22 research outputs found
The Earliest Stages of Isolated Low-Mass Star Formation
The knowledge of the initial conditions of low-mass star formation is a key to understand the process of protostellar collapse and protostar formation. This thesis focuses on the earliest stages of isolated low-mass star formation and covers spatial scales which spread over three orders of magnitude. It includes the study of a filament with a length of a few parsecs, down to the in-depth analysis of an isolated prestellar core, where structures of the order of a few hundred AU are resolved. In the first part, a NIR extinction map of the filament L 1495 reveals its small scale, highly fragmented structure, and a high star-forming potential in all its parts. In some regions, the initial filament collapse and fragmentation is still taking place and star formation is yet to occur, whereas in other parts, I identify a population of dense cores with separations of the order of the locals Jeans length, which are likely to collapse and form stars. The second part of the thesis focuses on the isolated core CB17, which hosts two sources at very different evolutionary stages (Class I vs. prestellar core) at a projected distance of only 5500AU. With the aid of FIR and (sub)mm observations of the dust emission, I determine its density and temperature structure and draw conclusions about the dust properties. Interferometric observations at high angular resolution reveal a complex velocity structure in the prestellar core, which might be result of interaction with the protostar
The Earliest Stages of Isolated Low-Mass Star Formation
The knowledge of the initial conditions of low-mass star formation is a key to understand the process of protostellar collapse and protostar formation. This thesis focuses on the earliest stages of isolated low-mass star formation and covers spatial scales which spread over three orders of magnitude. It includes the study of a filament with a length of a few parsecs, down to the in-depth analysis of an isolated prestellar core, where structures of the order of a few hundred AU are resolved. In the first part, a NIR extinction map of the filament L 1495 reveals its small scale, highly fragmented structure, and a high star-forming potential in all its parts. In some regions, the initial filament collapse and fragmentation is still taking place and star formation is yet to occur, whereas in other parts, I identify a population of dense cores with separations of the order of the locals Jeans length, which are likely to collapse and form stars. The second part of the thesis focuses on the isolated core CB17, which hosts two sources at very different evolutionary stages (Class I vs. prestellar core) at a projected distance of only 5500AU. With the aid of FIR and (sub)mm observations of the dust emission, I determine its density and temperature structure and draw conclusions about the dust properties. Interferometric observations at high angular resolution reveal a complex velocity structure in the prestellar core, which might be result of interaction with the protostar
A Major Asymmetric Dust Trap in a Transition Disk
The statistics of discovered exoplanets suggest that planets form
efficiently. However, there are fundamental unsolved problems, such as
excessive inward drift of particles in protoplanetary disks during planet
formation. Recent theories invoke dust traps to overcome this problem. We
report the detection of a dust trap in the disk around the star Oph IRS 48
using observations from the Atacama Large Millimeter/submillimeter Array
(ALMA). The 0.44-millimeter-wavelength continuum map shows high-contrast
crescent-shaped emission on one side of the star originating from
millimeter-sized grains, whereas both the mid-infrared image (micrometer-sized
dust) and the gas traced by the carbon monoxide 6-5 rotational line suggest
rings centered on the star. The difference in distribution of big grains versus
small grains/gas can be modeled with a vortex-shaped dust trap triggered by a
companion.Comment: 25 pages, 7 figures (accepted version prior to language editing
The Initial Mass Function of the Stellar Association NGC 602 in the Small Magellanic Cloud with Hubble Space Telescope ACS Observations
We present our photometric study of the stellar association NGC 602 in the
wing of the Small Magellanic Cloud (SMC). The data were taken in the filters
F555W and F814W using the Advanced Camera for Surveys (ACS) on-board the Hubble
Space Telescope (HST). Photometry was performed using the ACS module of the
stellar photometry package DOLPHOT. We detected more than 5,500 stars with a
magnitude range of 14 \lsim m_{555} \lsim 28 mag. Three prominent stellar
concentrations are identified with star counts in the observed field, the
association NGC 602 itself, and two clusters, one of them not being currently
in any known catalog. The Color-Magnitude Diagrams (CMDs) of both clusters show
features typical for young open clusters, while that of the association reveals
bright main sequence (MS) and faint pre-main sequence (PMS) stars as the
members of the system. We construct the initial mass spectrum (IMS) of the
association by applying an age-independent method of counting the PMS stars
within evolutionary tracks, while for the bright MS stars we transform their
magnitudes to masses with the use of mass-luminosity relations. The IMS of NGC
602 is found to be well represented by a single-power law, corresponding to an
Initial Mass Function (IMF) of slope \Gamma\approx -1.2 for 1 \lsim M/M{\solar}
\lsim 45. This indicates that the shape of the IMF of a star forming system in
the SMC for stars with masses higher than 1 M{\solar} seems to be quite similar
to the field IMF in the solar neighborhood.Comment: Accepted for publication in ApJ, 13 pages, 14 figures, emulateapj.cls
LaTeX style, full resolution version available on
http://www.astro.uni-bonn.de/~dgoulier/Science/NGC602/ms.pd
Star Formation in the Taurus filament L 1495: from Dense Cores to Stars
We present a study of dense structures in the L1495 filament in the Taurus Molecular Cloud and examine its star-forming properties. In particular we construct a dust extinction map of the filament using deep near-infrared observations, exposing its small-scale structure in unprecedented detail. The filament shows highly fragmented substructures and a high mass-per-length value of , reflecting star-forming potential in all parts of it. However, a part of the filament, namely B211, is remarkably devoid of young stellar objects. We argue that in this region the initial filament collapse and fragmentation is still taking place and star formation is yet to occur. In the star-forming part of the filament, we identify 39 cores with masses from and preferred separations in agreement with the local Jeans length. Most of these cores exceed the Bonnor-Ebert critical mass, and are therefore likely to collapse and form stars. The Dense Core Mass Function follows a power law with exponent , a form commonly observed in star-forming regions.Astronom
L1448 IRS2E: A candidate first hydrostatic core
Intermediate between the prestellar and Class 0 protostellar phases, the
first core is a quasi-equilibrium hydrostatic object with a short lifetime and
an extremely low luminosity. Recent MHD simulations suggest that the first core
can even drive a molecular outflow before the formation of the second core
(i.e., protostar). Using the Submillimeter Array and the Spitzer Space
Telescope, we present high angular resolution observations towards the embedded
dense core IRS2E in L1448. We find that source L1448 IRS2E is not visible in
the sensitive Spitzer infrared images (at wavelengths from 3.6 to 70 um), and
has weak (sub-)millimeter dust continuum emission. Consequently, this source
has an extremely low bolometric luminosity (< 0.1 L_sun). Infrared and
(sub-)millimeter observations clearly show an outflow emanating from this
source; L1448 IRS2E represents thus far the lowest luminosity source known to
be driving a molecular outflow. Comparisons with prestellar cores and Class 0
protostars suggest that L1448 IRS2E is more evolved than prestellar cores but
less evolved than Class 0 protostars, i.e., at a stage intermediate between
prestellar cores and Class 0 protostars. All these results are consistent with
the theoretical predictions of the radiative/magneto hydrodynamical
simulations, making L1448 IRS2E the most promising candidate of the first
hydrostatic core revealed so far.Comment: 20 pages, 4 figures, to be published by Ap
SMA and Spitzer Observations of Bok Glouble CB17: A Candidate First Hydrostatic Core?
We present high angular resolution SMA and Spitzer observations toward the
Bok globule CB17. SMA 1.3mm dust continuum images reveal within CB17 two
sources with an angular separation of about 21" (about 5250 AU at a distance of
250 pc). The northwestern continuum source, referred to as CB17 IRS, dominates
the infrared emission in the Spitzer images, drives a bipolar outflow extending
in the northwest-southeast direction, and is classified as a low luminosity
Class0/I transition object (L_bol ~ 0.5 L_sun). The southeastern continuum
source, referred to as CB17 MMS, has faint dust continuum emission in the SMA
1.3mm observations (about 6 sigma detection; ~3.8 mJy), but is not detected in
the deep Spitzer infrared images at wavelengths from 3.6 to 70 micron. Its
bolometric luminosity and temperature, estimated from its spectral energy
distribution, are less than 0.04 L_sun and 16 K, respectively. The SMA CO(2-1)
observations suggest that CB17 MMS may drive a low-velocity molecular outflow
(about 2.5 km/s), extending in the east-west direction. Comparisons with
prestellar cores and Class0 protostars suggest that CB17 MMS is more evolved
than prestellar cores but less evolved than Class0 protostars. The observed
characteristics of CB17 MMS are consistent with the theoretical predictions
from radiative/magneto hydrodynamical simulations of a first hydrostatic core,
but there is also the possibility that CB17 MMS is an extremely low luminosity
protostar deeply embedded in an edge-on circumstellar disk. Further
observations are needed to study the properties of CB17 MMS and to address more
precisely its evolutionary stage.Comment: 33 pages, 11 figures, to be published by Ap
Perceptual and Memorial Contributions to Developmental Prosopagnosia
Developmental prosopagnosia (DP) is commonly associated with the failure to properly perceive individuating facial properties, notably those conveying configural or holistic
content. While this may indicate that the primary impairment is perceptual, it is conceivable that some cases of DP are instead caused by a memory impairment, with any perceptual
complaint merely allied rather than causal. To investigate this possibility, we administered a battery of face perception tasks to 11 individuals who reported that their face recognition difficulties disrupt daily activity and who also performed poorly on two formal tests of face recognition. Group statistics identified, relative to age- and gender-matched controls, difficulties in apprehending global-local relations and the holistic properties of faces, and in matching across viewpoints, but these were mild in nature and were not consistently evident at the level of individual participants. Six of the 11 individuals failed to show any evidence of perceptual impairment. In the remaining five individuals, no single perceptual deficit, or combination of deficits, was necessary or sufficient for poor recognition performance. These data suggest that some cases of DP are better explained by a memorial rather than perceptual deficit, and highlight the relevance of the apperceptive/associative distinction more commonly applied to the allied syndrome of acquired prosopagnosia
The Mid-infrared E-ELT Imager and Spectrograph (METIS)
METIS will be among the first generation of scientific instruments on the
E-ELT. Focusing on highest angular resolution and high spectral resolution,
METIS will provide diffraction limited imaging and coronagraphy from 3-14um
over an 20"x20" field of view, as well as integral field spectroscopy at R ~
100,000 from 2.9-5.3um. In addition, METIS provides medium-resolution (R ~
5000) long slit spectroscopy, and polarimetric measurements at N band. While
the baseline concept has already been discussed, this paper focuses on the
significant developments over the past two years in several areas: The science
case has been updated to account for recent progress in the main science areas
circum-stellar disks and the formation of planets, exoplanet detection and
characterization, Solar system formation, massive stars and clusters, and star
formation in external galaxies. We discuss the developments in the adaptive
optics (AO) concept for METIS, the telescope interface, and the instrument
modelling. Last but not least, we provide an overview of our technology
development programs, which ranges from coronagraphic masks, immersed gratings,
and cryogenic beam chopper to novel approaches to mirror polishing, background
calibration and cryo-cooling. These developments have further enhanced the
design and technology readiness of METIS to reliably serve as an early
discovery machine on the E-ELT.Comment: 18 pages, 14 figures, paper presented at the conference 'Astronomical
Telescopes and Instrumentation' in Montreal (2014
An ammonia spectral map of the L1495-B218 filaments in the Taurus molecular cloud. I. Physical properties of filaments and dense cores
We present deep NH3 observations of the L1495-B218 filaments in the Taurus molecular cloud covering over a 3° angular range using the K-band focal plane array on the 100 m Green Bank Telescope. The L1495-B218 filaments form an interconnected, nearby, large complex extending over 8 pc. We observed NH3 (1, 1) and (2, 2) with a spectral resolution of 0.038 km s−1 and a spatial resolution of 31''. Most of the ammonia peaks coincide with intensity peaks in dust continuum maps at 350 and 500 μm. We deduced physical properties by fitting a model to the observed spectra. We find gas kinetic temperatures of 8–15 K, velocity dispersions of 0.05–0.25 km s−1, and NH3 column densities of 5 × 1012 to 1 × 1014 cm−2. The CSAR algorithm, which is a hybrid of seeded-watershed and binary dendrogram algorithms, identifies a total of 55 NH3 structures, including 39 leaves and 16 branches. The masses of the NH3 sources range from 0.05 to 9.5 . The masses of NH3 leaves are mostly smaller than their corresponding virial mass estimated from their internal and gravitational energies, which suggests that these leaves are gravitationally unbound structures. Nine out of 39 NH3 leaves are gravitationally bound, and seven out of nine gravitationally bound NH3 leaves are associated with star formation. We also found that 12 out of 30 gravitationally unbound leaves are pressure confined. Our data suggest that a dense core may form as a pressure-confined structure, evolve to a gravitationally bound core, and undergo collapse to form a protostar