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Search Results: 1 - 10 of 221262 matches for " Ralph C. Bohlin "
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The CALSPEC Stars P177D and P330E
Ralph C. Bohlin,Arlo U. Landolt
Physics , 2015, DOI: 10.1088/0004-6256/149/4/122
Abstract: Multicolor photometric data are presented for the CALSPEC stars P177D and P330E. Together with previously published photometry for nine other CALSPEC standards, the photometric observations and synthetic photometry from HST/STIS spectrophotometry agree in the B, V, R, and I bands to better than $\sim$1\% (10 mmag).
Mid-UV Determination of Elliptical Galaxy Abundances and Ages
Jennifer M. Lotz,Henry C. Ferguson,Ralph C. Bohlin
Physics , 1999,
Abstract: We investigate the effects of abundance and age on the mid-UV spectra and Mg_{2} strengths of stellar populations using simple population synthesis models. These models are used to constrain the star formation history of four nearby elliptical galaxies and spiral bulges. The mid-UV (1800 - 3200 \AA) light of evolved stellar populations (> 1 Gyr) is dominated by the main sequence turn-off, unlike the optical light which is dominated by the red giant branch. A detailed investigation of the mid-UV features of elliptical galaxies may help break the age-metallicity degeneracy that plagues optical techniques. Also, a better understanding of this wavelength region is useful for the studies of 0.5 $\leq$ z $\leq$ 1.5 galaxies for which the rest frame mid-UV is redshifted into the visible. We create simple, single age (3-20 Gyr), single metallicity (Z = 0.0004 - 0.05) spectral energy distributions (SEDs) extending into the UV using the Kurucz model stellar fluxes. Comparison to standard stars' mid-UV spectra reveals that the Kurucz model fluxes accurately model a blend feature of FeI and MgI at 2538 {\AA} (Bl2538) and the slope of the continuum between 2600 and 3100 {\AA} (S2850). We find that our simple single age, single metallicity SEDs agree well with these mid-UV features of globular clusters. However, the majority of the galaxies do not agree with the Bl2538, S2850, and Mg_{2} values given by these simple models. The mid-UV features appear to require both an old metal-rich and a small old metal-poor (Z $\leq$ 0.001) population. Despite being limited by the quality of the model stellar fluxes, our study has yielded two promising mid-UV spectral diagnostics (Bl2538 and S2850) and suggests unique and complex star formation histories for elliptical galaxies.
Techniques and Review of Absolute Flux Calibration from the Ultraviolet to the Mid-Infrared
Ralph C. Bohlin,Karl D. Gordon,P. -E. Tremblay
Physics , 2014, DOI: 10.1086/677655
Abstract: The measurement of precise absolute fluxes for stellar sources has been pursued with increased vigor since the discovery of the dark energy and the realization that its detailed understanding requires accurate spectral energy distributions (SEDs) of redshifted Ia supernovae in the rest frame. The flux distributions of spectrophotometric standard stars were initially derived from the comparison of stars to laboratory sources of known flux but are now mostly based on calculated model atmospheres. For example, pure hydrogen white dwarf (WD) models provide the basis for the HST CALSPEC archive of flux standards. The basic equations for quantitative spectrophotometry and photometry are explained in detail. Several historical lab based flux calibrations are reviewed; and the SEDs of stars in the major on-line astronomical databases are compared to the CALSPEC reference standard spectrophotometry. There is good evidence that relative fluxes from the visible to the near-IR wavelength of ~2.5 micron are currently accurate to 1% for the primary reference standards; and new comparisons with lab flux standards show promise for improving that precision.
A New Probe of the Molecular Gas in Galaxies: Application to M101
Denise A. Smith,Ronald J. Allen,Ralph C. Bohlin,Natalya Nicholson,Theodore P. Stecher
Physics , 2000, DOI: 10.1086/309172
Abstract: Recent studies of nearby spiral galaxies suggest that photodissociation regions (PDRs) are capable of producing much of the observed HI in galaxy disks. In that case, measurements of the HI column density and the far-ultraviolet (FUV) photon flux provide a new probe of the volume density of the local underlying H_2. We develop the method and apply it to the giant Scd spiral M101 (NGC 5457). We find that, after correction for the best-estimate gradient of metallicity in the ISM of M101 and for the extinction of the ultraviolet emission, molecular gas with a narrow range of density from 30-1000 cm^-3 is found near star- forming regions at all radii in the disk of M101 out to a distance of 12' (approximately 26 kpc), close to the photometric limit of R_25 = 13.5'. In this picture, the ISM is virtually all molecular in the inner parts of M101. The strong decrease of the HI column density in the inner disk of the galaxy at R_G < 10 kpc is a consequence of a strong increase in the dust-to-gas ratio there, resulting in an increase of the H_2 formation rate on grains and a corresponding disappearance of hydrogen in its atomic form.
A New Grid of Synthetic Spectra for the Analysis of [WC]-type Central Stars of Planetary Nebulae
Graziela R. Keller,James E. Herald,Luciana Bianchi,Walter J. Maciel,Ralph C. Bohlin
Physics , 2012, DOI: 10.1111/j.1365-2966.2011.19085.x
Abstract: We present a comprehensive grid of synthetic stellar-atmosphere spectra, suitable for the analysis of high resolution spectra of hydrogen-deficient post-Asymptotic Giant Branch (post-AGB) objects hotter than 50000 K, migrating along the constant luminosity branch of the Hertzsprung-Russell diagram (HRD). The grid was calculated with CMFGEN, a state-of-the-art stellar atmosphere code that properly treats the stellar winds, accounting for expanding atmospheres in non-LTE, line blanketing, soft X-rays, and wind clumping. We include many ionic species that have been previously neglected. Our uniform set of models fills a niche in an important parameter regime, i.e., high effective temperatures, high surface gravities, and a range of mass-loss values. The grid constitutes a general tool to facilitate determination of the stellar parameters and line identifications and to interpret morphological changes of the stellar spectrum as stars evolve through the central star of planetary nebula (CSPN) phase. We show the effect of major physical parameters on spectral lines in the far-UV, UV, and optical regimes. We analyse UV and far-UV spectra of the central star of NGC 6905 using the grid to constrain its physical parameters, and proceed to further explore other parameters not taken in consideration in the grid. This application shows that the grid can be used to constrain the main photospheric and wind parameters, as a first step towards a detailed analysis. The full grid of synthetic spectra, comprising far-UV, UV, optical, and IR spectral regions, is available on-line.
The White Dwarf Distance to the Globular Cluster NGC 6752 (and its Age) with the Hubble Space Telescope (Based on observations made with the NASA/ESA Hubble Space Telescope (HST))
Alvio Renzini,Angela Bragaglia,Francesco R. Ferraro,Roberto Gilmozzi,Sergio Ortolani,J. B. Holberg,James Liebert,F. Wesemael,Ralph C. Bohlin
Physics , 1996,
Abstract: Deep Hubble Space Telescope (HST) observations with WFPC2 of the nearby globular cluster NGC 6752 have allowed us to obtain accurate photometry for the cluster white dwarfs (WD). A sample of local WDs of known trigonometric parallax and mass close to that of the cluster WDs have also been observed with WFPC2. Matching the cluster and the local WD sequences provides a direct measure of the distance to the cluster: $(m-M)_\circ=13.05$, with an uncertainty less than $\pm0.1$ mag which allows a substantial reduction in the uncertainty in the age of the cluster. Indeed, coupling this value of the cluster distance to the cluster metallicity, helium abundance and $\alpha$-element enhancement [$\alpha$/Fe]=0.5 yields an age of 15.5 Gyr and 14.5 Gyr using evolutionary models that do not include or do include helium diffusion, respectively. The uncertainty affecting these age determinations is $\sim 10\%$. The majority of the cluster WDs appear to be of the DA variety, while the color-magnitude location of two WDs is consistent with the DB type. This suggests a cluster DB/DA ratio similar to that of WDs in the solar neighborhood.
Ultraviolet Imaging of the Irregular Galaxy NGC 4449 with UIT: Photometry and Recent Star-Formation History
Robert S. Hill,Michael N. Fanelli,Denise A. Smith,Ralph C. Bohlin,Susan G. Neff,Robert W. O'Connell,Morton S. Roberts,Andrew M. Smith,Theodore P. Stecher
Physics , 1998, DOI: 10.1086/306302
Abstract: The bright Magellanic irregular galaxy NGC 4449 was observed during the Astro-2 Space Shuttle mission by the Ultraviolet Imaging Telescope (UIT), which obtained images of a ~40 arcmin field centered on the galaxy in two broad far-ultraviolet (FUV) bands centered at 1520 A and 1620 A, with 3 arcsec - 5 arcsec spatial resolution. Together with H-alpha and H-beta fluxes from ground-based Fabry-Perot images, these data are analyzed in order to explore the recent star formation history of NGC 4449. Maps of the flux ratios H-alpha/FUV and FUV/blue continuum are presented and interpreted using evolutionary synthesis models. Photometry is presented both for 22 apertures containing large OB complexes and for 57 small apertures containing compact FUV-emitting knots. The OB complexes along the northern edge of the visible system have high H-alpha/FUV ratios, and thus appear to be more dominated by the current generation of stars than are other parts of the galaxy. However, young sources do exist elsewhere and are particularly conspicuous along the bar. The small aperture analysis shows three candidate regions for sequential star formation. Surface brightness profiles are consistent with an exponential disk in both the FUV and the optical continuum.
HST Observations of New Horizontal Branch Structures in the Globular Cluster omega Centauri
Noella L. D'Cruz,Robert W. O'Connell,Robert T. Rood,Jonathan H. Whitney,Ben Dorman,Wayne B. Landsman,Robert S. Hill,Theodore P. Stecher,Ralph C. Bohlin
Physics , 1999, DOI: 10.1086/308375
Abstract: The globular cluster omega Centauri contains the largest known population of very hot horizontal branch (HB) stars. We have used the Hubble Space Telescope to obtain a far-UV/optical color-magnitude diagram of three fields in omega Cen. We find that over 30% of the HB objects are ``extreme'' HB or hot post-HB stars. The hot HB stars are not concentrated toward the cluster center, which argues against a dynamical origin for them. A wide gap in the color distribution of the hot HB stars appears to correspond to gaps found earlier in several other clusters. This suggests a common mechanism, probably related to giant branch mass loss. The diagram contains a significant population of hot sub-HB stars, which we interpret as the ``blue-hook'' objects predicted by D'Cruz et al. (1996a). These are produced by late He-flashes in stars which have undergone unusually large giant branch mass loss. omega Cen has a well-known spread of metal abundance, and our observations are consistent with a giant branch mass loss efficiency which increases with metallicity.
HST Spectrophotometry and Models for Solar Analogs
R. C. Bohlin
Physics , 2010, DOI: 10.1088/0004-6256/139/4/1515
Abstract: Absolute flux distributions for seven solar analog stars are measured from 0.3 to 2.5 \mu m by HST spectrophotometry.In order to predict the longer wavelength mid-IR fluxes that are required for JWST calibration, the HST SEDs are fit with Castelli & Kurucz model atmospheres; and the results are compared with fits from the MARCS model grid. The rms residuals in 10 broad band bins are all <0.5% for the best fits from both model grids. However, the fits differ systematically: The MARCS fits are 40-100 K hotter in T_{eff}, 0.25-0.80 higher in log g, 0.01-0.10 higher in log z, and 0.008-0.021 higher in the reddening E(B-V), probably because their specifications include different metal abundances. Despite these differences in the parameters of the fits, the predicted mid-IR fluxes differ by only ~1%; and the modeled flux distributions of these G stars have an estimated ensemble accuracy of 2% out to 30 \mu m.
HST CALSPEC Flux Standards: Sirius (and Vega)
R. C. Bohlin
Physics , 2014,
Abstract: The Space Telescope Imaging Spectrograph (STIS) has measured the flux for Sirius from 0.17--1.01~$\mu$m on the \emph{HST} White Dwarf scale. Because of the cool debris disk around Vega, Sirius is commonly recommended as the primary IR flux standard. The measured STIS flux agrees well with predictions of a special Kurucz model atmosphere, adding confidence to the modeled IR flux predictions. The IR flux agrees to 2--3% with respect to the standard template of Cohen and to 2% with the MSX absolute flux measurements in the mid-IR. A weighted average of the independent visible and mid-IR absolute flux measures implies that the monochromatic flux at 5557.5~\AA\ (5556~\AA\ in air) for Sirius and Vega, respectively, is $1.35\times10^{-8}$ and $3.44\times10^{-9}$~erg cm$^{-2}$ s$^{-1}$ \AA$^{-1}$ with formal uncertainties of 0.5%. Contrary to previously published conclusions, the Hipparcos photometry offers no support for the variability of Vega. Pulse pileup severely affects the Hp photometry for the brightest stars.
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