1,452 research outputs found
Galactic Cepheids with Spitzer: I. Leavitt Law and Colors
Classical Cepheid variable stars have been important indicators of
extragalactic distance and Galactic evolution for over a century. The Spitzer
Space Telescope has opened the possibility of extending the study of Cepheids
into the mid- and far-infrared, where interstellar extinction is reduced. We
have obtained photometry from images of a sample of Galactic Cepheids with the
IRAC and MIPS instruments on Spitzer. Here we present the first mid-infrared
period-luminosity relations for Classical Cepheids in the Galaxy, and the first
ever Cepheid period-luminosity relations at 24 and 70 um. We compare these
relations with theoretical predictions, and with period-luminosity relations
obtained in recent studies of the Large Magellanic Cloud. We find a significant
period-color relation for the [3.6]-[8.0] IRAC color. Other mid-infrared colors
for both Cepheids and non-variable supergiants are strongly affected by
variable molecular spectral features, in particular deep CO absorption bands.
We do not find strong evidence for mid-infrared excess caused by warm (~500 K)
circumstellar dust. We discuss the possibility that recent detections with
near-infrared interferometers of circumstellar shells around delta Cep, l Car,
Polaris, Y Oph and RS Pup may be a signature of shocked gas emission in a
dust-poor wind associated to pulsation-driven mass loss.Comment: Accepted by The Astrophysical Journal on Nov 11, 200
Spitzer/IRAC Observations of AGB stars
We present here the first observation of galactic AGB stars with the InfraRed
Array Camera (IRAC) onboard the Spitzer Space Telescope. Our sample consists of
48 AGB stars of different chemical signature, mass loss rate and variability
class. For each star we have measured IRAC photometry and colors. Preliminary
results shows that IRAC colors are sensitive to spectroscopic features
associated to molecules and dust in the AGB wind. Period is only loosely
correlated to the brightness of the stars in the IRAC bands. We do find,
however, a tight period-color relation for sources classified as semiregular
variables. This may be interpreted as the lack of warm dust in the wind of the
sources in this class, as opposed to Mira variables that show higher infrared
excess in all IRAC bands.Comment: 8 pages, to be published in proceedings "IX Torino Workshop on
Evolution and Nucleosynthesis in AGB Stars", 22-26 October 2007, Perugia,
Ital
Spitzer/IRAC Limits to Planetary Companions of Fomalhaut and epsilon Eridani
Fomalhaut and epsilon Eridani are two young, nearby stars that possess
extended debris disks whose structures suggest the presence of perturbing
planetary objects. With its high sensitivity and stable point spread function,
Spitzer/IRAC is uniquely capable of detecting cool, Jupiter-like planetary
companions whose peak emission is predicted to occur near 4.5 um. We report on
deep IRAC imaging of these two stars, taken at 3.6 and 4.5 um using subarray
mode and in all four channels in wider-field full array mode. Observations
acquired at two different telescope roll angles allowed faint surrounding
objects to be separated from the stellar diffraction pattern. No companion
candidates were detected at the reported position of Fomalhaut b with 3 sigma
model-dependent mass upper limits of 3 MJ (for an age of 200 Myr). Around
epsilon Eridani we instead set a limit of 4 and <1 MJ (1 Gyr model age) at the
inner and outer edge of the sub-millimeter debris ring, respectively. These
results are consistent with non-detections in recent near-infrared imaging
searches, and set the strongest limits to date on the presence of planets
outside epsilon Eridani sub-millimeter ring.Comment: Accepted by The Astrophysical Journal. Request electronic-only plates
to M. Marengo ([email protected]
Optical and Near-Infrared UBVRIJHK Photometry for the RR Lyrae stars in the Nearby Globular Cluster M4 (NGC 6121)
We present optical and near-infrared UBVRIJHK photometry of stars in the
Galactic globular cluster M4 (NGC 6121) based upon a large corpus of
observations obtained mainly from public astronomical archives. We concentrate
on the RR Lyrae variable stars in the cluster, and make a particular effort to
accurately reidentify the previously discovered variables. We have also
discovered two new probable RR Lyrae variables in the M4 field: one of them by
its position on the sky and its photometric properties is a probable member of
the cluster, and the second is a probable background (bulge?) object. We
provide accurate equatorial coordinates for all 47 stars identified as RR
Lyraes, new photometric measurements for 46 of them, and new period estimates
for 45. We have also derived accurate positions and mean photometry for 34 more
stars previously identified as variable stars of other types, and for an
additional five non-RR Lyrae variable stars identified for the first time here.
We present optical and near-infrared color-magnitude diagrams for the cluster
and show the locations of the variable stars in them. We present the Bailey
(period-amplitude) diagrams and the period-frequency histogram for the RR Lyrae
stars in M4 and compare them to the corresponding diagrams for M5 (NGC 5904).
We conclude that the RR Lyrae populations in the two clusters are quite similar
in all the relevant properties that we have considered. The mean periods,
pulsation-mode ratios, and Bailey diagrams of these two clusters show support
for the recently proposed "Oosterhoff-neutral" classification.Comment: 33 pages, 16 figures, 7 table
- âŠ