98 research outputs found
Peering through the veil: near-infrared photometry and extinction for the Galactic nuclear star cluster
The aims of this work are to provide accurate photometry in multiple
near-infrared broadband filters, to determine the power-law index of the
extinction-law toward the central parsec of the Galaxy, to provide measurements
of the absolute extinction toward the Galactic center, and finally to measure
the spatial variability of extinction on arcsecond scales.We use adaptive
optics observations of the central parsec of the Milky Way. Absolute values for
the extinction in the H, Ks, and L'-bands as well as of the power-law indices
of the H to Ks and Ks to L' extinction-laws are measured based on the
well-known properties of red clump stars. Extinction maps are derived based on
H-Ks and Ks-L' colors. We present Ks-band photometry for ~7700 stars (H and L'
photometry for a subset). From a number of recently published values we compute
a mean distance of the Galactic center of R_0=8.03+-0.15 kpc, which has an
uncertainty of just 2%. Based on this R_0 and on the RC method, we derive
absolute mean extinction values toward the central parsec of the Galaxy of
A_H=4.48+-0.13 mag, A_Ks=2.54+-0.12$ mag, and A_L'=1.27+-0.18 mag. We estimate
values of the power-law indices of the extinction-law of
alpha_{H-Ks}=2.21+-0.24 and alpha_{Ks-L'}=1.34+-0.29. A Ks-band extinction map
for the Galactic center is computed based on this extinction law and on stellar
H-Ks colors. Mean extinction values in a circular region with 0.5" radius
centered on Sagittarius A* are A_{H, SgrA*}=4.35+-0.12, A_{Ks,
SgrA*}=2.46+-0.03, and A_{L', SgrA*}=1.23+-0.08.Comment: accepted for publication by Astronomy & Astrophysics; please contact
RS for higher quality figure
Properties of bow-shock sources at the Galactic center
There are an enigmatic population of massive stars around the Galactic Center
(GC) that were formed some Ma ago. A fraction of these stars has been found to
orbit the supermassive black hole, SgrA*, in a projected clockwise disk, which
suggests that they were formed in a formerly existing dense disk around SgrA*.
We focus on the extended, near-infrared (NIR) sources IRS1W, IRS5, IRS10W, and
IRS21 that have been suggested to be young, massive stars that form bow-shocks
through their interaction with the ISM. Their nature has impeded accurate
determination of their orbital parameters. We aim at establishing their nature
and kinematics to test whether they form part of the clockwise disk. We
performed NIR multi-wavelength imaging using adaptive optics (AO) and sparse
aperture masking (SAM). We introduce a new method for self-calibration of the
SAM PSF in dense stellar fields. The emission mechanism, morphology and
kinematics of the targets were examined via 3D bow-shock models. We confirm
previous findings that IRS21, IRS1W, and IRS5 are bow-shocks created by the
interaction between mass-losing stars and the interstellar gas. The nature of
IRS10W remains unclear. Our modeling shows that the bow-shock-emission is
caused by thermal emission while the scattering of stellar light does not play
any significant role. IRS 1W appears to be a bow-shock produced by an
anisotropic stellar wind or by locally inhomogeneous ISM density. Our best-fit
models provide an estimate of the local proper motion of the ISM in the NA in
agreement with the published models. Assuming that all of the sources are tied
to SgrA*, their orbital planes were obtained via a Monte-Carlo simulation. Our
orbital analysis suggests that they are not part of any of the clockwise disk.
We thus add more evidence to recent findings that a large part of the massive
stars show apparently random orbital orientations.Comment: accepted for publication by A&A, 17 pages, 11 figures, 1 appendi
The SONYC survey: Towards a complete census of brown dwarfs in star forming regions
SONYC, short for "Substellar Objects in Nearby Young Clusters", is a survey
program to provide a census of the substellar population in nearby star forming
regions. We have conducted deep optical and near-infrared photometry in five
young regions (NGC1333, rho Ophiuchi, Chamaeleon-I, Upper Sco, and Lupus-3),
combined with proper motions, and followed by extensive spectroscopic campaigns
with Subaru and VLT, in which we have obtained more than 700 spectra of
candidate low-mass objects. We have identified and characterized more than 60
new substellar objects, among them a handful of objects with masses close to,
or below the Deuterium burning limit. Through SONYC and surveys by other
groups, the substellar IMF is now well characterized down to ~ 5 - 10 MJup, and
we find that the ratio of the number of stars with respect to brown dwarfs lies
between 2 and 6. A comprehensive survey of NGC 1333 reveals that, down to
~5MJup, free-floating objects with planetary masses are 20-50 times less
numerous than stars, i.e. their total contribution to the mass budget of the
clusters can be neglected.Comment: to appear in the proceedings of the conference 'Brown dwarfs come of
age', May 20-24 2013, Memorie della Societa Astronomica Italian
Near-infrared photometry of WISE J085510.74-071442.5
Indexación: Web of ScienceAims. We aim at obtaining near-infrared photometry and deriving the mass, age, temperature, and surface gravity of WISE J085510.74 071442.5 (J0855 0714), which is the coolest object beyond the solar system currently known.
Methods. We used publicly available data from the archives of the Hubble Space Telescope (HST) and the Very Large Telescope (VLT) to determine the emission of this source at 1.153 mu m (F110W) and 1.575 mu m (CH4-o ff). J0855 0714 was detected at both wavelengths with a signal-to-noise ratio of approximate to 10 (F110W) and approximate to 4 (CH4-off) at the peak of the corresponding point-spread-functions.
Results. This is the first detection of J0855 0714 in the H-band wavelengths. We measured 26.31 +/- 0.10 and 23.22 +/- 0.35 mag in F110W and CH4-o ff (Vega system). J0855 0714 remains unresolved in the HST images that have a spatial resolution of 0.22 0 0. Companions at separations of 0.5 AU (similar mass and brightness) and at similar to 1 AU approximate to 1 mag fainter in the F110W filter) are discarded. By combining the new data with published photometry, including non-detections, we build the spectral energy distribution of J0855 0714 from 0.89 through 22.09 mu m, and contrast it against current solar-metallicity models of planetary atmospheres. We determine that the best spectral fit yields a temperature of 225 250 K, a bolometric luminosity of log L/L-circle dot = 8 : 57, and a high surface gravity of log g = 5 : 0 (cm s(2)), which suggests an old age although a gravity this high is not fully compatible with evolutionary models. After comparing our data with the cooling theory for brown dwarfs and planets, we infer a mass in the interval 2 10 MJup for ages of 1 12 Gyr and high atmospheric gravities of log g greater than or similar to 3.5 (cm s(2)). If it had the age of the Sun, J0855 0714 would be a approximate to 5-M-Jup free-floating planetary-mass object.
Conclusions. J0855 0714 meets the mass values previously determined for free-floating planetary-mass objects discovered in starforming regions and young stellar clusters. Based on extrapolations of the substellar mass functions of young clusters to the field, as many J0855 0714-like objects as M5-L2 stars may be expected to populate the solar neighborhood.http://www.aanda.org/articles/aa/pdf/2016/08/aa28662-16.pd
The Galactic centre mini-spiral in the mm-regime
The mini-spiral is a feature of the interstellar medium in the central ~2 pc
of the Galactic center. It is composed of several streamers of dust and ionised
and atomic gas with temperatures between a few 100 K to 10^4 K. There is
evidence that these streamers are related to the so-called circumnuclear disk
of molecular gas and are ionized by photons from massive, hot stars in the
central parsec. We attempt to constrain the emission mechanisms and physical
properties of the ionized gas and dust of the mini-spiral region with the help
of our multiwavelength data sets. Our observations were carried out at 1.3 mm
and 3 mm with the mm interferometric array CARMA in California in March and
April 2009, with the MIR instrument VISIR at ESO's VLT in June 2006, and the
NIR Br-gamma with VLT NACO in August 2009. We present high resolution maps of
the mini-spiral, and obtain a spectral index of 0.5 for Sgr A*, indicating an
inverted synchrotron spectrum. We find electron densities within the range
0.8-1.5x10^4 cm-3 for the mini-spiral from the radio continuum maps, along with
a dust mass contribution of ~0.25 solar masses from the MIR dust continuum, and
extinctions ranging from 1.8-3 at 2.16 micron in the Br-gamma line. We observe
a mixture of negative and positive spectral indices in our 1.3 mm and 3 mm
observations of the extended emission of the mini-spiral, which we interpret as
evidence that there are a range of contributions to the thermal free-free
emission by the ionized gas emission and by dust at 1.3 mm.Comment: 9 pages, 11 figures, accepted to A&
Cometary shaped sources at the Galactic Center - Evidence for a wind from the central 0.2 pc
In 2007 we reported two cometary shaped sources in the vicinity of Sgr A*
(0.8" and 3.4" projected distance), named X7 and X3. The symmetry axes of the
two sources are aligned to within 5 degrees in the plane of the sky and the
tips of their bow-shocks point towards Sgr A*. Our measurements show that the
proper motion vectors of both features are pointing in directions more than 45
deg away from the line that connects them with Sgr A*. This misalignment of the
bow-shock symmetry axes and their proper motion vectors, combined with the high
proper motion velocities of several 100 km/s, suggest that the bow-shocks must
be produced by an interaction with some external fast wind, possibly coming
from Sgr A*, or stars in its vicinity. We have developed a bow-shock model to
fit the observed morphology and constrain the source of the external wind. The
result of our modeling allows us to estimate the velocity of the external wind,
making sure that all likely stellar types of the bow-shock stars are
considered. We show that neither of the two bow-shocks (one of which is clearly
associated with a stellar source) can be produced by influence of a stellar
wind of a single mass-losing star in the central parsec. Instead, an outflow
carrying a momentum comparable to the one contributed by the ensemble of the
massive young stars, can drive shock velocities capable of producing the
observed cometary features. We argue that a collimated outflow arising
perpendicular to the plane of the clockwise rotating stars (CWS), can easily
account for the two features and the mini-cavity. However, the collective wind
from the CWS has a scale of >10''. The presence of a strong, mass-loaded
outbound wind at projected distances from Sgr A* of <1'' is in fact in
agreement with models that predict a highly inefficient accretion onto the
central black hole due to a strongly radius dependent accretion flow.Comment: to appear in A&
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