47 research outputs found
The Carnegie Hubble Program
We present an overview of and preliminary results from an ongoing
comprehensive program that has a goal of determining the Hubble constant to a
systematic accuracy of 2%. As part of this program, we are currently obtaining
3.6 micron data using the Infrared Array Camera (IRAC) on Spitzer, and the
program is designed to include JWST in the future. We demonstrate that the
mid-infrared period-luminosity relation for Cepheids at 3.6 microns is the most
accurate means of measuring Cepheid distances to date. At 3.6 microns, it is
possible to minimize the known remaining systematic uncertainties in the
Cepheid extragalactic distance scale. We discuss the advantages of 3.6 micron
observations in minimizing systematic effects in the Cepheid calibration of the
Hubble constant including the absolute zero point, extinction corrections, and
the effects of metallicity on the colors and magnitudes of Cepheids. We are
undertaking three independent tests of the sensitivity of the mid-IR Cepheid
Leavitt Law to metallicity, which when combined will allow a robust constraint
on the effect. Finally, we are providing a new mid-IR Tully-Fisher relation for
spiral galaxies
Cool Companions to White Dwarf Stars from the Two Micron All Sky Survey All Sky Data Release
We present the culmination of our near-infrared survey of the optically spectroscopically identified white dwarf stars from the McCook and Sion catalog, conducted using photometric data from the Two Micron All Sky Survey final All Sky Data Release. The color selection technique, which identifies candidate binaries containing a white dwarf and a low-mass stellar (or substellar) companion via their distinctive locus in the near-infrared color-color diagram, is demonstrated to be simple to apply and to yield candidates with a high rate of subsequent confirmation. We recover 105 confirmed binaries, and identify 27 firm candidates (19 of which are new to this work) and 21 tentative candidates (17 of which are new to this work) from the 2MASS data. Only a small number of candidates from our survey have likely companion spectral types later than M5, none of which is an obvious L-type (i.e., potential brown dwarf) companion. Only one previously known white dwarf + brown dwarf binary is detected. This result is discussed in the context of the 2MASS detection limits, as well as other recent observational surveys that suggest a very low rate of formation (or survival) for binary stars with extreme mass ratios
The Cepheid Period-Luminosity Relation at Mid-Infrared Wavelengths: I. First-Epoch LMC Data
We present the first mid-infrared Period-Luminosity (PL) relations for Large
Magellanic Cloud (LMC) Cepheids. Single-epoch observations of 70 Cepheids were
extracted from Spitzer IRAC observations at 3.6, 4.5, 5.8 and 8.0 microns,
serendipitously obtained during the SAGE (Surveying the Agents of a Galaxy's
Evolution) imaging survey of the LMC. All four mid-infrared PL relations have
nearly identical slopes over the period range 6 - 88 days, with a small scatter
of only +/-0.16 mag independent of period for all four of these wavelengths. We
emphasize that differential reddening is not contributing significantly to the
observed scatter, given the nearly two orders of magnitude reduced sensitivity
of the mid-IR to extinction compared to the optical. Future observations,
filling in the light curves for these Cepheids, should noticeably reduce the
residual scatter. These attributes alone suggest that mid-infrared PL relations
will provide a practical means of significantly improving the accuracy of
Cepheid distances to nearby galaxies.Comment: 19 pages, 4 figures, 1 table, Accepted for publication in the
Astrophysical Journa
Anchors for the Cosmic Distance Scale: the Cepheid QZ Normae in the Open Cluster NGC 6067
Cepheids are key to establishing the cosmic distance scale. Therefore it's
important to assess the viability of QZ Nor, V340 Nor, and GU Nor as
calibrators for Leavitt's law via their purported membership in the open
cluster NGC 6067. The following suite of evidence confirms that QZ Nor and V340
Nor are members of NGC 6067, whereas GU Nor likely lies in the foreground: (i)
existing radial velocities for QZ Nor and V340 Nor agree with that established
for the cluster (-39.4+-1.2 km/s) to within 1 km/s, whereas GU Nor exhibits a
markedly smaller value; (ii) a steep velocity-distance gradient characterizes
the sight-line toward NGC 6067, thus implying that objects sharing common
velocities are nearly equidistant; (iii) a radial profile constructed for NGC
6067 indicates that QZ Nor is within the cluster bounds, despite being 20' from
the cluster center; (iv) new BVJH photometry for NGC 6067 confirms the cluster
lies d=1.75+-0.10 kpc distant, a result that matches Wesenheit distances
computed for QZ Nor/V340 Nor using the Benedict et al. (2007, HST parallaxes)
calibration. QZ Nor is a cluster Cepheid that should be employed as a
calibrator for the cosmic distance scale.Comment: To appear in ApS
The Cepheid Period-Luminosity Relation at Mid-Infrared Wavelengths: II. Second-Epoch LMC Data
We present revised and improved mid-infrared Period-Luminosity (PL) relations
for Large Magellanic Cloud (LMC) Cepheids based on double-epoch data of 70
Cepheids observed by Spitzer at 3.6, 4.5, 5.8 and 8.0um. The observed scatter
at all wavelengths is found to decrease from +/-0.17 mag to +/-0.14 mag, which
is fully consistent with the prediction that the total scatter is made up of
roughly equal contributions from random sampling of the light curve and
nearly-uniform samplings of stars across the instability strip. It is
calculated that the Cepheids in this sample have a full amplitude of about 0.4
mag and that their fully-sampled, time-averaged magnitudes should eventually
reveal mid-infrared PL relations that each have intrinsic scatter at most at
the +/-0.12 mag level, and as low as +/-0.08 mag after correcting for the tilt
of the LMC.Comment: 7 pages, 6 figures, 1 data tabl
Erratum: The Carnegie Hubble Program: The Leavitt Law at 3.6 micron and 4.5 micron in the Large Magellanic Cloud (2011, APJ, 743, 76)
Due to an error at the publisher, an incorrect version of Table 3 appeared in the published article. The updated version of
Table 3 is given
The Cepheid Period-Luminosity Relation (The Leavitt Law) at Mid-Infrared Wavelengths: III. Cepheids in NGC 6822
We present the first application of mid-infrared Period-Luminosity relations
to the determination of a Cepheid distance beyond the Magellanic Clouds. Using
archival IRAC imaging data on NGC 6822 from Spitzer we were able to measure
single-epoch magnitudes for sixteen long-period (10 to 100-day) Cepheids at
3.6um, fourteen at 4.5um, ten at 5.8um and four at 8.0um. The measured slopes
and the observed scatter both conform to the relations previously measured for
the Large Magellanic Cloud Cepheids, and fitting to those relations gives
apparent distance moduli of mod{3.6} = 23.57 +/- 0.06, mod{4.5} = 23.55 +/-
0.07, mod{5.8} = 23.60 +/- 0.09 and mod{8.0} = 23.51 +/-0.08 mag. A
multi-wavelength fit to the new IRAC moduli, and previously published BVRIJHK
moduli, allows for a final correction for interstellar reddening and gives a
true distance modulus of 23.49 +/- 0.03 mag with E(B-V) = 0.26 mag,
corresponding to a metric distance of 500 +/-8 kpc.Comment: Accepted to ApJ, 2 figure
The Cepheid Period-Luminosity Relation (The Leavitt Law) at Mid-Infrared Wavelengths: IV. Cepheids in IC 1613
We present mid-infrared Period-Luminosity relations for Cepheids in the Local
Group galaxy IC1613. Using archival IRAC imaging data from Spitzer we were able
to measure single-epoch magnitudes for five, 7 to 50-day, Cepheids at 3.6 and
4.5 microns. When fit to the calibrating relations, measured for the Large
Magellanic Cloud Cepheids, the data give apparent distance moduli of 24.29 +/-
0.07 and 24.28 +/- 0.07 at 3.6 and 4.5 microns, respectively. A
multi-wavelength fit to previously published BVRIJHK apparent moduli and the
two IRAC moduli gives a true distance modulus of 24.27 +/- 0.02 mag with E(B-V)
= 0.08 mag, and a corresponding metric distance of 715 kpc. Given that these
results are based on single-phase observations derived from exposures having
total integration times of only 1,000 sec/pixel we suggest that Cepheids out to
about 2 Mpc are accessible to Spitzer with modest integration times during its
warm mission. We identify the main limiting factor to this method to be
crowding/contamination induced by the ubiquitous population of infrared-bright
AGB stars.Comment: Accepted to ApJ December 2008: 9 pages, 3 figure
The Second-Generation Guide Star Catalog: Description and Properties
The GSC-II is an all-sky database of objects derived from the uncompressed
DSS that the STScI has created from the Palomar and UK Schmidt survey plates
and made available to the community. Like its predecessor (GSC-I), the GSC-II
was primarily created to provide guide star information and observation
planning support for HST. This version, however, is already employed at some of
the ground-based new-technology telescopes such as GEMINI, VLT, and TNG, and
will also be used to provide support for the JWST and Gaia space missions as
well as LAMOST, one of the major ongoing scientific projects in China. Two
catalogs have already been extracted from the GSC-II database and released to
the astronomical community. A magnitude-limited (R=18.0) version, GSC2.2, was
distributed soon after its production in 2001, while the GSC2.3 release has
been available for general access since 2007.
The GSC2.3 catalog described in this paper contains astrometry, photometry,
and classification for 945,592,683 objects down to the magnitude limit of the
plates. Positions are tied to the ICRS; for stellar sources, the all-sky
average absolute error per coordinate ranges from 0.2" to 0.28" depending on
magnitude. When dealing with extended objects, astrometric errors are 20% worse
in the case of galaxies and approximately a factor of 2 worse for blended
images. Stellar photometry is determined to 0.13-0.22 mag as a function of
magnitude and photographic passbands (B,R,I). Outside of the galactic plane,
stellar classification is reliable to at least 90% confidence for magnitudes
brighter than R=19.5, and the catalog is complete to R=20.Comment: 52 pages, 33 figures, to be published in AJ August 200
The Konkoly Blazhko Survey: Is light-curve modulation a common property of RRab stars?
A systematic survey to establish the true incidence rate of the Blazhko
modulation among short-period, fundamental-mode, Galactic field RR Lyrae stars
has been accomplished. The Konkoly Blazhko Survey (KBS) was initiated in 2004.
Since then more than 750 nights of observation have been devoted to this
project. A sample of 30 RRab stars was extensively observed, and light-curve
modulation was detected in 14 cases. The 47% occurrence rate of the modulation
is much larger than any previous estimate. The significant increase of the
detected incidence rate is mostly due to the discovery of small-amplitude
modulation. Half of the Blazhko variables in our sample show modulation with so
small amplitude that definitely have been missed in the previous surveys. We
have found that the modulation can be very unstable in some cases, e.g. RY Com
showed regular modulation only during one part of the observations while during
two seasons it had stable light curve with abrupt, small changes in the
pulsation amplitude. This type of light-curve variability is also hard to
detect in other Survey's data. The larger frequency of the light-curve
modulation of RRab stars makes it even more important to find the still lacking
explanation of the Blazhko phenomenon. The validity of the [Fe/H](P,phi_{31})
relation using the mean light curves of Blazhko variables is checked in our
sample. We have found that the formula gives accurate result for
small-modulation-amplitude Blazhko stars, and this is also the case for
large-modulation-amplitude stars if the light curve has complete phase
coverage. However, if the data of large-modulation-amplitude Blazhko stars are
not extended enough (e.g. < 500 data points from < 15 nights), the formula may
give false result due to the distorted shape of the mean light curve used.Comment: Accepted for publication in MNRAS, 14 pages, 7 Figure