31 research outputs found
The Sagittarius dwarf galaxy as a microlensing target
We estimate the optical depth, time-scale distribution and fraction of
microlensing events originating from sources in the Sagittarius dwarf galaxy
(Sgr) lensed by deflectors in the Milky Way. These events have a time-scale
longer by a factor ~1.3 than the MW/MW events and occur mainly on sources
fainter than V~21 mag below Sgr's turn off. The fraction of events involving a
source in Sgr depends on the location and extinction of the field and on the
limiting magnitude of the survey. The contribution of the MW/Sgr events is
negligible (<1%) at very low latitudes (|b|<2 deg.) but increases continuously
towards higher |b| and becomes dominant near the highest density region of the
dwarf galaxy. Sgr is present within the fields of current microlensing surveys
and any optical depth map inferred from observations will become biased by the
presence of Sgr towards higher |b| where the contribution of MW/Sgr events is
significant. Systematic spectroscopic measurements on the sources of all the
microlensing events may allow detection of this kind of event for which the
degeneracy on the lens mass can be significantly reduced.Comment: 9 pages, 8 figures. Accepted for publication in A&A Main Journa
HST Imaging of MEGA Microlensing Candidates in M31
We investigate /ACS and WFPC2 images at the positions of five candidate
microlensing events from a large survey of variability in M31 (MEGA). Three
closely match unresolved sources, and two produce only flux upper limits. All
are confined to regions of the color-magnitude diagram where stellar
variability is unlikely to be easily confused with microlensing. Red variable
stars cannot explain these events (although background supernova are possible
for two). If these lenses arise in M31's halo, they are due to masses (95% certainty, for a -function mass distribution),
brown dwarfs for disk lenses, and stellar masses for bulge lenses.Comment: Accepted for publication in ApJL. Higher resolution version available
at http://www.astro.columbia.edu/~patrick/hst/hst_ml.pd
Great Circle tidal streams: evidence for a nearly spherical massive dark halo around the Milky Way
An all high-latitude sky survey for cool carbon giant stars in the Galactic
halo has revealed 75 such stars, of which the majority are new detections. Of
these, more than half are clustered on a Great Circle on the sky which
intersects the center of Sagittarius dwarf galaxy (Sgr) and is parallel to its
proper motion vector, while many of the remainder are outlying Magellanic Cloud
C-stars. A pole-count analysis of the carbon star distribution clearly
indicates that the Great Circle stream we have isolated is statistically
significant, being a 5-6 sigma over-density. These two arguments strongly
support our conclusion that a large fraction of the Halo carbon stars
originated in Sgr. The stream orbits the Galaxy between the present location of
Sgr, 16 kpc from the Galactic center, and the most distant stream carbon star,
at ~60 kpc. It follows neither a polar nor a Galactic plane orbit, so that a
large range in both Galactic R and z distances are probed. That the stream is
observed as a Great Circle indicates that the Galaxy does not exert a
significant torque upon the stream, so the Galactic potential must be nearly
spherical in the regions probed by the stream. We present N-body experiments
simulating this disruption process as a function of the distribution of mass in
the Galactic halo. A likelihood analysis shows that, in the Galactocentric
distance range 16 kpc < R < 60 kpc, the dark halo is most likely almost
spherical. We rule out, at high confidence levels, the possibility that the
Halo is significantly oblate, with isodensity contours of aspect q_m < 0.7.
This result is quite unexpected and contests currently popular galaxy formation
models. (Abridged)Comment: 26 pages, 13 figures (6 in color, 8 chunky due to PS compression),
minor revisions, accepted by Ap
Multi-Periodic Oscillations in Cepheids and RR Lyrae-Type Stars
Classical Cepheids and RR Lyrae-type stars are usually considered to be
textbook examples of purely radial, strictly periodic pulsators. Not all the
variables, however, conform to this simple picture. In this review I discuss
different forms of multi-periodicity observed in Cepheids and RR Lyrae stars,
including Blazhko effect and various types of radial and nonradial multi-mode
oscillations.Comment: Proceedings of the 20th Stellar Pulsation Conference Series: "Impact
of new instrumentation & new insights in stellar pulsations", 5-9 September
2011, Granada, Spai
First Microlensing Events From The MEGA Survey Of M31
We present the first M31 candidate microlensing events from the Microlensing
Exploration of the Galaxy and Andromeda (MEGA) survey. MEGA uses several
telescopes to detect microlensing towards the nearby Andromeda galaxy, M31, in
order to establish whether massive compact objects are a significant
contribution to the mass budget of the dark halo of M31. The results presented
here are based on observations with the Isaac Newton Telescope on La Palma,
during the 1999/00 and 2000/01 observing seasons. In this data set, 14 variable
sources consistent with microlensing have been detected, 12 of which are new
and 2 have been reported previously by the POINT-AGAPE group. A preliminary
analysis of the spatial and timescale distributions of the candidate events
support their microlensing nature. We compare the spatial distributions of the
candidate events and of long-period variable stars, assuming the chances of
finding a long-period variable and a microlensing event are comparable. The
spatial distribution of our candidate microlensing events is more far/near side
asymmetric than expected from the detected long-period variable distribution.
The current analysis is preliminary and the asymmetry not highly significant,
but the spatial distribution of candidate microlenses is suggestive of the
presence of a microlensing halo.Comment: revised version, 16 pages, 12 figures, submitted to Astronomy &
Astrophysic
RR Lyrae stars in the Sagittarius dwarf galaxy: Period analysis
(Abridged) We carried out a period analysis on ~3700 RR Lyrae stars spread
over ~50 square degrees towards the Sagittarius dwarf galaxy (Sgr). The RR
Lyraes are distributed almost evenly between Sgr and the Milky Way. For Sgr
members, the average periods are 0.574 day and 0.322 day for RRab and RRc stars
respectively. This places Sgr in the long-period tail of the Oosterhoff I
group. We report the detection of 53 double-mode RR Lyrae stars (RRd) within
our sample. The magnitude of 40 of these stars is consistent with membership in
Sgr whereas 13 RRds are located within our Galaxy. We also found 13 RR Lyraes
(5 in Sgr and 8 in the Galaxy) exhibiting two closely spaced frequencies, most
probably related to non-radial pulsations. The period distribution of the RR
Lyrae variables in Sgr is compared to those of other Milky Way satellites. We
find a remarkable similarity between the RR Lyrae populations in Sgr and the
Large Magellanic Cloud (LMC), suggesting that these galaxies have similar
horizontal branch morphologies. This may indicate that Sgr and the LMC started
their formation under similar conditions. Using various photometric indicators,
we estimate the metallicity of the RR Lyrae stars in Sgr and find ~-1.6
dex with a dispersion of ~ +/-0.5 dex around this value and a minor but
significant population at <-2.0 dex. We do not find evidence for a spatial
metallicity gradient in the RR Lyrae population of Sgr. From the spatial
distribution of RR Lyraes, we estimate the RRab content and total luminosity of
this galaxy and find MV(Sgr)~-14.7 mag, a value that would be consistent with
the empirical metallicity/luminosity relation for Dsph galaxies.Comment: 13 pages, 10 figures, 4 tables. Accepted for publication in A&
PAndromeda - first results from the high-cadence monitoring of M31 with Pan-STARRS 1
The Pan-STARRS 1 (PS1) survey of M31 (PAndromeda) is designed to identify
gravitational microlensing events, caused by bulge and disk stars
(self-lensing) and by compact matter in the halos of M31 and the Milky Way
(halo lensing, or lensing by MACHOs). With the 7 deg2 FOV of PS1, the entire
disk of M31 can be imaged with one single pointing. Our aim is to monitor M31
with this wide FOV with daily sampling (20 mins/day). In the 2010 season we
acquired in total 91 nights towards M31, with 90 nights in the rP1 and 66
nights in the iP1. The total integration time in rP1 and iP1 are 70740s and
36180s, respectively. As a preliminary analysis, we study a 40'\times40'
sub-field in the central region of M31, a 20'\times20' sub-field in the disk of
M31 and a 20'\times20' sub-field for the investigation of astrometric
precision. We demonstrate that the PSF is good enough to detect microlensing
events. We present light curves for 6 candidate microlensing events. This is a
competitive rate compared to previous M31 microlensing surveys. We finally also
present one example light curve for Cepheids, novae and eclipsing binaries in
these sub-fields.Comment: 41 pages, 19 figures, 4 tables. Published in A
The Planetary Nebula population of the Sagittarius Dwarf Spheroidal Galaxy
The identification of two new Planetary Nebulae in the Sagittarius Dwarf
Spheroidal Galaxy (Sgr) is presented. This brings the total number to four.
The first, StWr 2-21, belongs to the main body of Sgr. The second, the halo
PN BoBn 1, has a location, distance and velocity in agreement with the leading
tidal tail of Sgr. We estimate that 10 per cent of the Galactic halo consists
of Sgr debris. The specific frequency of PNe indicates a total luminosity of
Sgr, including its tidal tails, of M_V=-14.1. StWr 2-21 shows a high abundance
of [O/H]=-0.23, which confirms the high-metallicity population in Sgr uncovered
by Bonaficio et al. (2004). The steep metallicity--age gradient in Sgr is due
to ISM removal during the Galactic plane passages, ISM reformation due to
stellar mass loss, and possibly accretion of metal-enriched gas from our
Galaxy. The ISM re-formation rate of Sgr, from stellar mass loss, is 5 X 10^-4
M_sun yr^-1, amounting to ~10^6 M_sun per orbital period. HST images reveal
well-developed bipolar morphologies, and provide clear detections of the
central stars. All three stars with deep spectra show WR-lines, suggesting that
the progenitor mass and metallicity determines whether a PN central star
develops a WR spectrum. One Sgr PN belongs to the class of IR-[WC] stars.
Expansion velocities are determined for three nebulae. Comparison with
hydrodynamical models indicates an initial density profile of rho ~ r^-3. This
is evidence for increasing mass-loss rates on the AGB. Peak mass-loss rates are
indicated of ~ 10^-4 M_sun yr^-1. The IR-[WC] PN, He 2-436, provides the sole
direct detection of dust in a dwarf spheroidal galaxy, to date.Comment: 16 pages. MNRAS, accepted for publicatio