215 research outputs found
Modeling Seven Years of Event Horizon Telescope Observations with Radiatively Inefficient Accretion Flow Models
An initial three-station version of the Event Horizon Telescope, a
millimeter-wavelength very-long baseline interferometer, has observed
Sagittarius A* (Sgr A*) repeatedly from 2007 to 2013, resulting in the
measurement of a variety of interferometric quantities. Of particular
importance, there is now a large set of closure phases, measured over a number
of independent observing epochs. We analyze these observations within the
context of a realization of semi-analytic radiatively inefficient disk models,
implicated by the low luminosity of Sgr A*. We find a broad consistency among
the various observing epochs and between different interferometric data types,
with the latter providing significant support for this class of models of Sgr
A*. The new data significantly tighten existing constraints on the spin
magnitude and its orientation within this model context, finding a spin
magnitude of , an inclination with respect to
the line of sight of
, and a position
angle of east of
north. These are in good agreement with previous analyses. Notably, the
previous degeneracy in the position angle has now been conclusively
broken by the inclusion of the closure phase measurements. A reflection
degeneracy in the inclination remains, permitting two localizations of the spin
vector orientation, one of which is in agreement with the orbital angular
momentum of the infrared gas cloud G2 and the clockwise disk of young stars.
This possibly supports a relationship between Sgr A*'s accretion flow and these
larger-scale features.Comment: 16 pages, 11 figures, accepted to Ap
A Combined Spitzer and Herschel Infrared Study of Gas and Dust in the Circumbinary Disk Orbiting V4046 Sgr
We present results from a spectroscopic Spitzer and Herschel
mid-to-far-infrared study of the circumbinary disk orbiting the evolved (age
~12-23 Myr) close binary T Tauri system V4046 Sgr. Spitzer IRS spectra show
emission lines of [Ne II], H_2 S(1), CO_2 and HCN, while Herschel PACS and
SPIRE spectra reveal emission from [O I], OH, and tentative detections of H_2O
and high-J transitions of CO. We measure [Ne III]/[Ne II] < 0.13, which is
comparable to other X-ray/EUV luminous T Tauri stars that lack jets. We use the
H_2 S(1) line luminosity to estimate the gas mass in the relatively warm
surface layers of the inner disk. The presence of [O I] emission suggests that
CO, H_2O, and/or OH is being photodissociated, and the lack of [C I] emission
suggests any excess C may be locked up in HCN, CN and other organic molecules.
Modeling of silicate dust grain emission features in the mid-infrared indicates
that the inner disk is composed mainly of large (r~5 um) amorphous pyroxene and
olivine grains (~86% by mass) with a relatively large proportion of crystalline
silicates. These results are consistent with other lines of evidence indicating
that planet building is ongoing in regions of the disk within ~30 AU of the
central, close binary.Comment: 33 pages, 9 figures, 3 tables. Accepted for publication in Ap
Detecting weak spectral lines in interferometric data through matched filtering
Funding: R.A.L. and J.H. gratefully acknowledge funding from National Science Foundation Graduate Research Fellowships (Grant No. DGE-1144152). R.A.L. also acknowledges funding from the NRAO Student Observing Support Program. K.I.Γ. acknowledges funding from the Alfred P. Sloan Foundation and the David and Lucile Packard Foundation. C.W. acknowledges financial support from the Netherlands Organisation for Scientific Research (NWO, grant 639.041.335) and start-up funds from the University of Leeds, UK.Modern radio interferometers enable observations of spectral lines with unprecedented spatial resolution and sensitivity. In spite of these technical advances, many lines of interest are still at best weakly detected and therefore necessitate detection and analysis techniques specialized for the low signal-to-noise ratio (S/N) regime. Matched filters can leverage knowledge of the source structure and kinematics to increase sensitivity of spectral line observations. Application of the filter in the native Fourier domain improves S/N while simultaneously avoiding the computational cost and ambiguities associated with imaging, making matched filtering a fast and robust method for weak spectral line detection. We demonstrate how an approximate matched filter can be constructed from a previously observed line or from a model of the source, and we show how this filter can be used to robustly infer a detection significance for weak spectral lines. When applied to ALMA Cycle 2 observations of CH3OH in the protoplanetary disk around TW Hya, the technique yields a β53% S/N boost over aperture-based spectral extraction methods, and we show that an even higher boost will be achieved for observations at higher spatial resolution. A Python-based open-source implementation of this technique is available under the MIT license at http://github.com/AstroChem/VISIBLE.Publisher PDFPeer reviewe
First Detection of c-C3H2 in a Circumstellar Disk
We report the first detection of c-C3H2 in a circumstellar disk. The c-C3H2 J = 6-5 line (217.882 GHz) is detected and imaged through Atacama Large Millimeter Array (ALMA) Science Verification observations toward the disk around the Herbig Ae star HD 163296 at 0.''8 resolution. The emission is consistent with that arising from a Keplerian rotating disk. Two additional c-C3H2 transitions are also tentatively detected, bolstering the identification of this species, but with insufficient signal-to-noise ratio to constrain the spatial distribution. Using a previously developed model for the physical structure of this disk, we fit a radial power-law distribution model to the c-C3H2 6-5 emission and find that c-C3H2 is present in a ring structure from an inner radius of about 30 AU to an outer radius of about 165 AU. The column density is estimated to be 1012-1013 cmβ2. The clear detection and intriguing ring structure suggest that c-C3H2 has the potential to become a useful probe of radiation penetration in disks.Astronom
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