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ABSTRACT: The circumstellar disk of AB Aurigae has garnered strong attention owing to
the apparent existence of spirals at a relatively young stage and also the
asymmetric disk traced in thermal dust emission. However, the physical
conditions of the spirals are still not well understood. The origin of the
asymmetric thermal emission is unclear.
We observed the disk at 230 GHz (1.3 mm) in both the continuum and the
spectral line ^12CO J=2-1 with IRAM 30-m, the Plateau de Bure interferometer,
and the Submillimeter Array to sample all spatial scales from 0.37" to about
50". To combine the data obtained from these telescopes, several methods and
calibration issues were checked and discussed.
The 1.3 mm continuum (dust) emission is resolved into inner disk and outer
ring. Molecular gas at high velocities traced by the CO line is detected next
to the stellar location. The inclination angle of the disk is found to decrease
toward the center. On a larger scale, based on the intensity weighted
dispersion and the integrated intensity map of ^12CO J=2-1, four spirals are
identified, where two of them are also detected in the near infrared. The total
gas mass of the 4 spirals (M_spiral) is 10^-7 < M_spiral < 10^-5 M_sun, which
is 3 orders of magnitude smaller than the mass of the gas ring. Surprisingly,
the CO gas inside the spiral is apparently counter-rotating with respect to the
CO disk, and it only exhibits small radial motion.
The wide gap, the warped disk, and the asymmetric dust ring suggest that
there is an undetected companion with a mass of 0.03 M_sun at a radius of 45
AU. Although an hypothetical fly-by cannot be ruled out, the most likely
explanation of the AB Aurigae system may be inhomogeneous accretion well above
or below the main disk plane from the remnant envelope, which can explain both
the rotation and large-scale motions detected with the 30-m image.
09/2012;
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ABSTRACT: FS Tau B is one of the few T Tauri stars that possess a jet and a counterjet
as well as an optically-visible cavity wall. We obtained images and spectra of
its jet-cavity system in the near-infrared H and K bands using Subaru/IRCS and
detected the jet and the counterjet in the [Fe II] 1.644 \mu m line for the
first time. Within the inner 2" the blueshifted jet is brighter, whereas beyond
~ 5" the redshifted counterjet dominates the [Fe II] emission. The innermost
blueshifted knot is spectrally resolved to have a large line width of ~ 110
km/s, while the innermost redshifted knot appears spectrally unresolved. The
velocity ratio of the jet to the counterjet is ~ 1.34, which suggests that FS
Tau B is driving an asymmetric jet, similar to those found in several T Tauri
Stars. Combining with optical observations in the literature, we showed that
the blueshifted jet has lower density and higher excitation than the redshifted
counterjet. We suggest that the asymmetry in brightness and velocity is the
manifestation of a bipolar outflow driving at different mass-loss rates, while
maintaining balance of linear momentum. A full explanation to the asymmetry in
the FS Tau B system awaits detail modeling and further investigation of the
kinematic structure of the wind-associated cavity walls.
01/2012;
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ABSTRACT: We made C18O (2-1) and CS (7-6) images of the protostellar envelope around
B335 with a high spatial dynamic range from ~10000 to ~400 AU, by combining the
Submillimeter Array and single-dish data. The C18O emission shows an extended
(~10000 AU) structure as well as a compact (~1500 AU) component concentrated at
the protostellar position. The CS emission shows a compact (~900 AU) component
surrounding the protostar, plus a halo-like (~3000 AU) structure elongated
along the east-west direction. At higher velocities (|dV| >~0.3 km s^-1), the
CS emission is stronger and more extended than the C18O emission. Physical
conditions of the envelope were examined through an LVG model. At |dV| >~0.3 km
s^-1, the gas temperature is higher (>40 K) than that at |dV| <~0.3 km s^-1,
whereas the gas density is lower (<10^6 cm^-3). We consider that the
higher-temperature and lower-density gas at |dV| >~0.3 km s^-1 is related to
the associated outflow, while the lower-temperature and higher-density gas at
|dV| <~0.3 km s^-1 is the envelope component. From the inspection of the
positional offsets in the velocity channel maps, the radial profile of the
specific angular momentum of the envelope rotation in B335 was revealed at
radii from ~10^4 down to ~10^2 AU. The specific angular momentum decreases down
to the radius of ~370 AU, and then appears to be conserved within that radius.
A possible scenario of the evolution of envelope rotation is discussed.
09/2011;
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ABSTRACT: We present \sim1" resolution 1.3 millimeter dust continuum and spectral line
(12CO and 13CO J=2-1) observations of the transitional disk system HD 135344B
obtained with the Submillimeter Array. The disk shows a Keplerian rotation
pattern with an inclination of \sim11{\deg}, based on the spatially and
spectrally resolved 12CO and 13CO emission. The data show clear evidence for
both dust and gas surface density reductions in the inner region of the disk
(radius \lesssim 50 AU) from the continuum and 13CO J=2-1 data, respectively.
The presence of this inner cavity in both the dust and gas is more consistent
with clearing by giant planet formation than by photoevaporation or by grain
growth. There is also an indication of global CO gas depletion in the disk, as
the mass estimated from 13CO emission (\sim3.8 \times 10^(-4) M\odot) is about
two orders of magnitude lower than that derived from the 1.3 mm continuum
(\sim2.8 \times 10^(-2) M\odot).
08/2011;
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ABSTRACT: We have observed dense gas around the Very Low-Luminosity Ob jects (VeLLOs) L1521F-IRS and IRAM 04191+1522 in carbon-chain and organic molecular lines with the Nobeyama 45 m telescope. Towards L1521F-IRS, carbon-chain lines of CH3CCH (50-40), C4H (17/2-15/2), and C3H2 (212-101) are 1.5 - 3.5 times stronger than those towards IRAM 04191+1522, and the abundances of the carbon-chain molecules towards L1521F-IRS are 2 to 5 times higher than those towards IRAM 04191+1522. Mapping observations of these carbon-chain molecular lines show that in L1521F the peak positions of these carbon-chain molecular lines are different from each other and there is no emission peak towards the VeLLO position, while in IRAM 04191+1522 these carbon-chain lines are as weak as the detection limits except for the C3H2 line. The observed chemical differentiation between L1521F and IRAM 04191+1522 suggests that the evolutionary stage of L1521F-IRS is younger than that of IRAM 04191+1522, consistent with the extent of the associated outflows seen in the 13CO (1-0) line. The non-detection of the organic molecular lines of CH3OH (6-2-7-1 E) and CH3CN (60-50) implies that the warm (~ 100 K) molecular-desorbing region heated by the central protostar is smaller than ~ 100 AU towards L1521F-IRS and IRAM 04191+1522, suggesting the young age of these VeLLOs. We propose that the chemical status of surrounding dense gas can be used to trace the evolutionary stages of VeLLOs. Comment: 25 pages, 4 figures
12/2010;
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ABSTRACT: FN Tau is a rare example of very low-mass T Tauri stars that exhibits a spatially resolved nebulosity in near-infrared scattering light. To directly derive the parameters of a circumstellar disk around FN Tau, observations of dust continuum emission at 340 GHz are carried out with the Submillimeter Array (SMA). A point-like dust continuum emission was detected with a synthesized beam of ~ 0.7" in FWHM. From the analysis of the visibility plot, the radius of the emission is estimated to be <= 0.29", corresponding to 41 AU. This is much smaller than the radius of the nebulosity, 1.85" for its brighter part at 1.6 micron. The 340 GHz continuum emission observed with the SMA and the photometric data at lambda <= 70 micron are explained by a power-law disk model whose outer radius and mass are 41 AU and (0.24 - 5.9) x 10^{-3} M_{sun}, respectively, if the exponent of dust mass opacity (beta) is assumed to be 0-2. The disk model cannot fully reproduce the flux density at 230 GHz obtained with the IRAM 30-meter telescope, suggesting that there is another extended "halo" component that is missed in the SMA observations. By requiring the halo not to be detected with the SMA, the lower limit to the size of the halo is evaluated to be between 174 AU and 574 AU, depending on the assumed beta value. The physical nature of the halo is unclear, but it may be the periphery of a flared circumstellar disk that is not described well in terms of a power-law disk model, or a remnant of a protostellar envelope having flattened structure. Comment: 21 pages with 5 figures; Accepted for publication in The Astrophysical Journal
02/2010;
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ABSTRACT: We have performed detailed imaging and analyses of SMA observations in 230 GHz continuum, 12CO (2-1), 13CO (2-1), and C18O (2-1) emission toward B335, a Bok globule with an embedded Class 0 source (Lbol ~ 1.5 Lsun). We report the first discover of high-velocity 12CO jets with a size of ~ 900 AU x 1500 AU along the E-W direction in B335. The estimated mass-loss rate (~ 2.3 x 10^-7 Msun yr^-1) and the momentum flux (~ 3.7 x 10^-5 Msun yr^-1 km s^-1) of the 12CO jets in B335 are one order of magnitude lower than those of other 12CO jets in more luminous sources such as HH 211 (Lbol ~ 3.6 Lsun) and HH 212 (Lbol ~ 14 Lsun). The weaker jet activity in B335 could be due to the lower active accretion onto the central protostar. The C18O emission shows a compact (~ 1500 AU) condensation associated with the central protostar, and it likely traces the protostellar envelope around B335. The envelope exhibits a velocity gradient from the east (blueshifted) to west (redshifted) that can be interpreted as an infalling motion. The estimated central stellar mass, the mass infalling rate, and the accretion luminosity are 0.04 Msun, 6.9 x 10^-6 Msun yr^-1, and 2.1 Lsun, respectively. On the other hand, there is no clear velocity gradient perpendicular to the outflow axis in the C18O envelope, suggesting little envelope rotation on a hundred-AU scale. The upper limits of the rotational velocity and specific angular momentum were estimated to be 0.04 km s^-1 and 7.0 x 10^-5 km s^-1 pc at a radius of 370 AU, respectively. The specific angular momentum and the inferred Keplerian radius (~ 6 AU) in B335 are 1 - 2 orders of magnitude smaller than those in other more-evolved sources. Possible scenarios to explain the lower specific angular momentum in B335 are discussed. Comment: 35 pages, 11 figures, and ApJ accepted; http://stacks.iop.org/0004-637X/710/1786
01/2010;
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ABSTRACT: 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. Comment: 16 pages, 2 figures, accepted to AJ
12/2009;
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ABSTRACT: We report interferometric observations of two embedded protostar candidates, IRAS 04169+2702 and IRAS 04365+2535 (TMC-1A), in the Taurus molecular cloud. The C18O J = 1-0 emission from IRAS 04169+2702 reveals a flattened envelope 2200 AU × 1100 AU in size; there is a velocity gradient along the elongation axis, which in turn is perpendicular to the outflow direction. Since the rotational velocity corrected for inclination, 0.23 km s-1 at 370 AU, gives an unacceptably small dynamical mass of 0.02 M☉ within that radius, we speculate that there is additional radial motion, possibly infall, in the flattened envelope. Around IRAS 04365+2535, a compact 13CO J = 1-0 condensation ~1400 AU in size was detected. Again, the velocity gradient is perpendicular to the associated molecular outflow, but a rotation velocity of 0.87 km s-1 at 580 AU radius is consistent with the condensation being a rotationally supported disk. Combining our new data for the two sources with published observations of rotationally supported disks and infalling envelopes around five young stars associated with the Taurus molecular cloud enables us to compare local specific angular momenta of a significant sample of these sources on scales of 200-2000 AU with those of dense cores on 6000-80,000 AU (0.03-0.4 pc) scales. The specific angular momenta for infalling envelopes and rotationally supported disks are relatively constant, ~10-3 km s-1 pc, and are typically an order of magnitude smaller than those for dense cores. These results can be explained if the dynamical collapse of dense star-forming cores takes place inside radii of ~0.03 pc while the region outside this radius remains dynamically stable.
The Astrophysical Journal 01/2009; 488(1):317. · 6.02 Impact Factor
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ABSTRACT: We report new interferometric observations of IRAS 04368+2557 (L1527) in 13CO (J = 1-0), C18O (J = 1-0), and 2.7 mm continuum emission using the Nobeyama Millimeter Array. The continuum map shows a well-defined emission peak with slightly extended features. The extended features are consistent with an 800 μm continuum map. The 13CO map shows blueshifted and redshifted outflowing shells characterized by a bipolar V-shape structure with a wide opening angle toward the east and west of the central source. Near the systemic velocity, a slightly blueshifted X-shaped condensation was detected in 13CO with its peak coincident with the central source. The symmetrical distribution of the X-shaped condensation centered on the central source suggests that it is a circumstellar envelope surrounding the central source. The C18O map shows a flattened structure elongated in the north-south direction, perpendicular to the outflow axis, centered on the central source. This flattened structure correlates spatially with the 13CO X-shaped condensation. Both eastern and western edges of the flattened structure are concave, as the 13CO X-shaped condensation also shows, and they are spatially well anticorrelated with the distribution of the outflowing shells in both blueshifted and redshifted velocities. The flattened structure is hence naturally interpreted as a disklike flattened envelope with an almost edge-on configuration. Its radius and gas mass are estimated to be ~2000 AU and ~0.038 M☉, respectively. The edge-on flattened envelope has both rotational and radial motions with the latter dominant. The large specific angular momentum carried by the envelope gas implies that the radial motion can be infall rather than outflow. The infall and rotation velocities are ~0.3 km s-1 and ~0.05 km s-1, respectively, at the envelope radius of 2000 AU. The flattened envelope is clearly not supported by rotation, but it is dynamically infalling. Its mass infall rate is ~1.1 × 10-6 M☉ yr-1 at 2000 AU in radius. This mass infall rate is consistent with that estimated from the bolometric luminosity of 1.4 L☉ and the mass of 0.1 M☉ of the central star. On the assumption that the mass infall rate is constant with time, the age of the central star is estimated to be ~105 yr, which is comparable to the typical age of protostars in Taurus, even though the central star in L1527 is identified as a very young class 0 source. The rotating motion of the flattened envelope is opposite to the large-scale rotation of the L1527 cloud, suggesting that the rotation of the flattened envelope did not originate from the large-scale rotation.
The Astrophysical Journal 01/2009; 475(1):211. · 6.02 Impact Factor
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ABSTRACT: We report aperture synthesis C18O J = 1-0 observations of L1551 IRS 5 with a spatial resolution of 28 × 25 using the Nobeyama Millimeter Array. We have detected an emission component centrally condensed around IRS 5, as well as a diffuse component extending in the north-south direction from the centrally condensed component. The centrally condensed component, 2380 × 1050 AU in size, is elongated in the direction perpendicular to the outflow axis, indicating the existence of a flattened circumstellar envelope around L1551 IRS 5. The mass of the centrally condensed component is estimated to be 0.062 M☉. The position-velocity (P-V) diagrams reveal that the velocity field in the centrally condensed component is composed of infall and slight rotation. The infall velocity in the outer part is equal to the free-fall velocity around a central mass of ~0.1 M☉, e.g., 0.5 km s-1 at r = 700 AU, whereas the rotation velocity, 0.24 km s-1 at the same radius, gets prominent at inner radii with a radial dependence of r-1. We make up P-V diagrams for the model envelopes with vertical structure, in which the matter falls under the gravity and eventually settles down in Keplerian rotation inside the centrifugal radius, and compare them with the observed P-V diagrams of the centrally condensed component. The main characteristics of the observed P-V diagrams are reproduced by either (1) an envelope with a moderately flattened density distribution, or (2) a spherical envelope with a bipolar cavity whose half-opening angle is about 50°. Detailed comparison of the observed and model P-V diagrams suggests that the C18O J = 1-0 emission from the outer part of the centrally condensed component is well reproduced with the models with the central mass ~0.15 M☉ and the mass infall rate ~6 × 10-6 M☉ yr-1. However, the higher velocity features of the emission near the star cannot be reproduced unless the central mass is taken to be ~0.5 M☉. These facts suggest either that the gas pressure and/or magnetic force dilute the effect of the gravity in the outer part of the envelope, or that the velocity structure inside the centrifugal radius deviates significantly from the Keplerian rotation.
The Astrophysical Journal 01/2009; 504(1):314. · 6.02 Impact Factor
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ABSTRACT: The circumstellar dust disk of the Herbig Ae star AB Aur has been found to exhibit complex spiral-like structures in the near-IR image obtained with the Subaru Telescope. We present maps of the disk in both 12CO (3-2) and dust continuum at 345 GHz with the Submillimeter Array at an angular resolution of 10 × 07 (144 × 100 AU). The continuum emission traces a dust disk with a central depression and a maximum overall dimension of 450 AU (FWHM). This dust disk exhibits several distinct peaks that appear to coincide with bright features in the near-IR image, in particular the brightest inner spiral arm. The CO emission traces a rotating gas disk of size 530 × 330 AU with a deprojected maximum velocity of 2.8 km s-1 at 450 AU. In contrast with the dust disk, the gas disk exhibits an intensity peak at the stellar position. Furthermore, the CO emission in several velocity channels traces the innermost spiral arm seen in the near-IR. We compare the observed spatial-kinematic structure of the CO emission to a simple model of a disk in Keplerian rotation and find that only the emission tracing the main spiral arm clearly lies outside the confines of our model. This emission has a net outward radial motion compared with the radial velocity predicted by the model at the location of the main spiral arms. The disk of AB Aur is therefore quite different from the Keplerian disks seen around many Herbig Ae stars. The spiral-like structures of the disk with non-Keplerian motions we revealed in 12CO (3-2), together with the central depression of the dust disk, could be explained to be driven by the possible existence of a giant planet forming in the disk.
The Astrophysical Journal 12/2008; 645(2):1297. · 6.02 Impact Factor
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ABSTRACT: We report a dual-band observation at 223 and 654 GHz (460 μm) toward an ultracompact (UC) H II region, G240.31+0.07, using the Submillimeter Array. With a beam size of 1.5'' × 0.8'', the dust continuum emission is resolved into two clumps, with clump A well coincident with an H2O maser and the UC H II region. The newly discovered clump, B, about 1.3'' (8.3 × 103 AU) to the southwest of clump A, is also associated with H2O masers and may be a more recent star-forming site. The continuum flux densities imply an opacity spectral index of β = 1.5 ± 0.3, suggestive of a value lower than the canonical 2.0 found in the interstellar medium and in cold, massive cores. The presence of hot (100 K) molecular gas is derived by the brightness ratio of two H2CO lines in the 223 GHz band. A radial velocity difference of 2.5 ± 0.4 km s-1 is found between the two clumps in C18O(6-5) emission. The total (nebular and stellar) mass of roughly 58 M☉ in the central region is close to, but not much larger than, the minimum mass required for the two clumps to be gravitationally bound for binary rotation. Our continuum data do not suggest a large amount of matter associated with the H2 knots that were previously proposed to arise from a massive disk or envelope.
The Astrophysical Journal 12/2008; 654(1):L87. · 6.02 Impact Factor
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Jes K. Jørgensen,
Tyler L. Bourke,
Philip C. Myers,
James Di Francesco,
Ewine F. van Dishoeck,
Chin-Fei Lee, Nagayoshi Ohashi,
Fredrik L. Schöier,
Shigehisa Takakuwa,
David J. Wilner,
and Qizhou Zhang
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ABSTRACT: This paper presents a large spectral line and continuum survey of eight deeply embedded, low-mass protostellar cores using the SMA. High-excitation line emission from 11 molecular species originating in warm and dense gas has been imaged at high angular resolution (1''-3'', typically 200-600 AU) together with continuum emission at 230 GHz (1.3 mm) and 345 GHz (0.8 mm). Compact continuum emission is observed for all sources, which likely originates in marginally optically thick circumstellar disks, with typical lower limits to their masses of 0.1 M☉ (1%-10% of the masses of their envelopes) and a dust opacity law, κν νβ, with β ≈ 1. Prominent collimated outflows are present in CO 2-1 observations in all sources. The most diffuse outflows are found in the sources with the lowest ratios of disk to envelope mass, and it is suggested that these sources are in a phase where accretion of matter from the envelope has almost finished and the remainder of the envelope material is being dispersed by the outflows. Other characteristic dynamical signatures are inverse P Cygni profiles indicative of infalling motions seen in the 13CO 2-1 lines toward NGC 1333 IRAS 4A and NGC 1333 IRAS 4B. Outflow-induced shocks are present on all scales in the protostellar environments and are most clearly traced by the emission of CH3OH in NGC 1333 IRAS 4A and NGC 1333 IRAS 4B. These observations suggest that the emission of CH3OH and H2CO from these proposed ``hot corinos'' is related to the shocks caused by the protostellar outflows. Only one source, NGC 1333 IRAS 2A, has evidence for hot, compact CH3OH emission coincident with the embedded protostar.
The Astrophysical Journal 12/2008; 659(1):479. · 6.02 Impact Factor
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ABSTRACT: We have imaged the circumstellar envelope around the binary protostar L1551 IRS 5 in CS J = 7-6 and 343 GHz continuum emission at ~3'' resolution using the Submillimeter Array. The continuum emission shows an elongated structure (~220 × 100 AU) around the binary perpendicular to the axis of the associated radio jet. The CS emission extends over ~400 AU, appears approximately circularly symmetric, and shows a velocity gradient from southeast (blueshifted) to northwest (redshifted). The direction of the velocity gradient is different from that observed in C18O J = 1-0. This may be because rotation is more dominant in the CS envelope than the C18O envelope, in which both infall and rotation exist. The CS emission can be divided into two velocity components: (1) a "high"-velocity disklike structure surrounding the protostar ±1.0-1.5 km s-1 from the systemic velocity, and (2) a "low"-velocity structure located southwest of the protostar less than 1.0 km s-1 from the systemic velocity. The high-velocity component traces warm and dense gas with kinematics consistent with rotation around the protostar. The low-velocity component may arise from dense gas entrained in the outflow. Alternatively, this component may trace infalling and rotating gas in an envelope with a vertical structure.
The Astrophysical Journal 12/2008; 616(1):L15. · 6.02 Impact Factor
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Chunhua Qi,
Paul T. P. Ho,
David J. Wilner,
Shigehisa Takakuwa,
Naomi Hirano, Nagayoshi Ohashi,
Tyler L. Bourke,
Qizhou Zhang,
Geoffrey A. Blake,
Michiel Hogerheijde,
Masao Saito,
Minho Choi,
and Ji Yang
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ABSTRACT: We present ~2''-4'' aperture synthesis observations of the circumstellar disk surrounding the nearby young star TW Hya in the CO J = 2-1 and J = 3-2 lines and associated dust continuum obtained with the partially completed Submillimeter Array. The extent and peak flux of the 230 and 345 GHz dust emission follow closely the predictions of the irradiated accretion disk model of Calvet et al. The resolved molecular line emission extends to a radius of at least 200 AU, the full extent of the disk visible in scattered light, and shows a clear pattern of Keplerian rotation. Comparison of the images with two-dimensional Monte Carlo models constrains the disk inclination angle to 7° ± 1°. The CO emission is optically thick in both lines, and the kinetic temperature in the line formation region is ~20 K. Substantial CO depletion, by an order of magnitude or more from canonical dark cloud values, is required to explain the characteristics of the line emission.
The Astrophysical Journal 12/2008; 616(1):L11. · 6.02 Impact Factor
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Yi-Jehng Kuan,
Hui-Chun Huang,
Steven B. Charnley,
Naomi Hirano,
Shigehisa Takakuwa,
David J. Wilner,
Sheng-Yuan Liu, Nagayoshi Ohashi,
Tyler L. Bourke,
Chunhua Qi,
and Qizhou Zhang
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ABSTRACT: Arcsecond-resolution spectral observations toward the protobinary system IRAS 16293-2422 at 344 and 354 GHz were conducted using the Submillimeter Array. Several complex organic molecules, such as CH3OH and HCOOCH3, were detected and mapped. Together with the rich organic inventory revealed, it clearly indicates the existence of two, rather than one, compact hot molecular cores (400 AU in radius) associated with each of the protobinary components identified by their dust continuum emission in the inner star-forming core.
The Astrophysical Journal 12/2008; 616(1):L27. · 6.02 Impact Factor
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ABSTRACT: We have mapped the protobinary source IRAS 16293–2422 in CO 2-1,13CO 2-1, and CO 3-2 with the Submillimeter Array (SMA). The maps with resolution of 1.5''-5'' reveal a single small-scale (~3000 AU) bipolar molecular outflow along the east-west direction. We found that the blueshifted emission of this small-scale outflow mainly extends to the east and the redshifted emission to the west from the position of IRAS 16293A. A comparison with the morphology of the large-scale outflows previously observed by single-dish telescopes at millimeter wavelengths suggests that the small-scale outflow may be the inner part of the large-scale (~15,000 AU) east-west outflow. On the other hand, there is no clear counterpart of the large-scale northeast-southwest outflow in our SMA maps. Comparing analytical models to the data suggests that the morphology and kinematics of the small-scale outflow can be explained by a wide-angle wind with an inclination angle of ~30°-40° with respect to the plane of the sky. The high-resolution CO maps show that there are two compact, bright spots in the blueshifted velocity range. An LVG analysis shows that the one located 1'' to the east of source A is extremely dense, n(H2) ~107 cm−3, and warm, Tkin > 55 K. The other one located 1'' southeast of source B has a higher temperature of Tkin > 65 K but slightly lower density of n(H2) ~106 cm−3. It is likely that these bright spots are associated with the hot core-like emission observed toward IRAS 16293. Since both bright spots are blueshifted from the systemic velocity and are offset from the protostellar positions, they are likely formed by shocks.
The Astrophysical Journal 12/2008; 675(1):454. · 6.02 Impact Factor
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ABSTRACT: We have investigated the evolution and distribution of molecules in collapsing prestellar cores via numerical chemical models, adopting the Larson-Penston solution and its delayed analogs to study collapse. Molecular abundances and distributions in a collapsing core are determined by the balance among the dynamical, chemical, and adsorption timescales. When the central density nH of a prestellar core with the Larson-Penston flow rises to 3 × 106 cm-3, the CCS and CO column densities are calculated to show central holes of radius 7000 and 4000 AU, respectively, while the column density of N2H+ is centrally peaked. These predictions are consistent with observations of L1544. If the dynamical timescale of the core is larger than that of the Larson-Penston solution owing to magnetic fields, rotation, or turbulence, the column densities of CO and CCS are smaller, and their holes are larger than in the Larson-Penston core with the same central gas density. On the other hand, N2H+ and NH3 are more abundant in the more slowly collapsing core. Therefore, molecular distributions can probe the collapse timescale of prestellar cores. Deuterium fractionation has also been studied via numerical calculations. The deuterium fraction in molecules increases as a core evolves and molecular depletion onto grains proceeds. When the central density of the core is nH = 3 × 106 cm-3, the ratio DCO+/HCO+ at the center is in the range 0.06-0.27, depending on the collapse timescale and adsorption energy; this range is in reasonable agreement with the observed value in L1544.
The Astrophysical Journal 12/2008; 552(2):639. · 6.02 Impact Factor
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ABSTRACT: Studying the earliest stages in the birth of stars is crucial for understanding how they form. Brown dwarfs with masses between that of stars and planets are not massive enough to maintain stable hydrogen-burning fusion reactions during most of their lifetime. Their origins are subject to much debate in recent literature because their masses are far below the typical mass where core collapse is expected to occur. We present the first confirmed evidence that brown dwarfs undergo a phase of molecular outflow that is typical of young stars. Using the Submillimeter Array, we have obtained a map of a bipolar molecular outflow from a young brown dwarf. We estimate an outflow mass of 1.6 × 10−4 M☉ and a mass-loss rate of 1.4 × 10−9 M☉. These values are over 2 orders of magnitude smaller than the typical ones for T Tauri stars. From our millimeter continuum data and our own analysis of Spitzer infrared photometry, we estimate that the brown dwarf has a disk with a mass of 8 × 10−3 M☉ and an outer disk radius of 80 AU. Our results demonstrate that the bipolar molecular outflow operates down to planetary masses, occurring in brown dwarfs as a scaled-down version of the universal process seen in young stars.
The Astrophysical Journal 12/2008; 689(2):L141. · 6.02 Impact Factor
