284 research outputs found
The distance to the Galactic Centre based on Population-II Cepheids and RR Lyrae stars
Context: The distance to the Galactic Centre (GC) is of importance for the
distance scale in the Universe. The value derived by Eisenhauer et al. (2005)
of 7.62 +- 0.32 kpc based on the orbit of one star around the central black
hole is shorter than most other distance estimates based on a variety of
different methods. Aim: To establish an independent distance to the GC with
high accuracy. To this end Population-II Cepheids are used that have been
discovered in the OGLE-II and III surveys. Method: Thirty-nine Pop-II Cepheids
have been monitored on 4 nights spanning 14 days. Light curves have been fitted
using the known periods from the OGLE data to determine the mean K-band
magnitude. It so happens that 37 RR Lyrae stars are in the field-of-views and
mean K-band magnitudes are derived for this sample as well. Results: The
period-luminosity relation of Pop-II Cepheids in the K-band is determined, and
the derived slope of -2.24 +- 0.14 is consistent with the value derived by
Matsunaga et al. (2006). Fixing the slope to their more accurate value results
in a zero point, and implies a distance modulus to the GC of 14.51 +- 0.12,
with an additional systematic uncertainty of 0.07 mag. Similarly, from the RR
Lyrae K-band PL-relation we derive a value of 14.48 +- 0.17 (random) +- 0.07
(syst.). The two independent determinations are averaged to find 14.50 +- 0.10
(random) +- 0.07 (syst.), or 7.94 +- 0.37 +- 0.26 kpc.Comment: A&A accepte
Time Dependent Models of Flares from Sagittarius A*
The emission from Sgr A*, the supermassive black hole in the Galactic Center,
shows order of magnitude variability ("flares") a few times a day that is
particularly prominent in the near-infrared (NIR) and X-rays. We present a
time-dependent model for these flares motivated by the hypothesis that
dissipation of magnetic energy powers the flares. We show that episodic
magnetic reconnection can occur near the last stable circular orbit in
time-dependent magnetohydrodynamic simulations of black hole accretion - the
timescales and energetics of these events are broadly consistent with the
flares from Sgr A*. Motivated by these results, we present a spatially one-zone
time-dependent model for the electron distribution function in flares,
including energy loss due to synchrotron cooling and adiabatic expansion.
Synchrotron emission from transiently accelerated particles can explain the
NIR/X-ray lightcurves and spectra of a luminous flare observed 4 April 2007. A
significant decrease in the magnetic field strength during the flare
(coincident with the electron acceleration) is required to explain the
simultaneity and symmetry of the simultaneous lightcurves. Our models predict
that the NIR and X-ray spectral indices differ by 0.5 and that there is only
modest variation in the spectral index during flares. We also explore
implications of this model for longer wavelength (radio-submm) emission
seemingly associated with X-ray and NIR flares; we argue that a few hour
decrease in the submm emission is a more generic consequence of large-scale
magnetic reconnection than delayed radio emission from adiabatic expansion.Comment: 18 pages, 10 figures, ApJ accepte
On the dissolution of star clusters in the Galactic centre. I. Circular orbits
We present N-body simulations of dissolving star clusters close to galactic
centres. For this purpose, we developed a new N-body program called nbody6gc
based on Aarseth's series of N-body codes. We describe the algorithm in detail.
We report about the density wave phenomenon in the tidal arms which has been
recently explained by Kuepper et al. (2008). Standing waves develop in the
tidal arms. The wave knots or clumps develop at the position, where the
emerging tidal arm hits the potential wall of the effective potential and is
reflected. The escaping stars move through the wave knots further into the
tidal arms. We show the consistency of the positions of the wave knots with the
theory in Just et al. (2009). We also demonstrate a simple method to study the
properties of tidal arms. By solving many eigenvalue problems along the tidal
arms, we construct numerically a 1D coordinate system whose direction is always
along a principal axis of the local tensor of inertia. Along this coordinate
system, physical quantities can be evaluated. The half-mass or dissolution
times of our models are almost independent of the particle number which
indicates that two-body relaxation is not the dominant mechanism leading to the
dissolution. This may be a typical situation for many young star clusters. We
propose a classification scheme which sheds light on the dissolution mechanism.Comment: 18 pages, 20 figures; accepted by MNRA
Star Formation in the Starburst Cluster in NGC 3603
We have used new, deep, visible and near infrared observations of the compact
starburst cluster in the giant HII region NGC 3603 and its surroundings with
the WFC3 on HST and HAWK-I on the VLT to study in detail the physical
properties of its intermediate mass (~ 1 - 3 M_sun) stellar population. We show
that after correction for differential extinction and actively accreting stars,
and the study of field star contamination, strong evidence remains for a
continuous spread in the ages of pre-main sequence stars in the range ~ 2 to ~
30 Myr within the temporal resolution available. Existing differences among
presently available theoretical models account for the largest possible
variation in shape of the measured age histograms within these limits. We also
find that this isochronal age spread in the near infrared and visible
Colour-Magnitude Diagrams cannot be reproduced by any other presently known
source of astrophysical or instrumental scatter that could mimic the luminosity
spread seen in our observations except, possibly, episodic accretion. The
measured age spread and the stellar spatial distribution in the cluster are
consistent with the hypothesis that star formation started at least 20-30 Myrs
ago progressing slowly but continuously up to at least a few million years ago.
All the stars in the considered mass range are distributed in a flattened
oblate spheroidal pattern with the major axis oriented in an approximate
South-East - North-West direction, and with the length of the equatorial axis
decreasing with increasing age. This asymmetry is most likely due to the fact
that star formation occurred along a filament of gas and dust in the natal
molecular cloud oriented locally in this direction.Comment: 21 pages, 19 figures, accepted for publication in Astrophysics &
Space Scienc
The impact of realistic models of mass segregation on the event rate of extreme-mass ratio inspirals and cusp re-growth
One of the most interesting sources of gravitational waves (GWs) for LISA is
the inspiral of compact objects on to a massive black hole (MBH), commonly
referred to as an "extreme-mass ratio inspiral" (EMRI). The small object,
typically a stellar black hole (bh), emits significant amounts of GW along each
orbit in the detector bandwidth. The slowly, adiabatic inspiral of these
sources will allow us to map space-time around MBHs in detail, as well as to
test our current conception of gravitation in the strong regime. The event rate
of this kind of source has been addressed many times in the literature and the
numbers reported fluctuate by orders of magnitude. On the other hand, recent
observations of the Galactic center revealed a dearth of giant stars inside the
inner parsec relative to the numbers theoretically expected for a fully relaxed
stellar cusp. The possibility of unrelaxed nuclei (or, equivalently, with no or
only a very shallow cusp) adds substantial uncertainty to the estimates. Having
this timely question in mind, we run a significant number of direct-summation
body simulations with up to half a million particles to calibrate a much
faster orbit-averaged Fokker-Planck code. We then investigate the regime of
strong mass segregation (SMS) for models with two different stellar mass
components. We show that, under quite generic initial conditions, the time
required for the growth of a relaxed, mass segregated stellar cusp is shorter
than a Hubble time for MBHs with
(i.e. nuclei in the range of LISA). SMS has a significant impact boosting the
EMRI rates by a factor of for our fiducial models of Milky Way type
galactic nuclei.Comment: Accepted by CQG, minor changes, a bit expande
The central black hole mass of the high-sigma but low-bulge-luminosity lenticular galaxy NGC 1332
The masses of the most massive supermassive black holes (SMBHs) predicted by
the M_BH-sigma and M_BH-luminosity relations appear to be in conflict. Which of
the two relations is the more fundamental one remains an open question. NGC
1332 is an excellent example that represents the regime of conflict. It is a
massive lenticular galaxy which has a bulge with a high velocity dispersion
sigma of ~320 km/s; bulge--disc decomposition suggests that only 44% of the
total light comes from the bulge. The M_BH-sigma and the M_BH-luminosity
predictions for the central black hole mass of NGC 1332 differ by almost an
order of magnitude. We present a stellar dynamical measurement of the SMBH mass
using an axisymmetric orbit superposition method. Our SINFONI integral-field
unit (IFU) observations of NGC 1332 resolve the SMBH's sphere of influence
which has a diameter of ~0.76 arcsec. The sigma inside 0.2 arcsec reaches ~400
km/s. The IFU data allow us to increase the statistical significance of our
results by modelling each of the four quadrants separately. We measure a SMBH
mass of (1.45 \pm 0.20) x 10^9 M_sun with a bulge mass-to-light ratio of 7.08
\pm 0.39 in the R-band. With this mass, the SMBH of NGC 1332 is offset from the
M_BH-luminosity relation by a full order of magnitude but is consistent with
the M_BH-sigma relation.Comment: 15 pages, 12 figures, accepted for publication in MNRA
ERIS: revitalising an adaptive optics instrument for the VLT
ERIS is an instrument that will both extend and enhance the fundamental
diffraction limited imaging and spectroscopy capability for the VLT. It will
replace two instruments that are now being maintained beyond their operational
lifetimes, combine their functionality on a single focus, provide a new
wavefront sensing module that makes use of the facility Adaptive Optics System,
and considerably improve their performance. The instrument will be competitive
with respect to JWST in several regimes, and has outstanding potential for
studies of the Galactic Center, exoplanets, and high redshift galaxies. ERIS
had its final design review in 2017, and is expected to be on sky in 2020. This
contribution describes the instrument concept, outlines its expected
performance, and highlights where it will most excel.Comment: 12 pages, Proc SPIE 10702 "Ground-Based and Airborne Instrumentation
for Astronomy VII
Stellar Processes Near the Massive Black Hole in the Galactic Center
A massive black hole resides in the center of most, perhaps all galaxies. The
one in the center of our home galaxy, the Milky Way, provides a uniquely
accessible laboratory for studying in detail the connections and interactions
between a massive black hole and the stellar system in which it grows; for
investigating the effects of extreme density, velocity and tidal fields on
stars; and for using stars to probe the central dark mass and probe
post-Newtonian gravity in the weak- and strong-field limits. Recent results,
open questions and future prospects are reviewed in the wider context of the
theoretical framework and physical processes that underlie them.
Contents: [1] Introduction (1.1) Astrophysical context (1.2) Science
questions (1.3) Scope and connections to related topics [2] Observational
overview: Stars in the Galactic center (2.1) The central 100 parsecs (2.2) The
central parsec [3] Stellar dynamics at extreme densities (3.1) Physical
processes and scales (3.2) The stellar cusp in the Galactic center (3.3) Mass
segregation (3.4) Stellar Collisions [4] Probing the dark mass with stellar
dynamics (4.1) Weighing and pinpointing the dark mass (4.2) Constraints on
non-BH dark mass alternatives (4.3) Limits on MBH binarity (4.4) High-velocity
runaway stars [5] Probing post-Newtonian gravity near the MBH (5.1)
Relativistic orbital effects (5.2) Gravitational lensing [6] Strong star-MBH
interactions (6.1) Tidal disruption (6.2) Dissipative interactions with the MBH
[7] The riddle of the young stars (7.1) The difficulties of forming or
importing stars near a MBH (7.2) Proposed solutions (7.3) Feeding the MBH with
stellar winds [8] Outlook (8.1) Progress report (8.2) Future directionsComment: Invited review article, to appear in Physics Reports. 101 p
Orbital effects of a monochromatic plane gravitational wave with ultra-low frequency incident on a gravitationally bound two-body system
We analytically compute the long-term orbital variations of a test particle
orbiting a central body acted upon by an incident monochromatic plane
gravitational wave. We assume that the characteristic size of the perturbed
two-body system is much smaller than the wavelength of the wave. Moreover, we
also suppose that the wave's frequency is much smaller than the particle's
orbital one. We make neither a priori assumptions about the direction of the
wavevector nor on the orbital geometry of the planet. We find that, while the
semi-major axis is left unaffected, the eccentricity, the inclination, the
longitude of the ascending node, the longitude of pericenter and the mean
anomaly undergo non-vanishing long-term changes. They are not secular trends
because of the slow modulation introduced by the tidal matrix coefficients and
by the orbital elements themselves. They could be useful to indepenedently
constrain the ultra-low frequency waves which may have been indirectly detected
in the BICEP2 experiment. Our calculation holds, in general, for any
gravitationally bound two-body system whose characteristic frequency is much
larger than the frequency of the external wave. It is also valid for a generic
perturbation of tidal type with constant coefficients over timescales of the
order of the orbital period of the perturbed particle.Comment: LaTex2e, 24 pages, no figures, no tables. Changes suggested by the
referees include
Observations and modelling of a clumpy galaxy at z=1.6: Spectroscopic clues to the origin and evolution of chain galaxies
We investigate the properties of a clump-cluster galaxy at redshift 1.57. The
morphology of this galaxy is dominated by eight star-forming clumps in optical
observations, and has photometric properties typical of most clump-cluster and
chain galaxies. Its complex asymmetrical morphology has led to the suggestion
that this system is a group merger of several initially separate
proto-galaxies. We performed H_alpha integral field spectroscopy of this system
using SINFONI on VLT UT4. These observations reveal a large-scale velocity
gradient throughout the system, but with large local kinematic disturbances.
Using a numerical model of gas-rich disk fragmentation, we find that clump
interactions and migration can account for the observed disturbed rotation. On
the other hand, the global rotation would not be expected for a multiply
merging system. We further find that this system follows the stellar mass vs.
metallicity, star formation rate and size relations expected for a disk at this
redshift, and exhibits a disk-like radial metallicity gradient, so that the
scenario of internal disk fragmentation is the most likely one. A red and
metallic central concentration appears to be a bulge in this proto-spiral
clumpy galaxy. A chain galaxy at redshift 2.07 in the same field also shows
disk-like rotation. Such systems are likely progenitors of the present-day
bright spiral galaxies, forming their exponential disks through clump migration
and disruption and fueling their bulges. Our present results show that
disturbed morphologies and kinematics are not necessarily signs of galaxy
mergers and interactions, and can instead result from the internal evolution of
primordial disks.Comment: A&A, accepted. Version with full resolution figures available at
http://aramis.obspm.fr/~bournaud/udfr2.pdf -- Animation of clumpy galaxy
models available at http://aramis.obspm.fr/~bournaud/cc/cc.htm
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