403 research outputs found
Sigma One
We demonstrate that it is possible to calculate not only the mean of an
underlying population but also its dispersion, given only a single observation
and physically reasonable constraints (i.e., that the quantities under
consideration are non-negative and bounded). We suggest that this
counter-intuitive conclusion is in fact at the heart of most modeling of
astronomical data.Comment: Accepted to the Astronomical Journal. 13 pages, 4 figure
GALEX: Galaxy Evolution Explorer
We review recent scientific results from the Galaxy Evolution Explorer with
special emphasis on star formation in nearby resolved galaxies.Comment: invited review; to be published in "The Spectral Energy Distribution
of Gas-Rich Galaxies: Confronting Models with Data", 4-8 October 2004,
Heidelberg, eds. Cristina C. Popescu and Richard J. Tuffs, AIP Conf. Ser.;
for a high resolution version see
http://nedwww.ipac.caltech.edu/level5/March05/Madore/frames.htm
Two New Tests of the Metallicity Sensitivity of the Cepheid Period-Luminosity Relation (The Leavitt Law)
We undertake a new test of the metallicity sensitivity of the Leavitt Law for
Classical Cepheids. We derive an empirical calibration of the apparent
luminosities of Cepheids as measured from the optical through the mid-infrared
(0.45-8.0um) as a function of spectroscopic [Fe/H] abundances of individual
Cepheids in the Large Magellanic Cloud from Romaniello et al. (2008). The
cumulative trend over the entire wavelength range shows a nearly monotonic
behavior. The sense of the trend is consistent with differential
line-blanketing in the optical, leading to stars of high metallicity being
fainter in the optical. This is followed by a reversal in the trend at longer
wavelengths, with the cross-over occurring near the K band at about 2.2um,
consistent with a subsequent redistribution of energy resulting in a mild
brightening of Cepheids (with increased metallicity) at mid-infrared
wavelengths. This conclusion agrees with that of Romaniello et al. based on a
differential comparison of the mean V- and K-band Leavitt Laws for the Galaxy,
SMC and LMC, but is opposite in sign to most other empirical tests of the
sensitivity of Cepheid distances to mean [O/H] HII region abundances. We also
search for a correlation of Cepheid host-galaxy metallicity with deviations of
the galaxy's Cepheid distance from that predicted from a pure Hubble flow.
Based on Cepheid distances to 26 nearby galaxies in the local flow, only a very
weak signal is detected giving Dmu_o = -0.17 (+/- 0.31) ([O/H] - 8.80) - 0.21
(+/-0.10). This is in agreement with previous determinations, but statistically
inconclusive.Comment: Accepted to the Astrophysical Journal. 8 pages, 5 figures, 1 tabl
The Carnegie Supernova Project. I. Third Photometry Data Release of Low-redshift Type Ia Supernovae and Other White Dwarf Explosions
We present final natural-system optical (ugriBV) and near-infrared (YJH) photometry of 134 supernovae (SNe) with probable white dwarf progenitors that were observed in 2004–2009 as part of the first stage of the Carnegie Supernova Project (CSP-I). The sample consists of 123 Type Ia SNe, 5 Type Iax SNe, 2 super-Chandrasekhar SN candidates, 2 Type Ia SNe interacting with circumstellar matter, and 2 SN 2006bt-like events. The redshifts of the objects range from z = 0.0037 to 0.0835; the median redshift is 0.0241. For 120 (90%) of these SNe, near-infrared photometry was obtained. Average optical extinction coefficients and color terms are derived and demonstrated to be stable during the five CSP-I observing campaigns. Measurements of the CSP-I near-infrared bandpasses are also described, and near-infrared color terms are estimated through synthetic photometry of stellar atmosphere models. Optical and near-infrared magnitudes of local sequences of tertiary standard stars for each supernova are given, and a new calibration of Y-band magnitudes of the Persson et al. standards in the CSP-I natural system is presented
Cepheid and Tip of the Red Giant Branch Distances To the Dwarf Irregular Galaxy IC10
We present color-magnitude diagrams and luminosity functions of stars in the
nearby galaxy IC 10, based on VI CCD photometry acquired with the COSMIC
prime-focus camera on the Palomar 5m telescope. The apparent I-band luminosity
function of stars in the halo of IC 10 shows an identifiable rise at I~21.7
mag. This is interpreted as being the tip of the red giant branch (TRGB) at
M_V~-4 mag. Since IC 10 is at a very low Galactic latitude, its foreground
extinction is expected to be high and the uncertainty associated with that
correction is the largest contributor to the error associated with its distance
determination. Multi-wavelength observations of Cepheid variable stars in IC 10
give a Population I distance modulus of 24.1 +- 0.2 mag, which corresponds to a
linear distance of 660 +- 66 kpc for a total line-of-sight reddening of E(B-V)
= 1.16 +- 0.08 mag, derived self-consistently from the Cepheid data alone.
Applying this Population I reddening to the Population II halo stars gives a
TRGB distance modulus of 23.5 +- 0.2 mag, corresponding to 500 +- 50 kpc. We
consider this to be a lower limit on the TRGB distance. Reconciling the Cepheid
and TRGB distances would require that the reddening to the halo is
E(B-V) = 0.31 mag lower than that into the main body of the galaxy.
This then suggests that the Galactic extinction in the direction of IC10 is
(B-V) ~ 0.85
The Cepheid Extragalactic Distance Scale: Past, Present and Future
Cepheids have been the cornerstone of the extragalactic distance scale for a
century. With high-quality data, these luminous supergiants exhibit a small
dispersion in their Leavitt (period-luminosity) relation, particularly at
longer wavelengths, and few methods rival the precision possible with Cepheid
distances. In these proceedings, we present an overview of major observational
programs pertaining to the Cepheid extragalactic distance scale, its progress
and remaining challenges. In addition, we present preliminary new results on
Cepheids from the James Webb Space Telescope (JWST). The launch of JWST has
opened a new chapter in the measurement of extragalactic distances and the
Hubble constant. JWST offers a resolution three times that of the Hubble Space
Telescope (HST) with nearly 10 times the sensitivity. It has been suggested
that the discrepancy in the value of the Hubble constant based on Cepheids
compared to that inferred from measurements of the cosmic microwave background
requires new and additional physics beyond the standard cosmological model.
JWST observations will be critical in reducing remaining systematics in the
Cepheid measurements and for confirming if new physics is indeed required.
Early JWST data for the galaxy, NGC 7250 show a decrease in scatter in the
Cepheid Leavitt law by a factor of two relative to existing HST data and
demonstrate that crowding/blending effects are a significant issue in a galaxy
as close as 20 Mpc.Comment: 12 pages, 9 figures, Invited Review for IAU Symposium 376, Richard de
Grijs, Patricia Whitelock and Marcio Catelan, ed
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