11,246 research outputs found
The Tip of the Red Giant Branch and Distance of the Magellanic Clouds: results from the DENIS survey
We present a precise determination of the apparent magnitude of the tip of
the red giant branch (TRGB) in the I (0.8 micron), J (1.25 micron), and K_S
(2.15 micron) bands from the luminosity function of a sample of data extracted
from the DENIS catalogue towards the Magellanic Clouds (Cioni et al. 2000).
From the J and Ks magnitudes we derive bolometric magnitudes m_bol. We
present a new algorithm for the determination of the TRGB magnitude, which we
describe in detail and test extensively using Monte-Carlo simulations. We note
that any method that searches for a peak in the first derivative (used by most
authors) or the second derivative (used by us) of the observed luminosity
function does not yield an unbiased estimate for the actual magnitude of the
TRGB discontinuity. We stress the importance of correcting for this bias, which
is not generally done. We combine the results of our algorithm with theoretical
predictions to derive the distance modulus of the Magellanic Clouds. We obtain
m-M = 18.55 (0.04 formal, 0.08 systematic) for the Large Magellanic Cloud
(LMC), and m-M = 18.99 (0.03 formal, 0.08 systematic) for the Small Magellanic
Cloud (SMC). These are among the most accurate determinations of these
quantities currently available, which is a direct consequence of the large size
of our sample and the insensitivity of near infrared observations to dust
extinction.Comment: 16 pages, 8 figures, revised version, accepted for publication in A&
Period-magnitude relations for M giants in Baade's Window NGC6522
A large and complete sample of stars with K < 9.75 in the NGC6522 Baade's
Window is examined using light curves from MACHO and IJK from DENIS. All 4 of
the sequences ABCD in the K vs logP diagram of the LMC are seen in the Bulge.
The Bulge sequences however show some differences from the Magellanic Clouds.
The sequences may be useful as distance indicators. A new diagram of the
frequency of late-type variables is presented. The catalogued SR variables of
the solar nbd are found to be a subset of the total of SRs, biased towards
large amplitude.Comment: 11 pages 11 fig
Long Period Variables in the Magellanic Clouds: OGLE + 2MASS + DENIS
(abridged) The 68000 I-band light curves of variable stars detected by the
OGLE survey in the Large and Small Magellanic Clouds (MCs) are fitted by
Fourier series, and also correlated with the DENIS and 2MASS databases and with
lists of spectroscopically confirmed M-, S- and C-stars. Lightcurves and the
results of the lightcurve fitting (periods and amplitudes) and DENIS and 2MASS
magnitudes are presented for 2277 M-,S-,C-stars in the MCs. The following
aspects are discussed: the K-band period-luminosity relations for the
spectroscopically confirmed AGB stars, period changes over a timespan of about
17 years in a subset of about 400 LPVs, and candidate obscured AGB stars.Comment: Astronomy and Astrophysics, accepte
Mid Infrared Photometry of Mass-Losing AGB Stars
We present ground-based mid-infrared imaging for 27 M-, S- and C-type
Asymptotic Giant Branch (AGB) stars. The data are compared with those of the
database available thanks to the IRAS, ISO, MSX and 2MASS catalogues. Our goal
is to establish relations between the IR colors, the effective temperature
, the luminosity and the mass loss rate , for improving
the effectiveness of AGB modelling. Bolometric (absolute) magnitudes are
obtained through distance compilations, and by applying previously-derived
bolometric corrections; the variability is also studied, using data accumulated
since the IRAS epoch. The main results are: i) Values of and for C
stars fit relations previously established by us, with Miras being on average
more evolved and mass losing than Semiregulars. ii) Moderate IR excesses (as
compared to evolutionary tracks) are found for S and M stars in our sample:
they are confirmed to originate from the dusty circumstellar environment. iii)
A larger reddening characterizes C-rich Miras and post-AGBs. In this case, part
of the excess is due to AGB models overestimating for C-stars, as a
consequence of the lack of suitable molecular opacities. This has a large
effect on the colors of C-rich sources and sometimes disentangling the
photospheric and circumstellar contributions is difficult; better model
atmospheres should be used in stellar evolutionary codes for C stars. iv) The
presence of a long-term variability at mid-IR wavelengths seems to be limited
to sources with maximum emission in the 8 -- 20 m region, usually Mira
variables (1/3 of our sample). Most Semiregular and post-AGB stars studied here
remained remarkably constant in mid-IR over the last twenty years.Comment: Accepted for publication in the Astronomical Journal - 35 pages (in
preprint), 9 figures, 5 table
Advanced channel coding for space mission telecommand links
We investigate and compare different options for updating the error
correcting code currently used in space mission telecommand links. Taking as a
reference the solutions recently emerged as the most promising ones, based on
Low-Density Parity-Check codes, we explore the behavior of alternative schemes,
based on parallel concatenated turbo codes and soft-decision decoded BCH codes.
Our analysis shows that these further options can offer similar or even better
performance.Comment: 5 pages, 7 figures, presented at IEEE VTC 2013 Fall, Las Vegas, USA,
Sep. 2013 Proc. IEEE Vehicular Technology Conference (VTC 2013 Fall), ISBN
978-1-6185-9, Las Vegas, USA, Sep. 201
Magellanic Cloud Structure from Near-IR Surveys I: The Viewing Angles of the LMC
We present a detailed study of the viewing angles of the LMC disk plane. We
find that our viewing direction differs considerably from the commonly accepted
values, which has important implications for the structure of the LMC. The
discussion is based on an analysis of spatial variations in the apparent
magnitude of features in the near-IR color-magnitude diagrams extracted from
the DENIS and 2MASS surveys. Sinusoidal brightness variations with a
peak-to-peak amplitude of approximately 0.25 mag are detected as function of
position angle, for both AGB and RGB stars. This is naturally interpreted as
the result of distance variations, due to one side of the LMC plane being
closer to us than the opposite side. The best fitting geometric model of an
inclined plane yields an inclination angle i = 34.7 +/- 6.2 degrees and
line-of-nodes position angle Theta = 122.5 +/- 8.3 degrees. There is tentative
evidence that the LMC disk plane may be warped. Traditional methods to estimate
the position angle of the line of nodes have used either the major axis
position angle Theta_maj of the spatial distribution of tracers on the sky, or
the position angle Theta_max of the line of maximum gradient in the velocity
field, given that for a circular disk Theta_maj = Theta_max = Theta. The
present study does not rely on the assumption of circular symmetry, and is
considerably more accurate than previous studies of its kind. We find that the
actual position angle of the line of nodes differs considerably from both
Theta_maj and Theta_max, for which measurements have fallen in the range
140-190 degrees. This indicates that the intrinsic shape of the LMC disk is not
circular, but elliptical, as discussed further in Paper II. [Abridged]Comment: Astronomical Journal, in press. 44 pages, LaTeX, with 8 PostScript
figures. Contains minor revisions with respect to previously posted version.
Check out http://www.stsci.edu/~marel/lmc.html for a large scale (23x21
degree) stellar number-density image of the LMC constructed from RGB and AGB
stars in the 2MASS and DENIS surveys. The paper is available with higher
resolution figures from http://www.stsci.edu/~marel/abstracts/abs_R31.htm
Observations of GRB 060526 Optical Afterglow with Russian-Turkish 1.5-m Telescope
We present the results of the photometric multicolor observations of GRB
060526 optical afterglow obtained with Russian-Turkish 1.5-m Telescope (RTT150,
Mt. Bakirlitepe, Turkey). The detailed measurements of afterglow light curve,
starting from about 5 hours after the GRB and during 5 consecutive nights were
done. In addition, upper limits on the fast variability of the afterglow during
the first night of observations were obtained and the history of afterglow
color variations was measured in detail. In the time interval from 6 to 16
hours after the burst, there is a gradual flux decay, which can be described
approximately as a power law with an index of -1.14+-0.02. After that the
variability on the time scale \delta t < t is observed and the afterglow
started to decay faster. The color of the afterglow, V-R=~0.5, is approximately
the same during all our observations. The variability is detected on time
scales up to \delta t/t =~ 0.0055 at \Delta F_\nu/F_\nu =~ 0.3, which violates
some constraints on the variability of the observed emission from
ultrarelativistic jet obtained by Ioka et al. (2005). We suggest to explain
this variability by the fact that the motion of the emitting shell is no longer
ultrarelativistic at this time.Comment: 6 pages, 7 figures, Astronomy Letters, 2007, 33, 797, The on-line
data tables and the original text in Russian can be found at
http://hea.iki.rssi.ru/grb/060526/indexeng.htm
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