2,228 research outputs found
Spitzer spectroscopy of circumstellar disks in the 5 Myr old upper Scorpius OB association
We present mid-infrared spectra between 5.2 and 38 ÎŒm for 26 disk-bearing members of the ~5 Myr old Upper Scorpius OB association obtained with the Infrared Spectrograph (IRS) onboard the Spitzer Space Telescope. We find clear evidence for changes in the spectral characteristics of dust emission between the early-type (B+A) and late-type (K+M) infrared excess stars. The early-type members exhibit featureless continuum excesses that become apparent redward of ~8 ÎŒm. In contrast, 10 and 20 ÎŒm silicate features or polycyclic aromatic hydrocarbon emission are present in all but one of the late-type excess members of Upper Scorpius. The strength of silicate emission among late-type Upper Scorpius members is spectral-type dependent, with the most prominent features being associated with K5-M2-type stars. By fitting the spectral energy distributions (SED) of a representative sample of low-mass stars with accretion disk models, we find that the SEDs are consistent with models having inner disk radii ranging from ~0.2 to 1.2 AU. Complementary high-resolution (R ~ 33,000) optical (λλ4800-9200) spectra for the Upper Scorpius excess stars were examined for signatures of gaseous accretion. Of the 35 infrared excess stars identified in Upper Scorpius, only seven (all late-type) exhibit definitive signatures of accretion. Mass-accretion rates (M) for these stars were estimated to range from 10^â11 to 10^â8.9 Mâ yr^â1. Compared to Class II sources in Taurus-Auriga, the disk population in Upper Scorpius exhibits reduced levels of near- and mid-infrared excess emission and an order of magnitude lower mass-accretion rates. These results suggest that the disk structure has changed significantly over the 2-4 Myr in age separating these two stellar populations. The ubiquity of depleted inner disks in the Upper Scorpius excess sample implies that such disks are a common evolutionary pathway that persists for some time
An ALMA Continuum Survey of Circumstellar Disks in the Upper Scorpius OB Association
We present ALMA 880 micron continuum observations of 20 K and M-type stars in
the Upper Scorpius OB association that are surrounded by protoplanetary disks.
These data are used to measure the dust content in disks around low mass stars
(0.1-1.6 Msun) at a stellar age of 5-11 Myr. Thirteen sources were detected in
the 880 micron dust continuum at >3 sigma with inferred dust masses between 0.3
and 52 Mearth. The dust masses tend to be higher around the more massive stars,
but the significance is marginal in that the probability of no correlation is p
~ 0.03. The evolution in the dust content in disks was assessed by comparing
the Upper Sco observations with published continuum measurements of disks
around ~ 1-2 Myr stars in the Class II stage in the Taurus molecular cloud.
While the dust masses in the Upper Sco disks are on average lower than in
Taurus, any difference in the dust mass distributions is significant at less
than 3sigma. For stellar masses between 0.49 Msun and 1.6 Msun, the mean dust
mass in disks is lower in Upper Sco relative to Taurus by Delta log Mdust =
0.44 +/-0.26.Comment: Accepted by Ap
Near-Infrared Photometric Variability of Stars Toward the Orion A Molecular Cloud
We present an analysis of J, H, and K time series photometry obtained with
the southern 2MASS telescope over a 0.84 x 6 deg^2 region centered near the
Orion Nebula Cluster. These data are used to establish the near-infrared
variability properties of pre-main-sequence stars in Orion on time scales of
1-36 days, 2 months, and 2 years. A total of 1235 variable stars are
identified, ~93% of which are associated with the Orion A molecular cloud. The
variable stars exhibit a diversity of photometric behavior with time, including
cyclic fluctuations, aperiodic day-to-day fluctuations, eclipses, slow drifts
in brightness over one month, colorless variability, stars that become redder
as they fade, and stars that become bluer as they fade. We examine rotational
modulation of cool and hot star spots, variable obscuration from an inner
circumstellar disk, and changes in the mass accretion rate and other properties
in a circumstellar disk as possible origins of the variability. Cool spots can
explain the variability characteristics in 56-77% of the stars, while the
properties of the photometric fluctuations are more consistent with hot spots
or extinction changes in at least 23% of the stars, and with variations in the
disk mass accretion rate or inner disk radius in 1% of our sample. However,
differences between the details of the observations and the details of
variability predicted these models suggest either that another variability
mechanism not considered here may be operative, or that the observed
variability represents the net results of several of these phenomena. Analysis
of the star count data indicates that the ONC is part of a larger area of
enhanced stellar surface density which extends over a 0.4 x 2.4 deg^2 (3.4 x 20
pc^2) region containing 2700 stars brighter than K=14. (abridged version)Comment: 75 pages with 27 figures; to appear in AJ; see also
http://www.astro.caltech.edu/~jmc/variables/orio
Periodic Photometric Variability in the Becklin-Neugebauer Object
The Becklin-Neugebauer (BN) object in the Orion Nebula Cluster (ONC) is a
well-studied optically invisible, infrared-bright young stellar object, thought
to be an intermediate-mass protostar. We report here that BN exhibited
nearly-sinusoidal periodic variability at the near-infrared H- and Ks-bands
during a one month observing campaign in 2000 March/April. The period was 8.28
days and the peak-to-peak amplitude ~0.2 mag. Plausible mechanisms for
producing the observed variability characteristics are explored.Comment: Accepted by ApJ Letter
Recent Results from CARMA
The Combined Array for Research in Millimeter-wave Astronomy (CARMA) is a heterogeneous array of 23 telescopes designed to operate in the 1 cm, 3 mm, and 1 mm atmospheric windows. The array is a merger of the eight 3.5 m antennas from the Sunyaev-Zelâdovich Array, the nine 6.1 m antennas from the Berkeley-Illinois-Maryland array, and the six 10.4 m antennas from the Owens Valley Radio Observatory (OVRO). As a signature of Tom Phillipsâ legacy, three of the CARMA antennas are from the original 3-element OVRO interferometer built under Tomâs leadership in the early 1980âs. Recent CARMA results are presented on the structure of circumstellar disks and the molecular gas distribution in M 51
OrientationâAveraged Amplitude of the OneâQuantum Term in the Neutron Scattering Law for Molecular Gases
The amplitude of the oneâquantum term in the scattering law for coherent scattering of neutrons by molecular gases is calculated using a generatingâfunction approach. The phase factor exp (iÎșâ
b) is averaged jointly with the amplitude factor (Îșâ
ÎłÎœ) (Îșâ
ÎłÎœâČ), as opposed to the more common technique of approximating the joint average by the product of the average of each factor. Results of example calculations show that the correction is of the same magnitude as the result of the conventional calculation.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69801/2/JCPSA6-46-2-465-1.pd
Large-Scale Asymmetries in the Transitional Disks of SAO 206462 and SR 21
We present Atacama Large Millimeter/submillimeter Array (ALMA) observations
in the dust continuum (690 GHz, 0.45 mm) and 12CO J=6-5 spectral line emission,
of the transitional disks surrounding the stars SAO 206462 and SR 21. These
ALMA observations resolve the dust-depleted disk cavities and extended gaseous
disks, revealing large-scale asymmetries in the dust emission of both disks. We
modeled these disks structures with a ring and an azimuthal gaussian, where the
azimuthal gaussian is motivated by the steady-state vortex solution from Lyra &
Lin (2013). Compared to recent observations of HD 142527, Oph IRS 48, and LkHa
330, these are low-contrast (< 2) asymmetries. Nevertheless, a ring alone is
not a good fit, and the addition of a vortex prescription describes these data
much better. The asymmetric component encompasses 15% and 28% of the total disk
emission in SAO 206462 and SR 21 respectively, which corresponds to a lower
limit of 2 MJup of material within the asymmetry for both disks. Although the
contrast in the dust asymmetry is low, we find that the turbulent velocity
inside it must be large (~20% of the sound speed) in order to drive these
azimuthally wide and radially narrow vortex-like structures. We obtain
residuals from the ring and vortex fitting that are still significant, tracing
non-axisymmetric emission in both disks. We compared these submillimeter
observations with recently published H-band scattered light observations. For
SR 21 the scattered light emission is distributed quite differently from the
submillimeter continuum emission, while for SAO 206462 the submillimeter
residuals are suggestive of spiral-like structure similar to the near-IR
emission.Comment: ApJL, in press. 6 pages, 3 figures, 2 table
ALMA Observations of Circumstellar Disks in the Upper Scorpius OB Association
We present ALMA observations of 106 G-, K-, and M-type stars in the Upper
Scorpius OB Association hosting circumstellar disks. With these data, we
measure the 0.88 mm continuum and CO = 32 line fluxes of disks
around low-mass ( ) stars at an age of 5-11 Myr. Of the
75 primordial disks in the sample, 53 are detected in the dust continuum and 26
in CO. Of the 31 disks classified as debris/evolved transitional disks, 5 are
detected in the continuum and none in CO. The lack of CO emission in
approximately half of the disks with detected continuum emission can be
explained if CO is optically thick but has a compact emitting area ( au), or if the CO is heavily depleted by a factor of at least
relative to interstellar medium abundances and is optically thin. The continuum
measurements are used to estimate the dust mass of the disks. We find a
correlation between disk dust mass and stellar host mass consistent with a
power-law relation of . Disk dust
masses in Upper Sco are compared to those measured in the younger Taurus
star-forming region to constrain the evolution of disk dust mass. We find that
the difference in the mean of between Taurus and
Upper Sco is , such that is lower in Upper
Sco by a factor of .Comment: 18 pages of text, 11 Figures, 5 Tables. Published in Ap
Near-Infrared Photometric Variability of Stars Toward the Chamaeleon I Molecular Cloud
We present the results of a J, H, and K_s photometric monitoring campaign of
a 0.72 x 6 sq deg. area centered on the Chamaeleon I star forming region. Data
were obtained on 15 separate nights over a 4 month time interval using the
2MASS South telescope. Out of a total of 34,539 sources brighter than the
photometric completeness limits (J=16.0, H=15.2, K_s=14.8), 95 exhibit
near-infrared variability in one or more bands. The variables can be grouped
into a population of bright, red objects that are associated with the
Chamaeleon I association, and a population of faint, blue variables that are
dispersed over the full 6 deg of the survey and are likely field stars or older
pre-main-sequence stars unrelated to the present-day Chamaeleon I molecular
cloud. Ten new candidate members of Chamaeleon I, including 8 brown dwarf
candidates, have been identified based on variability and/or near-infrared
excess emission in the J-H vs. H-K_s color-color-diagram. We also provide a
compendium of astrometry and J, H, and K_s photometry for previously identified
members and candidate members of Chamaeleon I.Comment: To appear in AJ; see
http://www.astro.caltech.edu/~jmc/variables/cham1
Millimeter Imaging of MWC 758: Probing the Disk Structure and Kinematics
We investigate the structure and kinematics of the circumstellar disk around the Herbig Ae star MWC 758 using high-resolution observations of the ^(12)CO (3-2) and dust continuum emission at the wavelengths of 0.87 and 3.3 mm. We find that the dust emission peaks at an orbital radius of about 100 AU, while the CO intensity has a central peak coincident with the position of the star. The CO emission is in agreement with a disk in Keplerian rotation around a 2.0 M_â star, confirming that MWC 758 is indeed an intermediate-mass star. By comparing the observation with theoretical disk models, we derive that the disk surface density ÎŁ(r) steeply increases from 40 to 100 AU and decreases exponentially outward. Within 40 AU, the disk has to be optically thin in the continuum emission at millimeter wavelengths to explain the observed dust morphology, though our observations lack the angular resolution and sensitivity required to constrain the surface density on these spatial scales. The surface density distribution in MWC 758 disk is similar to that of "transition" disks, though no disk clearing has been previously inferred from the analysis of the spectral energy distribution (SED). Moreover, the asymmetries observed in the dust and CO emission suggest that the disk may be gravitationally perturbed by a low-mass companion orbiting within a radius of 30 AU. Our results emphasize that SEDs alone do not provide a complete picture of disk structure and that high-resolution millimeter-wave images are essential to reveal the structure of the cool disk mid-plane
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