211 research outputs found
Direct evidence of dust growth in L183 from mid-infrared light scattering
Context. Theoretical arguments suggest that dust grains should grow in the dense cold parts of molecular clouds. Evidence of larger grains has so far been gathered in near/mid infrared extinction and millimeter observations. Interpreting the data is, however, aggravated by the complex interplay of density and dust properties (as well as temperature for thermal emission).
Aims. Direct evidence of larger particles can be derived from scattered mid-infrared (MIR) radiation from a molecular cloud observed in a spectral range where little or no emission from polycyclic aromatic hydrocarbons (PAHs) is expected.
Methods. We present new Spitzer data of L183 in bands that are sensitive and insensitive to PAHs. The visual extinction AV map derived in a former paper was fitted by a series of 3D Gaussian distributions. For different dust models, we calculate the scattered MIR radiation images of structures that agree with the AV map and compare them to the Spitzer data.
Results. The Spitzer data of L183 show emission in the 3.6 and 4.5 μm bands, while the 5.8 μm band shows slight absorption. The emission layer of stochastically heated particles should coincide with the layer of strongest scattering of optical interstellar radiation, which is seen as an outer surface on I band images different from the emission region seen in the Spitzer images. Moreover, PAH emission is expected to strongly increase from 4.5 to 5.8 μm, which is not seen. Hence, we interpret this emission to be MIR scattered light from grains located further inside the core, and call it ”coreshine”. Scattered light modeling when assuming interstellar medium dust grains without growth does not reproduce flux measurable by Spitzer. In contrast, models with grains growing with density yield images with a flux and pattern comparable to the Spitzer images in the bands 3.6, 4.5, and 8.0 μm.
Conclusions. There is direct evidence of dust grain growth in the inner part of L183 from the scattered light MIR images seen by Spitzer
The large-scale disk fraction of brown dwarfs in the Taurus cloud as measured with Spitzer
Aims. The brown dwarf (BD) formation process has not yet been completely
understood. To shed more light on the differences and similarities between star
and BD formation processes, we study and compare the disk fraction among both
kinds of objects over a large angular region in the Taurus cloud. In addition,
we examine the spatial distribution of stars and BD relative to the underlying
molecular gas Methods. In this paper, we present new and updated photometry
data from the Infrared Array Camera (IRAC) aboard the Spitzer Space Telescope
on 43 BDs in the Taurus cloud, and recalculate of the BD disk fraction in this
region. We also useed recently available CO mm data to study the spatial
distribution of stars and BDs relative to the cloud's molecular gas. Results.
We find that the disk fraction among BDs in the Taurus cloud is 41 \pm 12%, a
value statistically consistent with the one among TTS (58 \pm 9%). We find that
BDs in transition from a state where they have a disk to a diskless state are
rare, and we study one isolated example of a transitional disk with an inner
radius of \approx 0.1 AU (CFHT BD Tau 12, found via its relatively small mid-IR
excess compared to most members of Taurus that have disks. We find that BDs are
statistically found in regions of similar molecular gas surface density to
those associated with stars. Furthermore, we find that the gas column density
distribution is almost identical for stellar and substellar objects with and
without disks.Comment: 8 page, 6 figures, Accepted in Astronomy & Astrophysics
On the circum(sub)stellar environment of brown dwarfs in Taurus
Aims : We want to investigate whether brown dwarfs (BDs) form like stars or
are ejected embryos. We study the presence of disks around BDs in the Taurus
cloud, and discuss implications for substellar formation models. Methods : We
use photometric measurements from the visible to the far infrared to determine
the spectral energy distributions (SEDs) of Taurus BDs. Results: We use Spitzer
color indices, Halpha as an accretion indicator, and models fit to the SEDs in
order to estimate physical parameters of the disks around these BDs. We study
the spatial distribution of BDs with and without disks across the Taurus
aggregates, and we find that BDs with and without disks are not distributed
regularly across the Taurus cloud. Conclusions: We find that 48%+/- 14% of
Taurus BDs have a circumstellar disk signature, a ratio similar to recent
results from previous authors in other regions. We fit the SEDs and find that
none of the disks around BDs in Taurus can be fitted convincingly with a
flaring index beta = 0, indicating that heating by the central object is
efficient and that the disks we observe retain a significant amount of gas. We
find that BDs with disks are proportionally more numerous in the northern
Taurus filament, possibly the youngest filament. We do not find such a clear
segregation for classical T Tauri stars (CTTS) and weak-lined T Tauri stars
(WTTS), suggesting that, in addition to the effects of evolution, any
segregation effects could be related to the mass of the object. A by-product of
our study is to propose a recalibration of the Barrado y Navascues & Martin
(2003) accretion limit in the substellar domain. The global shape of the limit
fits our data points if it is raised by a factor 1.25-1.30.Comment: 11 pages, 5 figures, A&A accepte
Masses of the components of SB2 binaries observed with Gaia. II. Masses derived from PIONIER interferometric observations for Gaia validation
In anticipation of the Gaia astrometric mission, a sample of spectroscopic
binaries is being observed since 2010 with the Sophie spectrograph at the
Haute--Provence Observatory. Our aim is to derive the orbital elements of
double-lined spectroscopic binaries (SB2s) with an accuracy sufficient to
finally obtain the masses of the components with relative errors as small as 1
% when combined with Gaia astrometric measurements. In order to validate the
masses derived from Gaia, interferometric observations are obtained for three
SB2s in our sample with F-K components: HIP 14157, HIP 20601 and HIP 117186.
The masses of the six stellar components are derived. Due to its edge-on
orientation, HIP 14157 is probably an eclipsing binary. We note that almost all
the derived masses are a few percent larger than the expectations from the
standard spectral-type-mass calibration and mass-luminosity relation. Our
calculation also leads to accurate parallaxes for the three binaries, and the
Hipparcos parallaxes are confirmed.Comment: 10 pages, 3 figures, accepted by MNRA
Young Stellar Object Variability (YSOVAR): Long Timescale Variations in the Mid-Infrared
The YSOVAR (Young Stellar Object VARiability) Spitzer Space Telescope
observing program obtained the first extensive mid-infrared (3.6 & 4.5 um)
time-series photometry of the Orion Nebula Cluster plus smaller footprints in
eleven other star-forming cores (AFGL490, NGC1333, MonR2, GGD 12-15, NGC2264,
L1688, Serpens Main, Serpens South, IRAS 20050+2720, IC1396A, and Ceph C).
There are ~29,000 unique objects with light curves in either or both IRAC
channels in the YSOVAR data set. We present the data collection and reduction
for the Spitzer and ancillary data, and define the "standard sample" on which
we calculate statistics, consisting of fast cadence data, with epochs about
twice per day for ~40d. We also define a "standard sample of members",
consisting of all the IR-selected members and X-ray selected members. We
characterize the standard sample in terms of other properties, such as spectral
energy distribution shape. We use three mechanisms to identify variables in the
fast cadence data--the Stetson index, a chi^2 fit to a flat light curve, and
significant periodicity. We also identified variables on the longest timescales
possible of ~6 years, by comparing measurements taken early in the Spitzer
mission with the mean from our YSOVAR campaign. The fraction of members in each
cluster that are variable on these longest timescales is a function of the
ratio of Class I/total members in each cluster, such that clusters with a
higher fraction of Class I objects also have a higher fraction of long-term
variables. For objects with a YSOVAR-determined period and a [3.6]-[8] color,
we find that a star with a longer period is more likely than those with shorter
periods to have an IR excess. We do not find any evidence for variability that
causes [3.6]-[4.5] excesses to appear or vanish within our data; out of members
and field objects combined, at most 0.02% may have transient IR excesses.Comment: Accepted to AJ; 38 figures, 93 page
Spitzer Observations of IC 2118
IC 2118, also known as the Witch Head Nebula, is a wispy, roughly cometary,
~5 degree long reflection nebula, and is thought to be a site of triggered star
formation. In order to search for new young stellar objects (YSOs), we have
observed this region in 7 mid- and far-infrared bands using the Spitzer Space
Telescope and in 4 bands in the optical using the U. S. Naval Observatory
40-inch telescope. We find infrared excesses in 4 of the 6 previously-known T
Tauri stars in our combined infrared maps, and we find 6 entirely new candidate
YSOs, one of which may be an edge-on disk. Most of the YSOs seen in the
infrared are Class II objects, and they are all in the "head" of the nebula,
within the most massive molecular cloud of the region.Comment: Accepted to Ap
The geometry of a vorticity model equation
We provide rigorous evidence of the fact that the modified
Constantin-Lax-Majda equation modeling vortex and quasi-geostrophic dynamics
describes the geodesic flow on the subgroup of orientation-preserving
diffeomorphisms fixing one point, with respect to right-invariant metric
induced by the homogeneous Sobolev norm and show the local existence
of the geodesics in the extended group of diffeomorphisms of Sobolev class
with .Comment: 24 page
Right-invariant Sobolev metrics of fractional order on the diffeomorphism group of the circle
In this paper, we study the geodesic flow of a right-invariant metric induced
by a general Fourier multiplier on the diffeomorphism group of the circle and
on some of its homogeneous spaces. This study covers in particular
right-invariant metrics induced by Sobolev norms of fractional order. We show
that, under a certain condition on the symbol of the inertia operator (which is
satisfied for the fractional Sobolev norm for ), the
corresponding initial value problem is well-posed in the smooth category and
that the Riemannian exponential map is a smooth local diffeomorphism.
Paradigmatic examples of our general setting cover, besides all traditional
Euler equations induced by a local inertia operator, the Constantin-Lax-Majda
equation, and the Euler-Weil-Petersson equation.Comment: 40 pages. Corrected typos and improved redactio
The North American and Pelican Nebulae II. MIPS Observations and Analysis
We present observations of ~7 square degrees of the North American and
Pelican Nebulae region at 24, 70, and 160 microns with the Spitzer Space
Telescope Multiband Imaging Photometer for Spitzer (MIPS). We incorporate the
MIPS observations with earlier Spitzer Infrared Array Camera (IRAC)
observations, as well as archival near-infrared (IR) and optical data. We use
the MIPS data to identify 1286 young stellar object (YSO) candidates. IRAC data
alone can identify 806 more YSO candidates, for a total of 2076 YSO candidates.
Prior to the Spitzer observations, there were only ~200 YSOs known in this
region. Three subregions within the complex are highlighted as clusters: the
Gulf of Mexico, the Pelican, and the Pelican's Hat. The Gulf of Mexico cluster
is subject to the highest extinction (Av at least ~30) and has the widest range
of infrared colors of the three clusters, including the largest excesses and by
far the most point-source detections at 70 microns. Just 3% of the cluster
members were previously identified; we have redefined this cluster as about
10-100 times larger (in projected area) than was previously realized.Comment: Accepted to ApJS. Full data tables available at
http://web.ipac.caltech.edu/staff/rebull/research.htm
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