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Post Info TOPIC: S Stars
Blobrana


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Posts: 131433
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RE: S Stars
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Title: Clocks around Sgr A*
Author: Raymond Angélil, Prasenjit Saha

The S stars near the Galactic centre and any pulsars that may be on similar orbits, can be modelled in a unified way as clocks orbiting a black hole, and hence are potential probes of relativistic effects, including block-hole spin. The high eccentricities of many S stars mean that relativistic effects peak strongly around pericentre; for example, orbit precession is not a smooth effect but almost a kick at pericentre. We argue that concentration around pericentre will be an advantage when analysing redshift or pulse-arrival data to measure relativistic effects, because cumulative precession will be drowned out by Newtonian perturbations from other mass in the Galactic-centre region. Wavelet decomposition may be a way to disentangle relativistic effects from Newton perturbations. Assuming a plausible model for Newtonian perturbations on S2, relativity appears to be strongest in a two-year interval around pericentre, in wavelet modes of timescale approximately 6 months.

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Blobrana


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Posts: 131433
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Radio Stars
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Title: The Discovery of Radio Stars within 10 arcseconds of Sgr A* at 7mm
Author: F. Yusef-Zadeh, D. A. Roberts, H. Bushouse, M. Wardle, W. Cotton, M. Royster, G. van Moorsel

Very Large Array observations of the Galactic Center at 7 mm have produced an image of the 30 arcseconds surrounding Sgr A* with a resolution of 82x42 milliarcseconds (mas). A comparison with IR images taken simultaneously with the Very Large Telescope (VLT) identifies 41 radio sources with L-band (3.8 microns) stellar counterparts. The well-known young, massive stars in the central Sgr A* cluster (e.g., IRS 16C, IRS 16NE, IRS 16SE2, IRS 16NW, IRS 16SW, AF, AFNW, IRS 34W and IRS 33E) are detected with peak flux densities between 0.2 and 1.3 mJy. The origin of the stellar radio emission in the central cluster is discussed in terms of ionized stellar winds with mass-loss rates in the range 0.8-5x10^{-5} solar mass per year. Radio emission from eight massive stars is used as a tool for registration between the radio and infrared frames with mas precision within a few arcseconds of Sgr A*. This is similar to the established technique of aligning SiO masers and evolved stars except that radio stars lie within a few arcseconds of Sgr A*. Our data show a scatter of ~6.5 mas in the positions of the eight radio sources that appear in both the L-band and 7 mm images. Lastly, we use the radio and IR data to argue that members of IRS 13N are Young Stellar Objects rather than dust clumps, supporting the hypothesis that recent star formation has occurred near Sgr A*.

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Blobrana


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Posts: 131433
Date:
RE: S Stars
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Title: S-stars in the Galactic center and hypervelocity stars in the Galactic halo: two faces of the tidal breakup of stellar binaries by the central massive black hole?
Authors: Fupeng Zhang (NAOC), Youjun Lu (NAOC), Qingjuan Yu (KIAA)

In this paper, we investigate the link between the hypervelocity stars (HVSs) discovered in the Galactic halo and the S-stars moving in the Galactic center (GC), under the hypothesis that they are both the products of the tidal breakup of the same population of stellar binaries by the central massive black hole (MBH). By adopting several hypothetical models for binaries to be injected into the vicinity of the MBH and doing numerical simulations, we realise the tidal breakup processes of the binaries and their follow-up evolution. We find that many statistical properties of the detected HVSs and S-stars can be reproduced under some binary injecting models, and their number ratio can be re-produced if the stellar initial mass function is top-heavy (e.g., with slope ~-1.6). The total number of the captured companions is ~50 that have masses in the range ~3-7solar masses and semimajor axes <~4000 AU and survive to the present within their main-sequence lifetime. The innermost one is expected to have a semimajor axis ~300-1500 AU and a pericenter distance ~10-200 AU, with a significant probability of being closer to the MBH than S2. Future detection of such a closer star would offer an important test to general relativity. The majority of the surviving ejected companions of the S-stars are expected to be located at Galactocentric distances <~20 kpc, and have heliocentric radial velocities ~-500-1500 km/s and proper motions up to ~5-20 mas/yr. Future detection of these HVSs may provide evidence for the tidal-breakup formation mechanism of the S-stars.

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Blobrana


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Posts: 131433
Date:
S0-102
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TMT Will Take Discoveries of Stars Orbiting the Milky Way's Monster Black Hole to the Next Level

Researchers have discovered a star that whips around the giant black hole at the center of our galaxy in record time, completing an orbit every 11.5 years. The finding, appearing today in the journal Science, points ahead to groundbreaking experiments involving Einstein's general theory of relativity. Those tests will be fully enabled by the Thirty Meter Telescope (TMT), slated to begin observations next decade.
The record-setting star, called S0-102, was detected with the twin 10-meter telescopes at the W.M. Keck Observatory in Hawaii. For the past 17 years, the telescopes have imaged the galactic core, where a team of astronomers have hunted for stars with short orbital periods. These stars offer a never-before-possible test of how a supermassive black hole's gravity warps the fabric of space-time.
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Blobrana


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Posts: 131433
Date:
RE: S Stars
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Title: Accumulated Tidal Heating of Stars Over Multiple Pericenter Passages Near SgrA*
Authors: Gongjie Li, Abraham Loeb

We consider the long-term tidal heating of stars by the supermassive black hole at the Galactic center, SgrA*. We show that the accumulated heat deposited by excitation of stellar modes over many pericenter passages can lead to a runaway disruption of a star at a pericenter distance that is 7-10 times farther than the standard tidal disruption radius. The accumulated heating may explain the lack of massive (\gtrsim 10 solar masses) S-stars closer than several tens of AU from SgrA*.

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Blobrana


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Posts: 131433
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Title: Adaptive Optics Observations of the Galactic Center Young Stars
Authors: S. Yelda, A. M. Ghez, J. R. Lu, T. Do, L. Meyer, M. R. Morris

Adaptive Optics observations have dramatically improved the quality and versatility of high angular resolution measurements of the center of our Galaxy. In this paper, we quantify the quality of our Adaptive Optics observations and report on the astrometric precision for the young stellar population that appears to reside in a stellar disk structure in the central parsec. We show that with our improved astrometry and a 16 year baseline, including 10 years of speckle and 6 years of laser guide star AO imaging, we reliably detect accelerations in the plane of the sky as small as 70 microarcsec/yr/yr (~2.5 km/s/yr) and out to a projected radius from the supermassive black hole of 1.5" (~0.06 pc). With an increase in sensitivity to accelerations by a factor of ~6 over our previous efforts, we are able to directly probe the kinematic structure of the young stellar disk, which appears to have an inner radius of 0.8". We find that candidate disk members are on eccentric orbits, with a mean eccentricity of < e > = 0.30 ± 0.07. Such eccentricities cannot be explained by the relaxation of a circular disk with a normal initial mass function, which suggests the existence of a top-heavy IMF or formation in an initially eccentric disk.

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Blobrana


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Posts: 131433
Date:
S-Star Cluster
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Title: The S-Star Cluster at the Center of the Milky Way: On the nature of diffuse NIR emission in the inner tenth of a parsec
Authors: Nadeen Sabha, Andreas Eckart, David Merritt, Mohammad Zamaninasab, Gunther Witzel, Macarena García-Marín, Behrang Jalali, Monica Valencia-S., Senol Yazici, Rainer Buchholz, Banafsheh Shahzamanian, Christian Straubmeier

Sagittarius A*, the super-massive black hole at the center of the Milky Way, is surrounded by a small cluster of high velocity stars, known as the S-stars. We aim at constraining the amount and nature of the stellar and dark mass associated with the cluster in the immediate vicinity of Sagittarius A*. We use near-infrared imaging to determine the K_s-band luminosity function of the S-star cluster members, and use the distribution of the diffuse background emission and the stellar number density counts around the central black hole. This allows us to determine the stellar light and mass contribution that we can expect from the faint members of the cluster. We then use post-Newtonian N-body techniques to investigate the effect of stellar perturbations on the motion of S2, as a means of detecting the number and masses of the perturbers. We find that the stellar mass derived from the K_s-band luminosity extrapolation is much smaller than the amount of mass that might be present considering the uncertainties in the orbital motion of the star S2. Also the amount of light from the fainter S-cluster members is below the amount of residual light at the position of the S-star cluster after removing the bright cluster members. If the distribution of stars and stellar remnants is strongly enough peaked near Sagittarius A*, observed changes in the orbital elements of S2 can be used to constrain both their masses and numbers. Based on simulations of the cluster of high velocity stars we find that at a wavelength of 2.2 \mu m close to the confusion level for 8 m class telescopes blend stars will occur (preferentially near the position of Sagittarius A*) that last for typically 3 years before they dissolve due to proper motions.

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Blobrana


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Posts: 131433
Date:
Galactic Centre S-Stars
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Title: Explaining the Orbits of the Galactic Centre S-Stars
Authors: David Merritt, Alessia Gualandris, Seppo Mikkola

The young stars near the supermassive black hole at the galactic centre follow orbits that are nearly random in orientation and that have an approximately thermal distribution of eccentricities, N(e)~e. We show that both of these properties are a natural consequence of a few million years' interaction with an intermediate-mass black hole (IBH), if the latter's orbit is mildly eccentric and if its mass exceeds approximately 1500 solar masses. Producing the most tightly-bound S-stars requires an IBH orbit with periastron distance less than about 10 mpc. Our results provide support for a model in which the young stars are carried to the galactic centre while bound to an IBH, and are consistent with the hypothesis that an IBH may still be orbiting within the nuclear star cluster.

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Blobrana


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Posts: 131433
Date:
S Stars
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Title: Origin of the S Stars in the Galactic Centre
Authors: Ulf Löckmann, Holger Baumgardt, Pavel Kroupa
(Version v2)

Over the last 15 years, around a hundred very young stars have been observed in the central parsec of our Galaxy. While the presence of young stars forming one or two stellar disks at approx. 0.1 pc from the supermassive black hole (SMBH) can be understood through star formation in accretion disks, the origin of the S stars observed a factor of 10 closer to the SMBH has remained a major puzzle. Here we show the S stars to be a natural consequence of dynamical interaction of two stellar disks at larger radii. Due to precession and Kozai interaction, individual stars achieve extremely high eccentricities at random orientation. Stellar binaries on such eccentric orbits are disrupted due to close passages near the SMBH, leaving behind a single S star on a much tighter orbit. The remaining star may be ejected from the vicinity of the SMBH, thus simultaneously providing an explanation for the observed hypervelocity stars in the Milky Way halo.

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