M_sun, demonstrates several star formation signatures, and hence likely represents an intermediate-mass (or even high-mass) star forming core. Due to the low star formation efficiency of ~10%, we suggest that star formation in the ON 1 region will continue. For example, SMA4 and SMA5 are not associated with any star formation signatures and likely mark star formation cores at very early evolutionary stages.

A SPATIALLY RESOLVED INNER HOLE IN THE DISK AROUND GM AURIGAE

We present 0.3 arcsec resolution observations of the disk around GM Aurigae with the Submillimeter Array (SMA) at a wavelength of 860 um and with the Plateau de Bure Interferometer at a wavelength of 1.3 mm. These observations probe the distribution of disk material on spatial scales commensurate with the size of the inner hole predicted by models of the spectral energy distribution. The data clearly indicate a sharp decrease in millimeter optical depth at the disk center, consistent with a deficit of material at distances less than ~20 AU from the star. We refine the accretion disk model of Calvet et al. (2005) based on the unresolved spectral energy distribution (SED) and demonstrate that it reproduces well the spatially resolved millimeter continuum data at both available wavelengths. We also present complementary SMA observations of CO J=3-2 and J=2-1 emission from the disk at 2" resolution. The observed CO morphology is consistent with the continuum model prediction, with two significant deviations: (1) the emission displays a larger CO J=3-2/J=2-1 line ratio than predicted, which may indicate additional heating of gas in the upper disk layers; and (2) the position angle of the kinematic rotation pattern differs by 11 +/- 2 degrees from that measured at smaller scales from the dust continuum, which may indicate the presence of a warp. We note that photoevaporation, grain growth, and binarity are unlikely mechanisms for inducing the observed sharp decrease in opacity or surface density at the disk center. The inner hole plausibly results from the dynamical influence of a planet on the disk material. Warping induced by a planet could also potentially explain the difference in position angle between the continuum and CO data sets.

FRAGMENTATION AT THE EARLIEST PHASE OF MASSIVE STAR FORMATION

We present 1.3 mm continuum and spectral line images of two massive molecular clumps P1 and P2 in the G28.34+0.06 region with the Submillimeter Array (SMA). While the two clumps contain masses of 1000 and 880 M☉, respectively, P1 has a luminosity OF <102 L☉, and a lower gas temperature and smaller line width than P2. Thus, P1 appears to be at a much earlier stage of massive star formation than P2. The high-resolution SMA observations reveal two distinctive cores in P2 with masses of 97 and 49 M☉, respectively. The 4 GHz spectral bandpass captures line emission from CO isotopologues, SO, CH3OH, and CH3CN, similar to hot molecular cores harboring massive young stars. The P1 clump, on the other hand, is resolved into five cores along the filament with masses from 22 to 64 M☉ and an average projected separation of 0.19 pc. Except 12CO, no molecular line emission is detected toward the P1 cores at a 1σ rms of 0.1 K. Since strong 12CO and C18O emissions are seen with the single-dish telescope at a resolution of 11'', the nondetection of these lines with the SMA indicates a depletion factor up to 103. While the spatial resolution of the SMA is better than the expected Jeans length, the masses in P1 cores are much larger than the thermal Jeans mass, indicating the importance of turbulence and/or magnetic fields in cloud fragmentation. The hierarchical structures in the P1 region provide a glimpse of the initial phase of massive star and cluster formation.

SUBMILLIMETER ARRAY OBSERVATIONS OF THE MOLECULAR OUTFLOW IN HIGH-MASS STAR-FORMING REGION G240.31+0.07

We present Submillimeter Array observations toward the 10^{4.7} Lsun star-forming region G240.31+0.07, in the J=2-1 transition of 12CO and 13CO and at 1.3 mm continuum, as well as the 12CO and 13CO observations from the Caltech Submillimeter Observatory to recover the extended emission filtered out by the interferometer. Maps of the 12CO and 13CO emission show a bipolar, wide-angle, quasi-parabolic molecular outflow, roughly coincident with an IR nebula revealed by the Spitzer 3.6 and 4.5 micron emission. The outflow has ~98 Msun molecular gas, making it one of the most massive molecular outflows known, and resulting in a very high mass-loss rate of 4.1 by 10^{-3} Msun yr^{-1} over a dynamical timescale of 2.4 by 10^4 yr. The 1.3 mm continuum observations with a 4" by 3" beam reveal a flattened dusty envelope of ~150 Msun, which is further resolved with a 1.2" by 1" beam into three dense cores with a total mass of ~40 Msun. The central mm core, showing evidence of active star formation, approximately coincides with the geometric center of the bipolar outflow thus most likely harbors the powering source of the outflow. Overall our observations provide the best case to date of a well-defined wide-angle molecular outflow in a >10^4 Lsun star-forming region. The outflow is morphologically and kinematically similar to low-mass protostellar outflows but has two to three orders of magnitude greater mass, momentum, and energy, and is apparently driven by an underlying wide-angle wind, hence further supports that high-mass stars up to late-O types, even in a crowded clustering environment, can form as a scaled-up version of low-mass star formation.

EVOLUTION OF MAGNETIC FIELDS IN HIGH MASS STAR FORMATION: SUBMILLIMETER ARRAY DUST POLARIZATION IMAGE OF THE ULTRACOMPACT H II REGION G5.89–0.39

We report high angular resolution (3'') Submillimeter Array (SMA) observations of the molecular cloud associated with the Ultracompact H ii region G5.89 − 0.39. Imaged dust continuum emission at 870 μm reveals significant linear polarization. The position angles (P.A.s) of the polarization vary enormously but smoothly in a region of 2 × 104 AU. Based on the distribution of the P.A.s and the associated structures, the polarized emission can be separated roughly into two components. The component "x" is associated with a well-defined dust ridge at 870 μm, and is likely tracing a compressed B field. The component "o" is located at the periphery of the dust ridge and is probably from the original B field associated with a pre-existing extended structure. The global B field morphology in G5.89, as inferred from the P.A.s, is clearly disturbed by the expansion of the H ii region and the molecular outflows. Using the Chandrasekhar–Fermi method, we estimate from the smoothness of the field structures that the B field strength in the plane of sky can be no more than 2–3 mG. We then compare the energy densities in the radiation, the B field, and the mechanical motions as deduced from the C17O 3-2 line emission. We conclude that the B field structures are already overwhelmed and dominated by the radiation, outflows, and turbulence from the newly formed massive stars.

DETECTION OF C I IN ABSORPTION TOWARD PKS 1830 – 211 WITH THE eSMA

We report the first science observations and results obtained with the "extended" SMA (eSMA), which is composed of the SMA (Submillimeter Array), JCMT (James Clerk Maxwell Telescope), and CSO (Caltech Submillimeter Observatory). Redshifted absorptions at z = 0.886 of C i (3P1–3P0) were observed with the eSMA with an angular resolution of 055 × 022 at 1.1 mm toward the southwestern image of the remarkable lensed quasar PKS 1830 − 211, but not toward the northeastern component at a separation of ∼1''. Additionally, SMA observations of CO, 13CO, and C18O (all J = 4–3) were obtained toward this object: CO was also detected toward the southwest component, but none of the isotopologues were. This is the first time [C i] has been detected in this object, allowing the first direct determination of relative abundances of neutral atomic carbon to CO in the molecular clouds of a spiral galaxy at z > 0.1. The [C i] and CO profiles can be decomposed into two and three velocity components, respectively. We derive C/CO column density ratios ranging from ≲0.5 (representative of dense cores) to ∼2.5 (close to translucent cloud values). This could indicate that we are seeing environments with different physical conditions or that we are witnessing the chemical evolution of regions where C has not completely been converted into CO.

High-resolution HNC 3–2 SMA observations of Arp 220

Aims. We study the properties of the nuclear molecular gas of the ultra luminous merger Arp 220 and effects of the nuclear source on gas excitation and chemistry. Specifically, our aim is to investigate the spatial location of the luminous HNC 3-2 line emission and address the underlying cause of its unusual brightness. Methods. We present high resolution observations of HNC J=3-2 with the submillimeter array (SMA). Results. We find luminous HNC 3-2 line emission in the western part of Arp 220, centred on the western nucleus, while the eastern side of the merger shows relatively faint emission. A bright (36 K at $0\hbox{$.\!\!^{\prime\prime}$ }4$ resolution), narrow (60 ${\rm km~s}^$) emission feature emerges from the western nucleus, superposed on a broader spectral component. A possible explanation is weak maser emission through line-of-sight amplification of the background continuum source. There is also a more extended HNC 3-2 emission feature north and south of the nucleus. This feature resembles the bipolar OH maser morphology around the western nucleus. Substantial HNC abundances are required to explain the bright line emission from this warm environment - even when the high gas column density towards the western nucleus is taken into account. We discuss this briefly in the context of an X-ray affected chemistry and radiative excitation. Conclusions. The luminous and possibly amplified HNC emission of the western nucleus of the Arp 220 merger reflects the unusual, and perhaps transient environment of the starburst/AGN activity there. The faint HNC line emission towards Arp 220-east reveals a real difference in physical conditions between the two merger nuclei.

Submillimeter Observations of the Young Low‐Mass Object IRAS 04158+2805

We present high spatial resolution Submillimeter Array observations and supplementary single-dish photometry of the molecular gas and dust around IRAS 04158+2805, a young source with spectral type M5-M6 in the Taurus star-forming region. A bright, highly elongated dust structure that extends 8'' (~1120 AU) in diameter is revealed in a 883 μm thermal continuum image. This emission geometry is in good agreement with optical observations that show a similar structure in absorption, aligned perpendicular to bipolar scattered light nebulae. However, the interferometric data also clearly demonstrate that the submillimeter continuum emission is not centrally concentrated, but rather appears to have a toroidal geometry with substantially lower intensities inside a radius of ~250-300 AU. Spatially resolved emission from the CO J = 3–2 transition exhibits a velocity gradient along the major axis of the dust structure. If this kinematic pattern is interpreted as the signature of rotation around a central object, then a relatively low mass is inferred (M* ~ 0.3 M☉, with a ~50% uncertainty). We discuss several possible explanations for the observed gas and dust environment around IRAS 04158+2805, including a flattened envelope with an outflow cavity and a large circumbinary ring. This source offers unique views of the gas and dust environment surrounding a young low-mass stellar system. Its properties are generally not commensurate with formation scenarios for such low-mass objects that rely on dynamical ejection, but rather confirms that a single mechanism—molecular cloud core collapse and fragmentation—can produce stars over a wide range of stellar masses (at least an order of magnitude).

The Masses of Transition Circumstellar Disks: Observational Support for Photoevaporation Models

We report deep Sub-Millimeter Array observations of 26 pre-main-sequence (PMS) stars with evolved inner disks. These observations measure the mass of the outer disk (r ~20-100 AU) across every stage of the dissipation of the inner disk (r < 10 AU) as determined by the IR spectral energy distributions (SEDs). We find that only targets with high mid-IR excesses are detected and have disk masses in the 1-5 M_Jup range, while most of our objects remain undetected to sensitivity levels of M_DISK ~0.2-1.5 M_Jup. To put these results in a more general context, we collected publicly available data to construct the optical to millimeter wavelength SEDs of over 120 additional PMS stars. We find that the near-IR and mid-IR emission remain optically thick in objects whose disk masses span 2 orders of magnitude (~0.5-50 M_Jup). Taken together, these results imply that, in general, inner disks start to dissipate only after the outer disk has been significantly depleted of mass. This provides strong support for photoevaporation being one of the dominant processes driving disk evolution.

A Search for Molecular Gas in the Nucleus of M87 and Implications for the Fueling of Supermassive Black Holes

Supermassive black holes in giant elliptical galaxies are remarkably faint given their expected accretion rates. This motivates models of radiatively inefficient accretion, due to either ion-electron thermal decoupling, generation of outflows that inhibit accretion, or settling of gas to a gravitationally unstable disk that forms stars in preference to feeding the black hole. The latter model predicts the presence of cold molecular gas in a thin disk around the black hole. Here we report Submillimeter Array observations of the nucleus of the giant elliptical galaxy M87 that probe 230 GHz continuum and CO(J=2--1) line emission. Continuum emission is detected from the nucleus and several knots in the jet, including one that has been undergoing flaring behavior. We estimate a conservative upper limit on the mass of molecular gas within ~100pc and +-400km/s line of sight velocity of the central black hole of ~8x10^6Msun, which includes an allowance for possible systematic errors associated with subtraction of the continuum. Ignoring such errors, we have a 3 sigma sensitivity to about 3x10^6Msun. In fact, the continuum-subtracted spectrum shows weak emission features extending up to 4 sigma above the RMS dispersion of the line-free channels. These may be artifacts of the continuum subtraction process. Alternatively, if they are interpreted as CO emission, then the implied molecular gas mass is ~5x10^6Msun spread out over a velocity range of 700km/s. These constraints on molecular gas mass are close to the predictions of the model of self-gravitating, star-forming accretion disks fed by Bondi accretion (Tan & Blackman 2005).

MOLECULAR GAS AND STAR FORMATION IN ARP 302

We present the Submillimeter Array (SMA) observation of the CO J = 2-1 transition toward the northern galaxy, ARP 302N, of the early merging system, ARP 302. Our high angular resolution observation reveals the extended spatial distribution of the molecular gas in ARP 302N. We find that the molecular gas has a very asymmetric distribution with two strong concentrations on either side of the center together with a weaker one offset by about 8 kpc to the north. The molecular gas distribution is also found to be consistent with that from the hot dust as traced by the 24 mu m continuum emission observed by Spitzer. The line ratio of CO J = 2-1/1-0 is found to vary strongly from about 0.7 near the galaxy center to 0.4 in the outer part of the galaxy. Excitation analysis suggests that the gas density is low, less than 103 cm(-3), over the entire galaxy. By fitting the spectral energy distribution of ARP 302N in the far infrared we obtain a dust temperature of T(d) = 26-36 K and a dust mass of M(dust) = 2.0-3.6 x 10(8) M(circle dot). The spectral index of the radio continuum is around 0.9. The spatial distribution and spectral index of the radio continuum emission suggest that most of the radio continuum emission is synchrotron emission from the star-forming regions at the nucleus and ARP 302N-cm. The good spatial correspondence between the 3.6 cm radio continuum emission, the Spitzer 8 and 24 mu m data and the high-resolution CO J = 2-1 observation from the SMA shows that there is asymmetrical star-forming activity in ARP 302N.

Resolving the Chemistry in the Disk of TW Hydrae. I. Deuterated Species

We present Submillimeter Array observations of several deuterated species in the disk around the classical T Tauri star TW Hydrae at arcsecond scales, including detections of the DCN J = 3–2 and DCO+ J = 3–2 lines and upper limits to the HDO 31,2–22,1, ortho-H2D+ 11,0–11,1, and para-D2H+ 11,0–10,1 transitions. We also present observations of the HCN J = 3–2, HCO+ J = 3–2, and H13CO+ J = 4–3 lines for comparison with their deuterated isotopologues. We constrain the radial and vertical distributions of various species in the disk by fitting the data using a model where the molecular emission from an irradiated accretion disk is sampled with a two-dimensional Monte Carlo radiative transfer code. We find that the distribution of DCO+ differs markedly from that of HCO+. The D/H ratios inferred change by at least 1 order of magnitude (0.01-0.1) for radii <30 to ≥70 AU, and there is a rapid falloff of the abundance of DCO+ at radii larger than 90 AU. Using a simple analytical chemical model, we constrain the degree of ionization, x(e−) = n(e−)/n(H2), to be ~10^−7 in the disk layer(s) where these molecules are present. Provided the distribution of DCN follows that of HCN, the ratio of DCN to HCN is determined to be (1.7 ± 0.5) × 10^−2; however, this ratio is very sensitive to the poorly constrained vertical distribution of HCN. The resolved radial distribution of DCO+ indicates that in situ deuterium fractionation remains active within the TW Hydrae disk and must be considered in the molecular evolution of circumstellar accretion disks.

The Structure of the DoAr 25 Circumstellar Disk

We present high spatial resolution (< 0.3 = 40$ AU) Submillimeter Array observations of the 865 micron continuum emission from the circumstellar disk around the young star DoAr 25. Despite its bright millimeter emission, this source exhibits only a comparatively small infrared excess and low accretion rate, suggesting that the material and structural properties of the inner disk may be in an advanced state of evolution. A simple model of the physical conditions in the disk is derived from the submillimeter visibilities and the complete spectral energy distribution using a Monte Carlo radiative transfer code. For the standard assumption of a homogeneous grain size distribution at all disk radii, the results indicate a shallow surface density profile, $\Sigma \propto r^{-p}$ with p = 0.34, significantly less steep than a steady-state accretion disk (p = 1) or the often adopted minimum mass solar nebula (p = 1.5). Even though the total mass of material is large (M_d = 0.10 M_sun), the densities inferred in the inner disk for such a model may be too low to facilitate any mode of planet formation. However, alternative models with steeper density gradients (p = 1) can explain the observations equally well if substantial grain growth in the planet formation region (r < 40 AU) has occurred. We discuss these data in the context of such models with dust properties that vary with radius and highlight their implications for understanding disk evolution and the early stages of planet formation.