arXiv subjects

Oliver Pfuhl

Publications and source records attributed to Oliver Pfuhl.

27 records · Page 2Linked to original sources

arXiv2014

On the Origin of Young Stars at the Galactic Center

The center of our galaxy is home to a massive black hole, SgrA*, and a nuclear star cluster containing stellar populations of various ages. While the late type stars may be too old to have retained memory of their initial orbital configuration, and hence formation mechanism, the kinematics of the early type stars should reflect their original distribution. In this contribution we present a new statistic which uses directly-observable kinematical stellar data to infer orbital parameters for stellar populations, and is capable of distinguishing between different origin scenarios. We use it on a population of B-stars in the Galactic center that extends out to large radii (0.5 pc) from the massive black hole. We find that the high K-magnitude population form an eccentric distribution, suggestive of a Hills binary-disruption origin.

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arXiv2014

On the origin of the B-stars in the Galactic center

We present a new directly-observable statistic which uses sky position and proper motion of stars near the Galactic center massive black hole to identify populations with high orbital eccentricities. It is most useful for stars with large orbital periods for which dynamical accelerations are difficult to determine. We apply this statistic to a data set of B-stars with projected radii 0."1 < p < 25" (~0.004 - 1 pc) from the massive black hole in the Galactic center. We compare the results with those from N-body simulations to distinguish between scenarios for their formation. We find that the scenarios favored by the data correlate strongly with particular K-magnitude intervals, corresponding to different zero-age main-sequence (MS) masses and lifetimes. Stars with 14 < mK < 15 (15 - 20 solar masses, t_{MS} = 8-13 Myr) match well to a disk formation origin, while those with mK > 15 (<15 solar masses, t_{MS} >13 Myr), if isotropically distributed, form a population that is more eccentric than thermal, which suggests a Hills binary-disruption origin.

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arXiv2013

Observations of the gas cloud G2 in the Galactic Center

In 2011, we discovered a compact gas cloud ("G2") with roughly three Earth masses that is falling on a near-radial orbit toward the massive black hole in the Galactic Center. The orbit is well constrained and pericenter passage is predicted for early 2014. Our data beautifully show that G2 gets tidally sheared apart due to the massive black hole's force. During the next months, we expect that in addition to the tidal effects, hydrodynamics get important, when G2 collides with the hot ambient gas around Sgr A*. Simulations show that ultimately, the cloud's material might fall into the massive black hole. Predictions for the accretion rate and luminosity evolution, however, are very difficult due to the many unknowns. Nevertheless, this might be a unique opportunity in the next years to observe how gas feeds a massive black hole in a galactic nucleus.

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arXiv2013

The Nuclear Cluster of the Milky Way: Total Mass and Luminosity

Here we present the fundamental properties of the nuclear cluster of the Milky Way. First, we derive its structural properties by constructing a density map of the central 1000'' using extinction-corrected star counts. We can describe the data with a two-component model built from Sersic profiles. The inner nearly spherical component is the nuclear cluster. The outer, strongly flattened component can be identified with the stellar component of the circumnuclear zone. Second, we enlarge the radius inside which detailed dynamics are available from $1\,$pc to $4\,$pc. We use more than 10000 individual proper motions and more than 2700 radial velocities. We determine the cluster mass by means of isotropic spherical Jeans modeling. We get a nuclear cluster mass within 100'' of $M_{100''}=(6.11 \pm 0.52|_{\mathrm{fix} R_0}\pm 0.97|_{R_0}) \times 10^6 $ M$_{\odot}$, which corresponds to a total cluster mass of M$_{\mathrm{NC}}=(13.08 \pm 2.51|_{\mathrm{fix} R_0}\pm 2.08|_{R_0}) \times 10^6 $ M$_{\odot}$. By combination of our mass with the flux we calculate $M/L=0.50 \pm 0.12 M_{\odot}/L_{\odot,\mathrm{Ks}}$ for the central 100''. That is broadly consistent with a Chabrier IMF. With its mass and a luminosity of M$_{\mathrm{Ks}}=-15.30\pm0.26$ the nuclear cluster is a bright and massive specimen with a typical size.

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arXiv2013

Constraining the dark cusp in the Galactic Center by long-period binaries

Massive black holes (MBHs) in galactic nuclei are believed to be surrounded by a high density stellar cluster, whose mass is mostly in hard-to-detect faint stars and compact remnants. Such dark cusps dominate the dynamics near the MBH: a dark cusp in the Galactic center (GC) of the Milky Way would strongly affect orbital tests of General Relativity there; on cosmic scales, dark cusps set the rates of gravitational wave emission events from compact remnants that spiral into MBHs, and they modify the rates of tidal disruption events, to list only some implications. A recently discovered long-period massive young binary (P_12 <~ 1 yr, M_12 ~ O(100 M_sun), T_12 ~ 6x10^6 yr), only ~0.1 pc from the Galactic MBH (Pfuhl et al 2013), sets a lower bound on the 2-body relaxation timescale there, min t_rlx ~ (P_12/M_12)^(2/3)T_12 ~ 10^7 yr, and correspondingly, an upper bound on the stellar number density, max n ~ few x 10^8/ 1/pc^3, based on the binary's survival against evaporation by the dark cusp. However, a conservative dynamical estimate, the drain limit, implies t_rlx > O(10^8) yr. Such massive binaries are thus too short-lived and tightly bound to constrain a dense relaxed dark cusp. We explore here in detail the use of longer-period, less massive and longer-lived binaries (P_12 ~ few yr, M_12 ~ 2-4 M_sun, T_12 ~ 10^8-10^10 yr), presently just below the detection threshold, for probing the dark cusp, and develop the framework for translating their future detections among the giants in the GC into dynamical constraints.

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arXiv2013

Pericenter passage of the gas cloud G2 in the Galactic Center

We have further followed the evolution of the orbital and physical properties of G2, the object currently falling toward the massive black hole in the Galactic Center on a near-radial orbit. New, very sensitive data were taken in April 2013 with NACO and SINFONI at the ESO VLT . The 'head' of G2 continues to be stretched ever further along the orbit in position-velocity space. A fraction of its emission appears to be already emerging on the blue-shifted side of the orbit, past pericenter approach. Ionized gas in the head is now stretched over more than 15,000 Schwarzschild radii RS around the pericenter of the orbit, at ~ 2000 RS ~ 20 light hours from the black hole. The pericenter passage of G2 will be a process stretching over a period of at least one year. The Brackett-γ luminosity of the head has been constant over the past 9 years, to within +- 25%, as have the line ratios Brackett-γ / Paschen-α and Brackett-γ / Helium-I. We do not see any significant evidence for deviations of G2's dynamical evolution, due to hydrodynamical interactions with the hot gas around the black hole, from a ballistic orbit of an initially compact cloud with moderate velocity dispersion. The constant luminosity and the increasingly stretched appearance of the head of G2 in the position-velocity plane, without a central peak, is not consistent with several proposed models with continuous gas release from an initially bound zone around a faint star on the same orbit as G2.

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arXiv2011

Line derived infrared extinction towards the Galactic Center

We derive the extinction curve towards the Galactic Center from 1 to 19 micron. We use hydrogen emission lines of the minispiral observed by ISO-SWS and SINFONI. The extinction free flux reference is the 2 cm continuum emission observed by the VLA. Towards the inner 14" * 20" we find an extinction of A(2.166 micron)=2.62 +/- 0.11, with a power-law slope of alpha=-2.11 +/- 0.06 shortward of 2.8 micron, consistent with the average near infrared slope from the recent literature. At longer wavelengths, however, we find that the extinction is grayer than shortward of 2.8 micron. We find it is not possible to fit the observed extinction curve with a dust model consisting of pure carbonaceous and silicate grains only, and the addition of composite particles, including ices, is needed to explain the observations. Combining a distance dependent extinction with our distance independent extinction we derive the distance to the GC to be R_0=7.94 +/- 0.65 kpc. Towards Sgr A* (r<0.5") we obtain A_H=4.21 +/- 0.10, A_Ks=2.42 +/- 0.10 and A_L'=1.09 +/- 0.13.

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arXiv2010

GC-IRS13E - A puzzling association of three early-type stars

We present a detailed analysis of high resolution near-infrared imaging and spectroscopy of the potential star cluster IRS13E very close to the massive black hole in the Galactic Center. We detect 19 objects in IRS13E from Ks-band images, 15 of which are also detected reliably in H-band. We derive consistent proper motions for these objects from the two bands. Most objects share a similar westward proper motion. We characterize the objects using spectroscopy (1.45 to 2.45 micrometer) and (narrow-band) imaging from H- (1.66 mircrometer) to L'-band (3.80 micrometer). Nine of the objects detected in both Ks- and H-band are very red, and we find that they are all consistent with being warm dust clumps. The dust emission may be caused by the colliding winds of the two Wolf-Rayet stars in the cluster. Three of the six detected stars do not share the motion or spectral properties of the three bright stars. This leaves only the three bright, early-type stars as potential cluster members. It is unlikely that these stars are a chance configuration. Assuming the presence of an IMBH, a mass of about 14000 solar masses follows from the velocities and positions of these three stars. However, our acceleration limits make such an IMBH nearly as unlikely as a chance occurrence of such a star association. Furthermore, there is no variable X-ray source in IRS13E despite the high density of dust and gas. Therefore, we conclude that is unlikely that IRS13E hosts a black hole massive enough to bind the three stars.

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arXiv2009

What is limiting near-infrared astrometry in the Galactic Center?

We systematically investigate the error sources for high-precision astrometry from adaptive optics based near-infrared imaging data. We focus on the application in the crowded stellar field in the Galactic Center. We show that at the level of <=100 micro-arcseconds a number of effects are limiting the accuracy. Most important are the imperfectly subtracted seeing halos of neighboring stars, residual image distortions and unrecognized confusion of the target source with fainter sources in the background. Further contributors to the error budget are the uncertainty in estimating the point spread function, the signal-to-noise ratio induced statistical uncertainty, coordinate transformation errors, the chromaticity of refraction in Earth's atmosphere, the post adaptive optics differential tilt jitter and anisoplanatism. For stars as bright as mK=14, residual image distortions limit the astrometry, for fainter stars the limitation is set by the seeing halos of the surrounding stars. In order to improve the astrometry substantially at the current generation of telescopes, an adaptive optics system with high performance and weak seeing halos over a relatively small field (r<=3") is suited best. Furthermore, techniques to estimate or reconstruct the seeing halo could be promising.

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