604 research outputs found
Using Narrow Band Photometry to Detect Young Brown Dwarfs in IC348
We report the discovery of a population of young brown dwarf candidates in
the open star cluster IC348 and the development of a new spectroscopic
classification technique using narrow band photometry. Observations were made
using FLITECAM, the First Light Camera for SOFIA, at the 3-m Shane Telescope at
Lick Observatory. FLITECAM is a new 1-5 micron camera with an 8 arcmin field of
view. Custom narrow band filters were developed to detect absorption features
of water vapor (at 1.495 microns) and methane (at 1.66 microns) characteristic
of brown dwarfs. These filters enable spectral classification of stars and
brown dwarfs without spectroscopy. FLITECAM's narrow and broadband photometry
was verified by examining the color-color and color-magnitude characteristics
of stars whose spectral type and reddening was known from previous surveys.
Using our narrow band filter photometry method, it was possible to identify an
object measured with a signal-to-noise ratio of 20 or better to within +/-3
spectral class subtypes for late-type stars. With this technique, very deep
images of the central region of IC348 (H ~ 20.0) have identified 18 sources as
possible L or T dwarf candidates. Out of these 18, we expect that between 3 - 6
of these objects are statistically likely to be background stars, with the
remainder being true low-mass members of the cluster. If confirmed as cluster
members then these are very low-mass objects (~5 Mjupiter). We also describe
how two additional narrow band filters can improve the contrast between M, L,
and T dwarfs as well as provide a means to determine the reddening of an
individual object.Comment: 43 pages, 17 figures. Accepted for publication in the Astrophysical
Journal 27 June 200
Ultraviolet-Selected Field and Pre-Main-Sequence Stars Towards Taurus and Upper Scorpius
We have carried out a Galaxy Evolution Explorer (GALEX) Cycle 1 guest
investigator program covering 56 square degrees near the Taurus T association
and 12 square degrees along the northern edge of the Upper Scorpius OB
association. We combined photometry in the GALEX FUV and NUV bands with data
from the Two Micron All Sky Survey to identify candidate young (<100 Myr old)
stars as those with an ultraviolet excess relative to older main sequence
stars. Follow-up spectroscopy of a partial sample of these candidates suggest 5
new members of Taurus, with 8-20 expected from additional observations, and 5
new members of Upper Scorpius, with 3-6 expected from additional observations.
These candidate new members appear to represent a distributed, non-clustered
population in either region, although our sample statistics are as of yet too
poor to constrain the nature or extent of this population. Rather, our study
demonstrates the ability of GALEX observations to identify young stellar
populations distributed over a wide area of the sky. We also highlight the
necessity of a better understanding of the Galactic ultraviolet source
population to support similar investigations. In particular, we report a large
population of stars with an ultraviolet excess but no optical indicators of
stellar activity or accretion, and briefly argue against several
interpretations of these sources.Comment: 46 pages, 16 figures, 13 tables; Accepted to the Astronomical Journa
Bispectrum speckle interferometry of the massive protostellar outflow source IRAS 23151+5912
We present bispectrum speckle interferometry of the massive protostellar
object IRAS 23151+5912 in the near-infrared K' band. The reconstructed image
shows the diffuse nebulosity north-east of two point-like sources in
unprecedented detail. The comparison of our near-infrared image with mm
continuum and CO molecular line maps shows that the brighter of the two point
sources lies near the center of the mm peak, indicating that it is a high-mass
protostar. The nebulosity coincides with the blue-shifted molecular outflow
component. The most prominent feature in the nebulosity is a bow-shock-like
arc. We assume that this feature is associated with a precessing jet which has
created an inward-pointed cone in the swept-up material. We present numerical
jet simulations that reproduce this and several other features observed in our
speckle image of the nebulosity. Our data also reveal a linear structure
connecting the central point source to the extended diffuse nebulosity. This
feature may represent the innermost part of a jet that drives the strong
molecular outflow (PA ~80 degr) from IRAS 23151+5912. With the aid of radiative
transfer calculations, we demonstrate that, in general, the observed inner
structures of the circumstellar material surrounding high-mass stars are
strongly influenced by the orientation and symmetry of the bipolar cavity.Comment: accepted by Astronomy & Astrophysics; preprints with high-resolution
images can be obtained from
http://www.mpifr-bonn.mpg.de/staff/tpreibis/iras23151.htm
Detection of an Inner Gaseous Component in a Herbig Be Star Accretion Disk: Near- and Mid-Infrared Spectrointerferometry and Radiative Transfer modeling of MWC 147
This is the author accepted manuscript. The final version is available from American Astronomical Society via the DOI in this record.We study the geometry and the physical conditions in the inner (AU-scale) circumstellar region around the young Herbig Be star MWC 147 using long-baseline spectrointerferometry in the near-infrared (NIR) K-band, VLTI/AMBER observations, and PTI archive data, as well as the mid-infrared (MIR) N-band, VLTI/MIDI observations. The emission from MWC 147 is clearly resolved and has a characteristic physical size of ~1.3 and ~9 AU at 2.2 and 11 ÎŒm, respectively (Gaussian diameter). The MIR emission reveals asymmetry consistent with a disk structure seen under intermediate inclination. The spectrally dispersed AMBER and MIDI interferograms both show a strong increase in the characteristic size toward longer wavelengths, much steeper than predicted by analytic disk models assuming power-law radial temperature distributions. We model the interferometric data and the spectral energy distribution of MWC 147 with two-dimensional, frequency-dependent radiation transfer simulations. This analysis shows that models of spherical envelopes or passive irradiated Keplerian disks (with vertical or curved puffed-up inner rim) can easily fit the SED, but predict much lower visibilities than observed; the angular size predicted by such models is 2-4 times larger than the size derived from the interferometric data, so these models can clearly be ruled out. Models of a Keplerian disk with optically thick gas emission from an active gaseous disk (inside the dust sublimation zone), however, yield a good fit of the SED and simultaneously reproduce the absolute level and the spectral dependence of the NIR and MIR visibilities. We conclude that the NIR continuum emission from MWC 147 is dominated by accretion luminosity emerging from an optically thick inner gaseous disk, while the MIR emission also contains contributions from the outer, irradiated dust disk.S. K. was
supported for this research through a fellowship from the International
Max Planck Research School ( IMPRS) for Radio and
Infrared Astronomy at the University of Bonn
Radio and X-ray variability of Young Stellar Objects in the Coronet Cluster
The Coronet Cluster in the nearby R CrA dark cloud offers the rare
opportunity to study at least four "class I" protostellar sources as well as
one candidate "class 0" source, a Herbig Ae star, and a candidate brown dwarf
within a few square arcminutes - most of them detected at radio- and X-ray
wavelengths. These sources were observed with the Very Large Array (VLA) at
3.5cm on nine occasions in 1998, spread over nearly four months. The source IRS
5, earlier shown to emit circularly polarized radio emission, was observed to
undergo a flux increase accompanied by changes in its polarization properties.
Comparison with VLA measurements taken in January 1997 allows for some analysis
of longer-term variability. In addition to this radio monitoring, we analyze
archival Chandra and XMM-Newton X-ray data of these sources. Three class I
protostars are bright enough for X-ray spectroscopy, and we perform a
variability analysis for these sources, covering a total of 154 ksec spread
over more than two and a half years. Also in X-rays, IRS 5 shows the most
pronounced variability, whilst the other two class I protostars IRS 1 and IRS 2
have more stable emission. X-ray data is also analyzed for the recently
identified candidate class 0 source IRS 7E, which shows strong variability as
well as for the Herbig Ae star R CrA for which we find extremely hot
X-ray-emitting plasma. For IRS 1,2 and 5, the hydrogen column densities derived
from the X-ray spectra are at about half the values derived with near-infrared
techniques, a situation similar to what has been observed towards some other
young stellar objects.Comment: 17 pages, 11 figures, accepted for publication in A&
Simultaneous X-ray, radio, near-infrared, and optical monitoring of Young Stellar Objects in the Coronet cluster
Multi-wavelength (X-ray to radio) monitoring of Young Stellar Objects (YSOs)
can provide important information about physical processes at the stellar
surface, in the stellar corona, and/or in the inner circumstellar disk regions.
While coronal processes should mainly cause variations in the X-ray and radio
bands, accretion processes may be traced by time-correlated variability in the
X-ray and optical/infrared bands. Several multi-wavelength studies have been
successfully performed for field stars and approx. 1-10 Myr old T Tauri stars,
but so far no such study succeeded in detecting simultaneous X-ray to radio
variability in extremely young objects like class I and class 0 protostars.
Here we present the first simultaneous X-ray, radio, near-infrared, and optical
monitoring of YSOs, targeting the Coronet cluster in the Corona Australis
star-forming region, which harbors at least one class 0 protostar, several
class I objects, numerous T Tauri stars, and a few Herbig AeBe stars. [...]
Seven objects are detected simultaneously in the X-ray, radio, and
optical/infrared bands; they constitute our core sample. While most of these
sources exhibit clear variability in the X-ray regime and several also display
optical/infrared variability, none of them shows significant radio variability
on the timescales probed. We also do not find any case of clearly
time-correlated optical/infrared and X-ray variability. [...] The absence of
time-correlated multi-wavelength variability suggests that there is no direct
link between the X-ray and optical/infrared emission and supports the notion
that accretion is not an important source for the X-ray emission of these YSOs.
No significant radio variability was found on timescales of days.Comment: 11 pages, 11 figures, accepted for publication in A&A (06 Dec 2006
Quasi-periodic X-ray Flares from the Protostar YLW15
With ASCA, we have detected three X-ray flares from the Class I protostar
YLW15. The flares occurred every ~20 hours and showed an exponential decay with
time constant 30-60 ks. The X-ray spectra are explained by a thin thermal
plasma emission. The plasma temperature shows a fast-rise and slow-decay for
each flare with kT_{peak}~4-6 keV. The emission measure of the plasma shows
this time profile only for the first flare, and remains almost constant during
the second and third flares at the level of the tail of the first flare. The
peak flare luminosities L_{X,peak} were ~5-20 * 10^{31} erg s^{-1}, which are
among the brightest X-ray luminosities observed to date for Class I protostars.
The total energy released in each flare was 3-6*10^{36} ergs. The first flare
is well reproduced by the quasi-static cooling model, which is based on solar
flares, and it suggests that the plasma cools mainly radiatively, confined by a
semi-circular magnetic loop of length ~14 Ro with diameter-to-length ratio
\~0.07. The two subsequent flares were consistent with the reheating of the
same magnetic structure as of the first flare. The large-scale magnetic
structure and the periodicity of the flares imply that the reheating events of
the same magnetic loop originate in an interaction between the star and the
disk due to the differential rotation.Comment: Accepted by ApJ, 9 pages incl. 4 ps figure
Investigating the effect of flow compensation and quantitative susceptibility mapping method on the accuracy of venous susceptibility measurement
Quantitative susceptibility mapping (QSM) is a promising non-invasive method for obtaining information relating to oxygen metabolism. However, the optimal acquisition sequence and QSM reconstruction method for reliable venous susceptibility measurements are unknown. Full flow compensation is generally recommended to correct for the influence of venous blood flow, although the effect of flow compensation on the accuracy of venous susceptibility values has not been systematically evaluated. In this study, we investigated the effect of different acquisition sequences, including different flow compensation schemes, and different QSM reconstruction methods on venous susceptibilities.
Ten healthy subjects were scanned with five or six distinct QSM sequence designs using monopolar readout gradients and different flow compensation schemes. All data sets were processed using six different QSM pipelines and venous blood susceptibility was evaluated in whole-brain segmentations of the venous vasculature and single veins. The quality of vein segmentations and the accuracy of venous susceptibility values were analyzed and compared between all combinations of sequences and reconstruction methods.
The influence of the QSM reconstruction method on average venous susceptibility values was found to be 2.7â11.6 times greater than the influence of the acquisition sequence, including flow compensation. The majority of the investigated QSM reconstruction methods tended to underestimate venous susceptibility values in the vein segmentations that were obtained.
In summary, we found that multi-echo gradient-echo acquisition sequences without full flow compensation yielded venous susceptibility values comparable to sequences with full flow compensation. However, the QSM reconstruction method had a great influence on susceptibility values and thus needs to be selected carefully for accurate venous QSM
The Properties of X-ray Luminous Young Stellar Objects in the NGC 1333 and Serpens Embedded Clusters
We present Chandra X-ray data of the NGC 1333 embedded cluster, combining
these data with existing Chandra data, Sptizer photometry and ground based
spectroscopy of both the NGC 1333 & Serpens North clusters to perform a
detailed study of the X-ray properties of two of the nearest embedded clusters
to the Sun. In NGC 1333, a total of 95 cluster members are detected in X-rays,
of which 54 were previously identified with Spitzer. Of the Spitzer sources, we
detect 23% of the Class I protostars, 53% of the Flat Spectrum sources, 52% of
the Class II, and 50% of the Transition Disk YSOs. Forty-one Class III members
of the cluster are identified, bringing the total identified YSO population to
178. The X-ray Luminosity Functions (XLFs) of the NGC 1333 and Serpens clusters
are compared to each other and the Orion Nebula Cluster. Based on this
comparison, we obtain a new distance for the Serpens cluster of 360+22/-13 pc.
The X-ray luminosity was found to depend on the bolometric luminosity as in
previous studies of other clusters, and that Lx depends primarily on the
stellar surface area. In the NGC 1333 cluster, the Class III sources have a
somewhat higher X-ray luminosity for a given surface area. We also find
evidence in NGC 1333 for a jump in the X-ray luminosity between spectral types
of M0 and K7, we speculate that this may result from the presence of radiative
zones in the K-stars. The gas column density vs. extinction in the NGC 1333 was
found to be N_H = 0.89 +/- 0.13 x 10^22 A_K, this is lower than expected of the
standard ISM but similar to that found previously in the Serpens Cloud Core.Comment: 58 pages, 14 figures, accepted by A
A deep wide-field sub-mm survey of the Carina Nebula complex
The Great Nebula in Carina is a superb location in which to study the physics
of violent massive star-formation and the resulting feedback effects, including
cloud dispersal and triggered star-formation. In order to reveal the cold dusty
clouds in the Carina Nebula complex, we used the Large APEX Bolometer Camera
LABOCA at the APEX telescope to map a 1.25 deg x 1.25 deg (= 50 x 50 pc^2)
region at 870 micrometer. From a comparison to Halpha images we infer that
about 6% of the 870 micrometer flux in the observed area is likely free-free
emission from the HII region, while about 94% of the flux is very likely
thermal dust emission. The total (dust + gas) mass of all clouds for which our
map is sensitive is ~ 60 000 Msun, in good agreement with the mass of the
compact clouds in this region derived from 13CO line observations. We generally
find good agreement in the cloud morphology seen at 870 micrometer and the
Spitzer 8 micrometer emission maps, but also identify a prominent infrared dark
cloud. Finally, we construct a radiative transfer model for the Carina Nebula
complex that reproduces the observed integrated spectral energy distribution
reasonably well. Our analysis suggests a total gas + dust mass of about 200000
Msun in the investigated area; most of this material is in the form of
molecular clouds, but a widely distributed component of (partly) atomic gas,
containing up to ~ 50% of the total mass, may also be present. Currently, only
some 10% of the gas is in sufficiently dense clouds to be immediately available
for future star formation, but this fraction may increase with time owing to
the ongoing compression of the strongly irradiated clouds and the expected
shockwaves of the imminent supernova explosions.Comment: Accepted for publication in Astronomy & Astrophysics; high-quality
pre-prints can be obtained from
http://www.usm.uni-muenchen.de/people/preibisch/publications.htm
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