arXiv subjects

T. Ott

Publications and source records attributed to T. Ott.

At least 163 records · Page 9Linked to original sources

arXiv2008

Kinematics of the old stellar population at the Galactic Center

We aim at a detailed description of the kinematic properties of the old, (several Gyrs) late-type CO-absorption star population among the Galactic centre (GC) cluster stars. We applied AO-assisted, near-infrared imaging and integral-field spectroscopy using the instruments NAOS/CONICA and SINFONI at the VLT. We obtained proper motions for 5445 stars, 3D velocities for 664 stars, and acceleration limits (in the sky plane) for 750 stars. We detect for the first time significant cluster rotation in the sense of the general Galactic rotation in proper motions. Out of the 3D velocity dispersion, we derive an improved statistical parallax for the GC of R0 = 8.07 +/- 0.32 (stat) +/- 0.13 (sys) kpc. The distribution of 3D stellar speeds can be approximated by local Maxwellian distributions. Kinematic modelling provides deprojected 3D kinematic parameters, including the mass profile of the cluster. We find an upper limit of 4% for the amplitude of fluctuations in the phase-space distribution of the cluster stars compared to a uniform, spherical model cluster. Using upper limits on accelerations, we constrain the minimum line-of-sight distances from the plane of SgrA* of five stars located within the innermost few (projected) arcsec. The stars within 0.7'' radius from the star group IRS13E do not co-move with this group, making it unlikely that IRS13E is the core of a substantial star cluster. Overall, the GC late-type cluster is described well as a uniform, isotropic, rotating, dynamically relaxed, phase-mixed system.

astro-ph↗

arXiv2008

Multiplicity of young stars in and around R Corona Australis

The R CrA star-forming region has a small dark cloud with quite a number of protostars, T Tauri stars, and some Herbig Ae/Be stars, plus a number of weak-line T Tauri stars around the cloud found by ROSAT follow-up observations. We searched for multiples among the young stars in and around the R CrA cloud in order to investigate multiplicity in this region. We performed interferometric and imaging observations with the speckle camera SHARP I at the ESO 3.5m NTT and adaptive optics observation with ADONIS at the ESO 3.6m telescope, all in the near-infrared bands JHK obtained in the years 1995, 2000, and 2001. We found 13 new binaries among the young stars in CrA between 0.13 arcsec (the diffraction limit) and 6 arcsec (set as an upper separation limit to avoid contamination by chance alignments). While most multiples in CrA are binaries, there are also one quadruple (TY CrA), and one triple (HR 7170) which may form a quintuple together with the binary HR 7169. One of the newly detected companions with a large magnitude difference found near the M3-5 type T Tauri star [MR 81] Ha 17 could be a brown dwarf or an infrared companion with an edge-on disk. Among seven Herbig Ae/Be stars in CrA, six are multiple. The multiplicity frequency in CrA is as high as in similar star forming regions. By comparing with the period distribution of main-sequence stars and extrapolating to separations not probed in this survey, we conclude that the companion-star frequency is 95+/-23 %; i.e. the average number of companions per primary is 0.95.

astro-ph↗

arXiv2008

From rings to bulges: evidence for rapid secular galaxy evolution at z~2 from integral field spectroscopy in the SINS survey

We present Ha integral field spectroscopy of well resolved, UV/optically selected z~2 star-forming galaxies as part of the SINS survey with SINFONI on the ESO VLT. Our laser guide star adaptive optics and good seeing data show the presence of turbulent rotating star forming rings/disks, plus central bulge/inner disk components, whose mass fractions relative to total dynamical mass appears to scale with [NII]/Ha flux ratio and star formation age. We propose that the buildup of the central disks and bulges of massive galaxies at z~2 can be driven by the early secular evolution of gas-rich proto-disks. High redshift disks exhibit large random motions. This turbulence may in part be stirred up by the release of gravitational energy in the rapid cold accretion flows along the filaments of the cosmic web. As a result dynamical friction and viscous processes proceed on a time scale of <1 Gyr, at least an order of magnitude faster than in z~0 disk galaxies. Early secular evolution thus drives gas and stars into the central regions and can build up exponential disks and massive bulges, even without major mergers. Secular evolution along with increased efficiency of star formation at high surface densities may also help to account for the short time scales of the stellar buildup observed in massive galaxies at z~2.

astro-ph↗

arXiv2008

Superdiffusion in quasi-two-dimensional Yukawa liquids

The emergence and vanishing of superdiffusion in quasi-two-dimensional Yukawa systems are investigated by molecular dynamics simulations. Using both the asymptotic behaviour of the mean-squared displacement of the particles and the long-time tail of the velocity autocorrelation function as indicators for superdiffusion, we confirm the existence of a transition from normal diffusion to superdiffusion in systems changing from a three-dimensional to a two-dimensional character. A connection between superdiffusion and dimensionality is established by the behaviour of the projected pair distribution function.

physics.plasm-ph↗

arXiv2008

Near-infrared polarization images of the Orion proplyds

We performed AO-assisted high-resolution imaging polarimetry on selected Orion proplyds close to the Trapezium stars in the J, H, and K bands. Differential polarimetric images of one of the largest and brightest proplyds are interpreted using 3D radiation transfer simulations based on the Monte Carlo method. Although not fully resolvable by ground-based observations, the circumstellar material can be mapped with polarimetry. We present constraints on the disk parameters of the giant proplyd 177-341. We tested whether dust models with different grain size distributions could explain the observed extent of the polarization patterns and find that simple models with larger grains will not reproduce the spectral polarization behavior. The technique of polarimetric differential imaging (PDI) in the NIR provides a good opportunity to study the structure of the Orion proplyds.

astro-ph↗

arXiv2007

On the nature of the fast moving star S2 in the Galactic Center

We analyze the properties of the star S2 orbiting the supermassive black hole at the center of the Galaxy. A high quality SINFONI H and K band spectrum obtained from coadding 23.5 hours of observation between 2004 and 2007 reveals that S2 is an early B dwarf (B0-2.5V). Using model atmospheres, we constrain its stellar and wind properties. We show that S2 is a genuine massive star, and not the core of a stripped giant star as sometimes speculated to resolve the problem of star formation so close to the supermassive black hole. We give an upper limit on its mass loss rate, and show that it is He enriched, possibly because of the presence of a magnetic field.

astro-ph↗

arXiv2007

The most massive stars in the Arches cluster

We study a sample composed of 28 of the brightest stars in the Arches cluster. We analyze K-band spectra obtained with the integral field spectrograph SINFONI on the VLT. Atmosphere models computed with the code CMFGEN are used to derive the effective temperatures, luminosities, stellar abundances, mass loss rates and wind terminal velocities. We find that the stars in our sample are either H-rich WN7-9 stars (WN7-9h) or O supergiants, two being classified as OIf+. All stars are 2-4 Myr old. There is marginal evidence for a younger age among the most massive stars. The WN7-9h stars reach luminosities as large as 2 x 1e6 Lsun, consistent with initial masses of ~ 120 Msun. They are still quite H-rich, but show both N enhancement and C depletion. They are thus identified as core H-burning objects showing products of the CNO equilibrium at their surface. Their progenitors are most likely supergiants of spectral types earlier than O4-6 and initial masses > 60 Msun. Their winds follow a well defined modified wind momentum - luminosity relation (WLR): this is a strong indication that they are radiatively driven. Stellar abundances tend to favor a slightly super solar metallicity, at least for the lightest metals. We note however that the evolutionary models seem to under-predict the degree of N enrichment.

astro-ph↗

arXiv2007

Evidence for a Long-Standing Top-Heavy IMF in the Central Parsec of the Galaxy

We classify 329 late-type giants within 1 parsec of Sgr A*, using the adaptive optics integral field spectrometer SINFONI on the VLT. These observations represent the deepest spectroscopic data set so far obtained for the Galactic Center, reaching a 50% completeness threshold at the approximate magnitude of the helium-burning red clump (Ks ~ 15.5 mag.). Combining our spectroscopic results with NaCo H and Ks photometry, we construct an observed Hertzsprung-Russell diagram, which we quantitatively compare to theoretical distributions of various star formation histories of the inner Galaxy, using a chi-squared analysis. Our best-fit model corresponds to continuous star formation over the last 12 Gyr with a top-heavy initial mass function (IMF). The similarity of this IMF to the IMF observed for the most recent epoch of star formation is intriguing and perhaps suggests a connection between recent star formation and the stars formed throughout the history of the Galactic Center.

astro-ph↗

arXiv2007

The Position of Sagittarius A*: III. Motion of the Stellar Cusp

In the first two papers of this series, we determined the position of Sgr A* on infrared images, by aligning the positions of red giant stars with positions measured at radio wavelengths for their circumstellar SiO masers. In this paper, we report detections of 5 new stellar SiO masers within 50" (2 pc) of Sgr A* and new and/or improved positions and proper motions of 15 stellar SiO masers. The current accuracies are ~1 mas in position and ~0.3 mas/y in proper motion. We find that the proper motion of the central stellar cluster with respect to Sgr A* is less than 45 km/s. One star, IRS 9, has a three-dimensional speed of ~370 km/s at a projected distance of 0.33 pc from Sgr A*. If IRS 9 is bound to the inner parsec, this requires an enclosed mass that exceeds current estimates of the sum of the mass of Sgr A* and luminous stars in the stellar cusp by ~0.8 x 10^6 Msun. Possible explanations include i) that IRS 9 is not bound to the central parsec and has "fallen" from a radius greater than 9 pc, ii) that a cluster of dark stellar remnants accounts for some of the excess mass, and/or iii) that Ro is considerably greater than 8 kpc.

astro-ph↗

arXiv2007

Stellar and wind properties of massive stars in the central parsec of the Galaxy

We study the stellar and wind properties of massive stars in the central cluster of the Galaxy. We use non-LTE atmosphere models including winds and line-blanketing to fit their H and K band spectra obtained with the 3D spectrograph SINFONI on the VLT. We derive the main stellar (Teff, L, abundances, ionizing flux) and wind (mass loss rate, terminal velocity) properties. They are found to be similar to other galactic massive stars. We show that a direct evolutionary link between Ofpe/WN9, WN8 and WN/C stars exists. Using individual SEDs for each massive star, we construct the total spectral energy distribution of the cluster and use it to compute photoionization models. We show that the nebular properties of the central HII region are well reproduced. We conclude that, contrary to previous claims, standard stellar evolution and atmosphere models are well suited to explain the properties of the central cluster. Our results indicate that massive stars in the central cluster do not have a peculiar evolution as could be expected from their proximity to the supermassive black hole SgrA*.

astro-ph↗

arXiv2007

The structure of the nuclear stellar cluster of the Milky Way

We present high-resolution seeing limited and AO NIR imaging observations of the stellar cluster within about one parsec of Sgr A*, the massive black hole at the centre of the Milky Way. Stellar number counts and the diffuse background light density were extracted from these observations in order to examine the structure of the nuclear stellar cluster.Our findings are as follows: (a) A broken-power law provides an excellent fit to the overall structure of the GC nuclear cluster. The power-law slope of the cusp is $Γ=0.19\pm0.05$, the break radius is $R_{\rm break} = 6.0'' \pm 1.0''$ or $0.22\pm0.04$ pc, and the cluster density decreases with a power-law index of $Γ=0.75\pm0.1$ outside of $R_{\rm break}$. (b) Using the best velocity dispersion measurements from the literature, we derive higher mass estimates for the central parsec than assumed until now. The inferred density of the cluster at the break radius is $2.8\pm1.3\times 10^{6} {\rm M_{\odot} pc^{-3}}$. This high density agrees well with the small extent and flat slope of the cusp. Possibly, the mass of the stars makes up only about 50% of the total cluster mass. (c) Possible indications of mass segregation in the cusp are found (d) The cluster appears not entirely homogeneous. Several density clumps are detected that are concentrated at projected distances of $R=3''$ and $R=7''$ from Sgr A*.(e) There appears to exist an under-density of horizontal branch/red clump stars near $R=5''$, or an over-density of stars of similar brightness at $R=3''$ and $R=7''$. (f) The extinction map in combination with cometary-like features in an L'-band image may provide support for the assumption of an outflow from Sgr A*.

astro-ph↗

arXiv2006

A polarised infrared flare from Sagittarius A* and the signatures of orbiting plasma hotspots

In this article we summarise and discuss the infrared, radio, and X-ray emission from the supermassive black hole in the Galactic Centre, SgrA*. We include new results from near-infrared polarimetric imaging observations obtained on May 31st, 2006. In that night, a strong flare in Ks band (2.08 microns) reaching top fluxes of ~16 mJy could be observed. This flare was highly polarised (up to ~40%) and showed clear sub-structure on a time scale of 15 minutes, including a swing in the polarisation angle of about 70 degrees. For the first time we were able to observe both polarised flux and short-time variability, with high significance in the same flare event. This result adds decisive information to the puzzle of the SgrA* activity. The observed polarisation angle during the flare peak is the same as observed in two events in 2004 and 2005. Our observations strongly support the dynamical emission model of a decaying plasma hotspot orbiting SgrA* on a relativistic orbit. The observed polarisation parameters and their variability with time might allow to constrain the orientation of accretion disc and spin axis with respect to the Galaxy.

astro-ph↗

arXiv2006

Polarimetry of near-infrared flares from Sagittarius A*

Context: We report new polarization measurements of the variable near-infrared emission of the SgrA* counterpart associated with the massive 3--4 10^6 solar masses Black Hole at the Galactic Center. Aims: We investigate the physical processes responsible for the variable emission from SgrA*. Methods: The observations have been carried out using the NACO adaptive optics (AO) instrument at the European Southern Observatory's Very Large Telescope Results: We find that the variable NIR emission of SgrA* is highly polarized and consists of a contribution of a non- or weakly polarized main flare with highly polarized sub-flares. The flare activity shows a possible quasi-periodicity of 20+-3 minutes consistent with previous observations. Conclusions: The highly variable and polarized emission supports that the NIR emission is non-thermal. The observations can be interpreted in a jet or temporary disk model. In the disk model the quasi-periodic flux density variations can be explained due to spots on relativistic orbits around the central MBH. Alternative explanations for the high central mass concentration involving boson or fermion balls are increasingly unlikely.

astro-ph↗

arXiv2006

GCIRS16SW: a massive eclipsing binary in the Galactic Center

We report on the spectroscopic monitoring of GCIRS16SW, an Ofpe/WN9 star and LBV candidate in the central parsec of the Galaxy. SINFONI observations show strong daily spectroscopic changes in the K band. Radial velocities are derived from the HeI 2.112 um line complex and vary regularly with a period of 19.45 days, indicating that the star is most likely an eclipsing binary. Under various assumptions, we are able to derive a mass of ~ 50 Msun for each component.

astro-ph↗

arXiv2006

Clockwise Stellar Disk and the Dark Mass in the Galactic Center

Two disks of young stars have recently been discovered in the Galactic Center. The disks are rotating in the gravitational field of the central black hole at radii r=0.1-0.3 pc and thus open a new opportunity to measure the central mass. We find that the observed motion of stars in the clockwise disk implies M=4.3+/-0.5 million solar masses for the fiducial distance to the Galactic Center R_0=8 kpc and derive the scaling of M with R_0. As a tool for our estimate we use orbital roulette, a recently developed method. The method reconstructs the three-dimensional orbits of the disk stars and checks the randomness of their orbital phases. We also estimate the three-dimensional positions and orbital eccentricities of the clockwise-disk stars.

astro-ph↗

arXiv2006

Variations in the spectral slope of Sgr A* during a NIR flare

We have observed a bright flare of Sgr A* in the near infrared with the adaptive optics assisted integral field spectrometer SINFON. Within the uncertainties, the observed spectrum is featureless and can be described by a power law. Our data suggest that the spectral index is correlated with the instantaneous flux and that both quantities experience significant changes within less than one hour. We argue that the near infrared flares from Sgr A* are due to synchrotron emission of transiently heated electrons, the emission being affected by orbital dynamics and synchrotron cooling, both acting on timescales of ~20 minutes.

astro-ph↗

arXiv2006

The Two Young Star Disks in the Central Parsec of the Galaxy: Properties, Dynamics and Formation

We report the definite spectroscopic identification of 41 OB supergiants, giants and main sequence stars in the central parsec of the Galaxy. Detection of their absorption lines have become possible with the high spatial and spectral resolution and sensitivity of the adaptive optics integral field spectrometer SPIFFI/SINFONI on the ESO VLT. Several of these OB stars appear to be helium and nitrogen rich. Almost all of the ~80 massive stars now known in the central parsec (central arcsecond excluded) reside in one of two somewhat thick (<|h|/R>~0.14) rotating disks. These stellar disks have fairly sharp inner edges (R~1") and surface density profiles that scale as R^{-2}. We do not detect any OB stars outside the central 0.5 pc. The majority of the stars in the clockwise system appear to be on almost circular orbits, whereas most of those in the `counter-clockwise' disk appear to be on eccentric orbits. Based on its stellar surface density distribution and dynamics we propose that IRS 13E is an extremely dense cluster (core density > 3x10^8 sunmass/pc^3), which has formed in the counter-clockwise disk. The stellar contents of both systems are remarkably similar, indicating a common age of ~6+/-2 Myr. The K-band luminosity function of the massive stars suggests a top-heavy mass function and limits the total stellar mass contained in both disks to ~1.5x10^4 sunmass. Our data strongly favor in situ star formation from dense gas accretion disks for the two stellar disks. This conclusion is very clear for the clockwise disk and highly plausible for the counter-clockwise system.

astro-ph↗

arXiv2006

Atmospheric Turbulence Compensation with Laser Phase Shifting Interferometry

Laser guide stars with adaptive optics allow astronomical image correction in the absence of a natural guide star. Single guide star systems with a star created in the earth's sodium layer can be used to correct the wavefront in the near infrared spectral regime for 8-m class telescopes. For possible future telescopes of larger sizes, or for correction at shorter wavelengths, the use of a single guide star is ultimately limited by focal anisoplanatism that arises from the finite height of the guide star. To overcome this limitation we propose to overlap coherently pulsed laser beams that are expanded over the full aperture of the telescope, traveling upwards along the same path which light from the astronomical object travels downwards. Imaging the scattered light from the resultant interference pattern with a camera gated to a certain height above the telescope, and using phase shifting interferometry we have found a method to retrieve the local wavefront gradients. By sensing the backscattered light from two different heights, one can fully remove the cone effect, which can otherwise be a serious handicap to the use of laser guide stars at shorter wavelengths or on larger telescopes. Using two laser beams multiconjugate correction is possible, resulting in larger corrected fields. With a proper choice of laser, wavefront correction could be expanded to the visible regime and, due to the lack of a cone effect, the method is applicable to any size of telescope. Finally the position of the laser spot could be imaged from the side of the main telescope against a bright background star to retrieve tip-tilt information, which would greatly improve the sky coverage of the system.

astro-ph↗