Search

To explore the properties of extreme outer stellar disks, we obtained ultra-deep V and GALEX UV images of 4 dwarf irregular galaxies and one Blue Compact Dwarf galaxy and ultra-deep B images of 3 of these. Our V-band surface photometry extends to 29.5 magnitudes arcsec^-2. We convert the FUV and V-band photometry, along with Halpha photometry, into radial star formation rate profiles that are sensitive to timescales from 10 Myrs to the lifetime of the galaxy. We also compare the stellar distributions, surface brightness profiles, and star formation rate profiles to HI-line emission maps, gas surface density profiles, and gas kinematics. Our data lead us to two general observations: First, the exponential disks in these irregular galaxies are extraordinarily regular. The stellar disks continue to decline exponentially as far as our measurements extend. In spite of lumpiness in the distribution of young stars and HI distributions and kinematics that have significant unordered motions, sporadic processes that have built the disks-star formation, radial movement of stars, and perhaps even perturbations from the outside-have, nevertheless, conspired to produce standard disk profiles. Second, there is a remarkable continuity of star formation throughout these disks over time. In four out of five of our galaxies the star formation rate in the outer disk measured from the FUV tracks that determined from the V-band, to within factors of 5, requiring star formation at a fairly steady rate over the galaxy's lifetime. Yet, the HI surface density profiles generally decline with radius more shallowly than the stellar light, and the gas is marginally gravitationally stable against collapse into clouds. Outer stellar disks are challenging our concepts of star formation and disk growth and provide a critical environment in which to understand processes that mold galaxy disks., Comment: AJ, in press

We present optical (UBVJ), ultraviolet (FUV, NUV), and high resolution atomic hydrogen (HI) observations of the nearby blue compact dwarf (BCD), VII Zw 403. We find that VII Zw 403 has a relatively high HI mass-to-light ratio for a BCD. The rotation velocity is nominally 10-15 km/s, but rises to ~20 km/s after correction for the ~8-10 km/s random motions present in the gas. The velocity field is complex; including a variation in the position angle of the major axis going from the NE to the SW parts of the galaxy. Our high resolution HI maps reveal structure in the central gas, including a large, low-density HI depression or hole between the southern and northern halves of the galaxy, coincident with an unresolved x-ray source. Although interactions have been proposed as the triggering mechanism for the vigorous star formation occurring in BCDs, VII Zw 403 does not seem to have been tidally triggered by an external interaction, as we have found no nearby possible perturbers. It also doesn't appear to fall in the set of galaxies that exhibit a strong central mass density concentration, as its optical scale length is large in comparison to similar systems. However, there are some features that are compatible with an accretion event: optical/HI axis misalignment, a change in position angle of the kinematic axis, and a complex velocity field., Comment: 62 pages, 19 figures

We have designed and constructed a ``dispersed Fourier Transform Spectrometer'' (dFTS), consisting of a conventional FTS followed by a grating spectrometer. By combining these two devices, we negate a substantial fraction of the sensitivity disadvantage of a conventional FTS for high resolution, broadband, optical spectroscopy, while preserving many of the advantages inherent to interferometric spectrometers. In addition, we have implemented a simple and inexpensive laser metrology system, which enables very precise calibration of the interferometer wavelength scale. The fusion of interferometric and dispersive technologies with a laser metrology system yields an instrument well-suited to stellar spectroscopy, velocimetry, and extrasolar planet detection, which is competitive with existing high-resolution, high accuracy stellar spectrometers. In this paper, we describe the design of our prototype dFTS, explain the algorithm we use to efficiently reconstruct a broadband spectrum from a sequence of narrowband interferograms, and present initial observations and resulting velocimetry of stellar targets.

We present sensitive and high resolution 21 cm observations of the dwarf irregular (dIm) galaxy DDO 43, in conjunction with optical broad and narrow band images in U, B, V, and Halpha. The observations are used to examine the relationship between its HI morphology and kinematics to past and present star formation. Optically, it is a small (R_25 = 990 pc), faint (M_B of -14.0) Im with a slightly boxy shape. In HI, DDO 43 has an extended (R_HI/R_Ho = 2.8) gas envelope. There is a high density ridge associated with the optical body of the galaxy, containing several higher density knots and lower density holes. The largest hole is ~850x530 pc. No expansion is detected, so it must be relatively old. The largest and potentially oldest (7-70 Myr) of the six identified star clusters is located at the western edge of the hole. Four of the other clusters are located near high density peaks. There are several HII regions, most of which are associated with peaks in the HI surface density. The overall star formation rate is average for its type. In many ways, DDO 43 is a very typical dwarf irregular galaxy. Its HI morphology is consistent with a history of episodes of localized star ormation that create holes and shells in the ISM, some of which can overlap. These features are located within the area of solid-body rotation in the galaxy; the lack of shear in these small systems allows such structures to persist for long periods of time., Comment: 50 pages, 22 figures, accepted by The Astronomical Journal, available at http://www.fiu.edu/~simpsonc/d43/

An asymmetric surface brightness distribution of the rapidly rotating A7IV-V star, Altair, has been measured by the Navy Prototype Optical Interferometer (NPOI). The observations were recorded simultaneously using a triangle of three long baselines of 30m, 37m, and 64m, on 19 spectral channels, covering the wavelength range of 520nm to 850nm. The outstanding characteristics of these observations are (a) high resolution with the minimum fringe spacing of 1.7mas, easily resolving the 3-milliarcsecond (mas) stellar disk, and (b) the measurement of closure phase which is a sensitive indicator to the asymmetry of the brightness distribution of the source. Uniform disk diameters fit to the measured squared visibility amplitudes confirms the Altair's oblate shape due to its rapid rotation. The measured observables of Altair showed two features which are inconsistent with both the uniform-disk and limb-darkened disk models, while the measured observable of the comparison star, Vega, are consistent with the limb-darkened disk model. The first feature is that measured squared visibility amplitudes at the first minimum do not reach 0.0 but rather remain at about 0.02, indicating the existence of a small bright region on the stellar disk. The other is that the measured closure phases show non-zero/180 degrees at all spectral channels, which requires an asymmetric surface brightness distribution. We fitted the measured observables to a model with a bright spot on a limb-darkened disk and found the observations are well reproduced by a bright spot, which has relative intensity of 4.7%, on a 3.38 mas limb-darkened stellar disk. Rapid rotation of Altair indicates that this bright region is a pole, which is brighter than other part of the star owing to gravity darkening., Comment: AAS TeX, 14 pages, 7 figures, accepted for publication in ApJ

The geometric Baade-Wesselink method is one of the most promising techniques for obtaining a better calibration of the Cepheid period-luminosity relation by means of interferometric measurements of accurate diameters. In this paper we present new wavelength- and phase-dependent limb darkening corrections based on our time-dependent hydrodynamic models of the classical Cepheid zeta Gem. We show that a model simulation of a Cepheid atmosphere, taking into account the hydrodynamic effects associated with the pulsation, shows strong departures from the limb darkening otherwise predicted by a static model. For most of its pulsational cycle the hydrodynamic model predicts a larger limb darkening then the equivalent static model. The hydrodynamics affects the limb darkening mainly at UV and optical wavelengths. Most of these effects evolve slowly as the star pulsates, but there are phases, associated with shocks propagating into the photosphere, in which significant changes in the limb darkening take place on time-scales of the order of less than a day. We assess the implication of our model LD corrections fitting the geometric Baade-Wesselink distance of zeta Gem for the available near-IR PTI data. We discuss the effects of our model limb darkening on the best fit result, and analyze the requirements needed to test the time-dependence of the limb darkening with future interferometric measurements., Comment: 22 pages, 5 figures, to be published on the Astrophysical Journal, June 1 2003 issue

Direct diameter observations of Cepheid variables are used to calibrate the Barnes-Evans Cepheid surface brightness relation. Fifty-nine separate Cepheid diameter measurements from four different optical interferometers are used to calculate surface brightnesses as a function of magnitude and color. The linear fit to Cepheid surface brightness versus color is in excellent agreement with functions in the literature found using interferometric observations of non-variable giant and supergiant stars (Fouque & Gieren 1997). Using these relations the distance is calculated to delta Cephei, for which an independent distance is known from trigonometric parallax. The distance from the relation in this paper differs from that derived by the previously published relation by 4% but is still marginally within the combined errors. Both distances are well within the errors of the distance derived from trigonometric parallax., Comment: 30 pages, 8 figures, accepted for publication in AJ

We present mean angular diameters for two cepheid variables, alpha Ursae Minoris and zeta Geminorum, determined with the Navy Prototype Optical Interferometer (NPOI). We present linear radii for these cepheids and two additional cepheids, delta Cephei and eta Aquilae, previously observed at the NPOI. We find the limb-darkened angular diameter of alpha Ursae Minoris and of zeta Geminorum to be 3.28 +/- 0.02 and 1.55 +/- 0.09 milliarcseconds respectively. Using trigonometric parallaxes, we find the linear radii of alpha Ursae Minoris, zeta Geminorum, delta Cephei and eta Aquilae to be 46 (+/- 3), 60 (+25, -14), 45 (+8, -6), and 69 (+28, -15) Solar radii respectively. We compare the pulsation periods and linear radii of this sample of cepheids, which range in period from three to 11 days, to theoretical and empirical period-radius and period-radius-mass relations found in the literature. We find that the observed diameter of alpha Ursae Minoris is in excellent agreement with the predicted diameter as determined from both surface brightness techniques and theory only if alpha Ursae Minoris is a first overtone pulsator.

We have measured the diameters of the Cepheid variables delta Cephei (18 nights) and eta Aquilae (11 nights) with the Navy Prototype Optical Interferometer. The primary results of these observations are the mean angular diameters of these Cepheids: 1.520 +/- 0.014 milliseconds of arc (mas) for delta Cep and 1.69 +/- 0.04 mas for eta Aql. We also report limb-darkened diameters for the check stars in this program: for beta Lac, theta(LD) = 1.909 +/- 0.011 mas, and for 12 Aql, theta(LD) = 2.418 +/- 0.010 mas. When combined with radius estimates from period-radius relations in the literature, the Cepheid angular diameters suggest distances slightly smaller than, but still consistent with, the Hipparcos distances. Pulsations are weakly detected at a level of about 1.5 sigma to 2 sigma for both Cepheids., Comment: 15 pages, including one figure; LaTeX 2e; uses aastex, aas_symbols, natbib; accepted for publication in the Astronomical Journal