arXiv subjects

Thomas Henning

Publications and source records attributed to Thomas Henning.

At least 595 records · Page 33Linked to original sources

arXiv2007

Discovery of the Pre-Main Sequence Population of the Stellar Association LH 95 in the Large Magellanic Cloud with Hubble Space Telescope ACS Observations

We report the discovery of an extraordinary number of pre-main sequence (PMS) stars in the vicinity of the stellar association LH 95 in the Large Magellanic Cloud (LMC). Using the {\em Advanced Camera for Surveys} on-board the {\em Hubble} Space Telescope in wide-field mode we obtained deep high-resolution imaging of the main body of the association and of a nearby representative LMC background field. These observations allowed us to construct the color-magnitude diagram (CMD) of the association in unprecedented detail, and to decontaminate the CMD for the average LMC stellar population. The most significant result is the direct detection of a substantial population of PMS stars and their clustering properties with respect to the distribution of the higher mass members of the association. Although LH 95 represents a rather modest star forming region, our photometry, with a detection limit $V$ \lsim 28 mag, reveals in its vicinity more than 2,500 PMS stars with masses down to $\sim 0.3$ M{\solar}. Our observations offer, thus, a new perspective of a typical LMC association: The stellar content of LH 95 is found to extend from bright OB stars to faint red PMS stars, suggesting a fully populated Initial Mass Function (IMF) from the massive blue giants down to the sub-solar mass regime.

astro-ph↗

arXiv2007

The dust, planetesimals and planets of HD 38529

HD 38529 is a post-main sequence G8III/IV star (3.5 Gyr old) with a planetary system consisting of at least two planets having Msin(i) of 0.8 MJup and 12.2 MJup, semimajor axes of 0.13 AU and 3.74 AU, and eccentricities of 0.25 and 0.35, respectively. Spitzer observations show that HD 38529 has an excess emission above the stellar photosphere, with a signal-to-noise ratio (S/N) at 70 micron of 4.7, a small excess at 33 micron (S/N=2.6) and no excess <30 micron. We discuss the distribution of the potential dust-producing planetesimals from the study of the dynamical perturbations of the two known planets, considering in particular the effect of secular resonances. We identify three dynamically stable niches at 0.4-0.8 AU, 20-50 AU and beyond 60 AU. We model the spectral energy distribution of HD 38529 to find out which of these niches show signs of harboring dust-producing plantesimals. The secular analysis, together with the SED modeling resuls, suggest that the planetesimals responsible for most of the dust emission are likely located within 20-50 AU, a configuration that resembles that of the Jovian planets + Kuiper Belt in our Solar System. Finally, we place upper limits (8E-6 lunar masses of 10 micron particles) to the amount of dust that could be located in the dynamically stable region that exists between the two planets (0.25--0.75 AU).

astro-ph↗

arXiv2007

Clustered Star Formation in the Small Magellanic Cloud. A Spitzer/IRAC View of the Star-Forming Region NGC 602/N 90

We present Spitzer/IRAC photometry on the star-forming HII region N 90, related to the young stellar association NGC 602 in the Small Magellanic Cloud. Our photometry revealed bright mid-infrared sources, which we classify with the use of a scheme based on templates and models of red sources in the Milky Way, and criteria recently developed from the Spitzer Survey of the SMC for the selection of candidate Young Stellar Objects (YSOs). We detected 57 sources in all four IRAC channels in a 6.2' x 4.8' field-of-view centered on N 90; 22 of these sources are classified as candidate YSOs. We compare the locations of these objects with the position of optical sources recently found in the same region with high-resolution HST/ACS imaging of NGC 602, and we find that 17 candidate YSOs have one or more optical counterparts. All of these optical sources are identified as pre-main sequence stars, indicating, thus, ongoing clustered star formation events in the region. The positions of the detected YSOs and their related PMS clusters give a clear picture of the current star formation in N 90, according to which the young stellar association photo-ionizes the surrounding interstellar medium, revealing the HII nebula, and triggering sequential star formation events mainly along the eastern and southern rims of the formed cavity of the parental molecular cloud.

astro-ph↗

arXiv2007

An Imaging Survey for Extrasolar Planets around 45 Close, Young Stars with SDI at the VLT and MMT

We present the results of a survey of 45 young (<250 Myr), close (<50 pc) stars with the Simultaneous Differential Imager (SDI) implemented at the VLT and the MMT for the direct detection of extrasolar planets. As part of the survey, we observed 54 objects total, of which 45 were close, young stars, 2 were somewhat more distant (<150 pc), very young (<10 Myr) stars, 3 were stars with known radial velocity planets, and 4 were older, very nearby (<20 pc) solar analogues. Our SDI devices use a double Wollaston prism and a quad filter to take images simultaneously at 3 wavelengths surrounding the 1.62 um methane absorption bandhead found in the spectrum of cool brown dwarfs and gas giant planets. In our VLT data, we achieved H band contrasts > 10 mag (5 sigma) at a separation of 0.5" from the primary star on 45% of our targets and H band contrasts of > 9 mag at a separation of 0.5'' on 80% of our targets. With this degree of attenuation, we should be able to image (5sigma detection) a 5 M_{Jup} planet 15 AU from a 70 Myr K1 star at 15 pc or a 5 M_{Jup} planet at 2 AU from a 12 Myr M star at 10 pc. We believe that our SDI images are the highest contrast astronomical images ever made from ground or space for methane rich companions <1'' from their star. For the best 20 of our survey stars, we attained 50% 5 sigma completeness for 6-10 M_Jup planets at semi-major axes of 20-40 AU. Thus, our completeness levels are sufficient to significantly test theoretical planet distributions. From our survey null result, we can rule out (at the 98% confidence/2.0sigma level) a model planet population using a planet distribution where N(a) $\propto$ constant out to a distance of 45 AU (further model assumptions discussed within).

astro-ph↗

arXiv2007

The Star-forming Region NGC 346 in the Small Magellanic Cloud with Hubble Space Telescope ACS Observations. II. Photometric Study of the Intermediate-Age Star Cluster BS 90

We present the results of our investigation of the intermediate-age star cluster BS 90, located in the vicinity of the HII region N 66 in the SMC, observed with HST/ACS. The high-resolution data provide a unique opportunity for a very detailed photometric study performed on one of the rare intermediate-age rich SMC clusters. The complete set of observations is centered on the association NGC 346 and contains almost 100,000 stars down to V ~28 mag. In this study we focus on the northern part of the region, which covers almost the whole stellar content of BS 90. We construct its stellar surface density profile and derive structural parameters. Isochrone fits on the CMD of the cluster results in an age of about 4.5 Gyr. The luminosity function is constructed and the present-day mass function of BS 90 has been obtained using the mass-luminosity relation, derived from the isochrone models. We found a slope between -1.30 and -0.95, comparable or somewhat shallower than a typical Salpeter IMF. Examination of the radial dependence of the mass function shows a steeper slope at larger radial distances, indicating mass segregation in the cluster. The derived half-mass relaxation time of 0.95 Gyr suggests that the cluster is mass segregated due to its dynamical evolution. From the isochrone model fits we derive a metallicity for BS 90 of [Fe/H]=-0.72, which adds an important point to the age-metallicity relation of the SMC. We discuss our findings on this relation in comparison to other SMC clusters.

astro-ph↗

arXiv2007

NACO-SDI direct imaging search for the exoplanet Eps Eri b

The active K2V star $ε$ Eri hosts the most nearby known extrasolar planet. With an angular separation of about 1" on average, and an age of a few to several hundred Myrs, $ε$ Eri b is one of the prime candidates for becoming the first definitive extrasolar planet imaged directly. We present a multi-epoch deep differential imaging survey performed with NACO-SDI at the VLT with the aim of finding the planet. The results are combined with recent astrometry in an attempt to further constrain the detection limits. No convincing candidate is found among the many coherent structures that constitute the residual speckle noise, which is the dominant noise at small angular scales. We present our detection limits, compare them with the estimated brightness of $ε$ Eri b, and analyze how the limits can be improved further. It is found that integration time remains a very important parameter for achieving good results, even in the speckle-dominated regimes. The results yield new, improved upper 3$σ$ limits on the absolute H-band (1.6 $μ$m) brightness of the 1.55 $M_{\rm jup}$ companion of 19.1 to 19.5 mag, depending on the specific age of the system.

astro-ph↗

arXiv2007

Silicate Dust in Evolved Protoplanetary Disks: Growth, Sedimentation, and Accretion

We present the Spitzer IRS spectra for 33 young stars in Tr 37 and NGC 7160. The sample includes the high- and intermediate-mass stars with MIPS 24 microns excess, the only known active accretor in the 12 Myr-old cluster NGC 7160, and 19 low-mass stars with disks in the 4 Myr-old cluster Tr 37. We examine the 10 microns silicate feature, present in the whole sample of low-mass star and in 3 of the high- and intermediate-mass targets, and we find that PAH emission is detectable only in the Herbig Be star. We analyze the composition and size of the warm photospheric silicate grains by fitting the 10 microns silicate feature, and study the possible correlations between the silicate characteristics and the stellar and disk properties (age, SED slope, accretion rate, spectral type). We find indications of dust settling with age and of the effect of turbulent enrichment of the disk atmosphere with large grains. Crystalline grains are only small contributors to the total silicate mass in all disks, and do not seem to correlate with any other property, except maybe binarity. We also observe that spectra with very weak silicate emission are at least 3 times more frequent among M stars than among earlier spectral types, which may be an evidence of inner disk evolution. Finally, we find that 5 of the high- and intermediate-mass stars have SEDs and IRS spectra consistent with debris disk models involving planet formation, which could indicate debris disk formation at ages as early as 4 Myr.

astro-ph↗

arXiv2007

The Formation and Evolution of Planetary Systems: Placing Our Solar System in Context with Spitzer

We provide an overview of the Spitzer Legacy Program ``Formation and Evolution of Planetary Systems" (FEPS) which was proposed in 2000, begun in 2001, and executed aboard the Spitzer Space Telescope between 2003 and 2006. This program exploits the sensitivity of Spitzer to carry out mid-infrared spectrophotometric observations of solar-type stars. With a sample of ~ 328 stars ranging in age from ~ 3 Myr to ~ 3 Gyr, we trace the evolution of circumstellar gas and dust from primordial planet-building stages in young circumstellar disks through to older collisionally generated debris disks. When completed, our program will help define the time scales over which terrestrial and gas giant planets are built, constrain the frequency of planetesimal collisions as a function of time, and establish the diversity of mature planetary architectures. In addition to the observational program, we have coordinated a concomitant theoretical effort aimed at understanding the dynamics of circumstellar dust with and without the effects of embedded planets, dust spectral energy distributions, and atomic and molecular gas line emission. Together with the observations, these efforts will provide astronomical context for understanding whether our Solar System - and its habitable planet - is a common or a rare circumstance. Additional information about the FEPS project can be found on the team website: feps.as.arizona.edu

astro-ph↗

arXiv2006

Chemistry in disks I - Deep search for N$_2$H$^+$ in the protoplanetary disks around LkCa 15, MWC 480, and DM Tau

Aims: To constrain the ionization fraction in protoplanetary disks, we present new high-sensitivity interferometric observations of N$_2$H$^+$ in three disks surrounding DM Tau, LkCa 15, and MWC 480. Methods: We used the IRAM PdBI array to observe the N$_2$H$^+$ J=1-0 line and applied a $χ^2$-minimization technique to estimate corresponding column densities. These values are compared, together with HCO$^+$ column densities, to results of a steady-state disk model with a vertical temperature gradient coupled to gas-grain chemistry. Results: We report two \ndhp detections for LkCa 15 and DM Tau at the $5 σ$ level and an upper limit for MWC 480. The column density derived from the data for LkCa 15 is much lower than previously reported. The [N$_2$H$^+$/HCO$^+$] ratio is on the order of 0.02--0.03. So far, HCO$^+$ remains the most abundant observed molecular ion in disks. Conclusions: All the observed values generally agree with the modelled column densities of disks at an evolutionary stage of a few million years (within the uncertainty limits), but the radial distribution of the molecules is not reproduced well. The low inferred concentration of N$_2$H$^+$ in three disks around low-mass and intermediate-mass young stars implies that this ion is not a sensitive tracer of the overall disk ionization fraction.

astro-ph↗

arXiv2006

Are Debris Disks and Massive Planets Correlated?

Using data from the Spitzer Space Telescope Legacy Science Program ``Formation and Evolution of Planetary Systems'' (FEPS), we have searched for debris disks around 9 FGK stars (2-10 Gyr), known from radial velocity (RV) studies to have one or more massive planets. Only one of the sources, HD 38529, has excess emission above the stellar photosphere; at 70 micron the signal-to-noise ratio in the excess is 4.7 while at wavelengths < 30 micron there is no evidence of excess. The remaining sources show no excesses at any Spitzer wavelengths. Applying survival tests to the FEPS sample and the results for the FGK survey published in Bryden et al. (2006), we do not find a significant correlation between the frequency and properties of debris disks and the presence of close-in planets. We discuss possible reasons for the lack of a correlation.

astro-ph↗

arXiv2006

Radial Velocity Follow-up of Planetary Transit Candidate MACHO.120.22303.5389

We present preliminary results on the radial velocity follow-up of a planetary transit candidate (P=2.43d, V=15.4) detected during the MACHO project. The photometry is consistent with a grazing transit of an object with radius >=1.8RJ orbiting a K dwarf star, and is the brightest best candidate detected from MACHO. Results from the 2.2m MPG/ESO telescope and FEROS (R=48,000) in May 2006 display an apparent radial velocity variation with amplitude ~650m/s with the same period as the transit, and a solar-type primary. This is consistent with an orbiting companion of mass ~4MJ. However, further observations display an additional secondary long-period variation with amplitude of several km/s, indicating the presence of a third body. The system is likely a low mass eclipsing binary orbiting the solar-type primary. Further observations are planned to fully characterize the system.

astro-ph↗

arXiv2006

Direct imaging of the young spectroscopic binary HD 160934

We report on the direct detection of a close companion to HD 160934, a young active star, SB1 spectroscopic binary, and suggested member of the AB Doradus moving group. High angular resolution at the Calar Alto 2.2m telescope was achieved by means of the Lucky Imaging technique, allowing direct imaging close to the diffraction limit in the SDSS z' band. Our results are combined with pre-discovery HST archive data, own UBVRI broadband photometry, published JHK magnitudes, and available radial velocity measurements to constrain the physical properties of the HD 160934 close binary. We suggest that the direct detection may be identical to the spectroscopically discovered companion.

astro-ph↗

arXiv2006

Improved age constraints for the AB Dor quadruple system - The binary nature of AB Dor B

We present resolved NACO photometry of the close binary AB Dor B in H- and Ks-band. AB Dor B is itself known to be a wide binary companion to AB Dor A, which in turn has a very low-mass close companion named AB Dor C. These four known components make up the young and dynamically interesting system AB Dor, which will likely become a benchmark system for calibrating theoretical pre-main sequence evolutionary mass tracks for low-mass stars. However, for this purpose the actual age has to be known, and this subject has been a matter of discussion in the recent scientific literature. We compare our resolved photometry of AB Dor Ba and Bb with theoretical and empirical isochrones in order to constrain the age of the system. This leads to an age estimate of about 50 to 100 Myr. We discuss the implications of such an age range for the case of AB Dor C, and compare with other results in the literature.

astro-ph↗

arXiv2006

Dust rings and filaments around the isolated young star V1331 Cygni

We characterize the small and large scale environment of the young star V1331 Cygni with high resolution HST/WFPC2 and Digitized Sky Survey images. In addition to a previously known outer dust ring (~30'' in diameter), the HST/WFPC2 scattered light image reveals an inner dust ring for the first time. This ring has a maximum radius of 6.5'' and is possibly related to a molecular envelope. Large-scale optical images show that V1331 Cyg is located at the tip of a long dust filament linking it to the dark cloud LDN 981. We discuss the origin of the observed dust morphology and analyze the object's relation to its parent dark cloud LDN 981. Finally, based on recent results from the literature, we investigate the properties of V1331 Cyg and conclude that in its current state the object does not show suffcient evidence to be characterized as an FU Ori object.

astro-ph↗

arXiv2006

Massive binaries in high-mass star-forming regions: A multi-epoch radial velocity survey of embedded O-stars

We present the first multi-epoch radial velocity study of embedded young massive stars using near-infrared spectra obtained with ISAAC mounted at the ESO Very Large Telescope, with the aim to detect massive binaries. Our 16 targets are located in high-mass star-forming regions and many of them are associated to known ultracompact HI I regions, whose young age ensures that dynamic evolution of the clusters did not influence the intrinsic binarity rate. We identify two stars with about 90 km/s velocity differences between two epochs proving the presence of close massive binaries. The fact that 2 out of the 16 observed stars are binary systems suggests that at least 20% of the young massive stars are formed in close multiple systems, but may also be consistent with most if not all young massive stars being binaries. In addition, we show that the radial velocity dispersion of the full sample is about 35 km/s, significantly larger than our estimated uncertainty (25 km/s). This finding is consistent with similar measurements of the young massive cluster 30 Dor which might have a high intrinsic binary rate. Furthermore, we argue that virial cluster masses derived from the radial velocity dispersion of young massive stars may intrinsically overestimate the cluster mass due to the presence of binaries.

astro-ph↗

arXiv2006

Atmosphere-like turbulence generation with surface-etched phase-screens

We built and characterized an optical system that emulates the optical characteristics of an 8m-class telescope like the Very Large Telescope. The system contains rotating glass phase-screens to generate realistic atmosphere-like optical turbulence, as needed for testing multi-conjugate adaptive optics systems. In this paper we present an investigation of the statistical properties of two phase-screens etched on glass-plate surfaces, obtained from Silios Technologies. Those etched screens are highly transmissive (above 85%) from 0.45 to 2.5 microns. From direct imaging, their Fried parameter r0 values (0.43+-0.04 mm and 0.81+-0.03 mm, respectively, at 0.633 microns) agree with the expectation to within 10%. This is also confirmed by a comparison of measured and expected Zernike coefficient variances. Overall, we find that those screens are quite reproducible, allowing sub-millimetre r0 values, which were difficult to achieve in the past. We conclude that the telescope emulator and phase-screens form a powerful atmospheric turbulence generator allowing systematic testing of different kinds of AO instrumentation.

physics.optics↗

arXiv2006

High-Resolution Spectroscopy in Tr37: Gas Accretion Evolution in Evolved Dusty Disks

Using the Hectochelle multifiber spectrograph, we have obtained high-resolution (R~34,000) spectra in the Halpha region for a large number of stars in the 4 Myr-old cluster Tr 37, containing 146 previously known members and 26 newly identified ones. We present the Halpha line profiles of all members, compare them to our IR observations of dusty disks (2MASS/JHK + IRAC + MIPS 24 micron), use the radial velocities as a membership criterion, and calculate the rotational velocities. We find a good correlation between the accretion-broadened profiles and the presence of protoplanetary disks, noting that a small fraction of the accreting stars presents broad profiles with Halpha equivalent widths smaller than the canonical limit separating CTTS and WTTS. The number of strong accretors appears to be lower than in younger regions, and a large number of CTTS have very small accretion rates (dM/dt<10^{-9} Msun/yr). Taking into account that the spectral energy distributions are consistent with dust evolution (grain growth/settling) in the innermost disk, this suggests a parallel evolution of the dusty and gaseous components. We also observe that about half of the "transition objects" (stars with no IR excesses at wavelengths shorter than ~6 micron) do not show any signs of active accretion, whereas the other half is accreting with accretion rates <10^{-9} Msun/yr. These zero or very low accretion rates reveal important gas evolution and/or gas depletion in the innermost disk, which could be related to grain growth up to planetesimal or even planet sizes. Finally, we examine the rotational velocities of accreting and non accreting stars, finding no significant differences that could indicate disk locking at these ages.

astro-ph↗

arXiv2006

Disks around young stars with VLTI/MIDI

We report on observations of circumstellar disks around young stars that have been obtained with the MIDI instrument, which is mounted on the VLT Interferometer and operates in the 10 micrometer atmospheric window. The maximum spatial resolution of 5 milli-arcsec corresponds to sub-AU scales at the distance to nearby star formation regions. Thus, we can study the disks on the spatial scales at which important processes occur, such as accretion, dust processing, and planet formation. The main results obtained so far can be summarized as follows: 1. The measured interferometric visibilities are in good qualitative agreement with those predicted by models of circumstellar disks. In particular, a predicted correlation between the strength of the far-infrared excess and the spatial structure of the disk is confirmed by direct measurements; 2. In several objects strong evidence for deviations from circular symmetry is present, indicating that an inclined disk is indeed the dominant component seen in the mid-infrared; 3. The dust properties are not uniform over the disk, but are instead a strong function of distance to the central star. The dust in the innermost disk regions is observed to be more ``processed'' than the dust further out, both in Herbig Ae star disks and in those around T-Tauri stars.

astro-ph↗