158 research outputs found
A homogeneous analysis of disks around brown dwarfs
We re-analyzed the Herschel/PACS data of a sample of 55 brown dwarfs (BDs)
and very low mass stars with spectral types ranging from M5.5 to L0. We
investigated the dependence of disk structure on the mass of the central object
in the substellar regime based on a homogeneous analysis of Herschel data from
flux density measurements to spectral energy distribution (SED) modeling. A
systematic comparison between the derived disk properties and those of sun-like
stars shows that the disk flaring of BDs and very low mass stars is generally
smaller than that of their higher mass counterparts, the disk mass is orders of
magnitude lower than the typical value found in T Tauri stars, and the disk
scale heights are comparable in both sun-like stars and BDs. We further divided
our sample into an early-type brown dwarf (ETBD) group and a late-type brown
dwarf (LTBD) group by using spectral type (=M8) as the border criterion. We
systematically compared the modeling results from Bayesian analysis between
these two groups, and found the trends of flaring index as a function of
spectral type also present in the substellar regime. The spectral type
independence of the scale height is also seen between high-mass and very
low-mass BDs. However, both the ETBD and LTBD groups feature a similar median
disk mass of 10^{-5}Msun and no clear trend is visible in the distribution,
probably due to the uncertainty in translating the far-IR photometry into disk
mass, the detection bias and the age difference among the sample. Unlike
previous studies, our analysis is completely homogeneous in Herschel/PACS data
reduction and modeling with a statistically significant sample. Therefore, we
present evidence of stellar-mass-dependent disk structure down to the
substellar mass regime, which is important for planet formation models.
(Abridged Version)Comment: Accepted for publication in A&
Herschel-PACS photometry of faint stars
Our aims are to determine flux densities and their photometric accuracy for a
set of seventeen stars that range in flux from intermediately bright (<2.5 Jy)
to faint (>5 mJy) in the far-infrared (FIR). We also aim to derive
signal-to-noise dependence with flux and time, and compare the results with
predictions from the Herschel exposure-time calculation tool. The PACS faint
star sample has allowed a comprehensive sensitivity assessment of the PACS
photometer. Accurate photometry allows us to establish a set of five FIR
primary standard candidates, namely alpha Ari, epsilon Lep, omega,Cap, HD41047
and 42Dra, which are 2 -- 20 times fainter than the faintest PACS fiducial
standard (gamma Dra) with absolute accuracy of <6%. For three of these primary
standard candidates, essential stellar parameters are known, meaning that a
dedicated flux model code may be run.Comment: 42 pages, 12 figure
Far-infrared photometric observations of the outer planets and satellites with Herschel-PACS
We present all Herschel PACS photometer observations of Mars, Saturn, Uranus,
Neptune, Callisto, Ganymede, and Titan. All measurements were carefully
inspected for quality problems, were reduced in a (semi-)standard way, and were
calibrated. The derived flux densities are tied to the standard PACS photometer
response calibration, which is based on repeated measurements of five fiducial
stars. The overall absolute flux uncertainty is dominated by the estimated 5%
model uncertainty of the stellar models in the PACS wavelength range between 60
and 210 micron. A comparison with the corresponding planet and satellite models
shows excellent agreement for Uranus, Neptune, and Titan, well within the
specified 5%. Callisto is brighter than our model predictions by about 4-8%,
Ganymede by about 14-21%. We discuss possible reasons for the model offsets.
The measurements of these very bright point-like sources, together with
observations of stars and asteroids, show the high reliability of the PACS
photometer observations and the linear behavior of the PACS bolometer source
fluxes over more than four orders of magnitude (from mJy levels up to more than
1000 Jy). Our results show the great potential of using the observed solar
system targets for cross-calibration purposes with other ground-based,
airborne, and space-based instruments and projects. At the same time, the PACS
results will lead to improved model solutions for future calibration
applications.Comment: 25 pages, 11 figures, 11 table
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