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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

COBE diffuse infrared background experiment observations of Galactic reddening and stellar populations

This Letter describes the results of an initial study of Galactic extinction and the colors of Galactic stellar populations in the near-IR using the Diffuse Infrared Background Experiment (DIRBE) aboard the Cosmic Background Explorer (COBE) spacecraft. The near-IR reddening observed by DIRBE is consistent with the extinction law tabulated by Rieke & Lebofsky (1985). The distribution of dust and stars in most of the first and fourth quadrants of the Galactic plane (0 deg less than l less than 90 deg, and 270 deg less than l less than 360 deg, respectively) can be modeled as a stellar background source seen through up to approximately 4 mag of extinction at 1.25 micrometers. The unreddened near-IR colors of the Galactic disk are similar to those of late-K and M giants. The Galactic bulge exhibits slightly bluer colors in the 2.2-3.5 micrometers range, as noted by Terndrup et al. (1991). Star-forming regions exhibit colors that indicate the presence of a approximately 900 K continuum produced by hot dust or polycyclic aromatic hydrocarbons (PAHs) contributing at wavelengths as short as 3.5 micrometers.

Arendt, R. G.↗
NASA NTRS1971

Infrared observations of the core of Centaurus A, NGC 5128.

Infrared radiation from the nucleus of NGC 5128 has been measured from 1 to 10 microns. Scans and aperture-size data show that there is only one bright infrared region within the central 1 min of the galaxy and that it is coincident to within 5 sec with the ?hot spot' of Kunkel and Bradt. The 3.5- and 2.2-micron radiation is more peaked toward the center than are the shorter wavelengths. The data are interpreted to indicate a small nonstellar infrared core superposed on an extended, reddened stellar component.

Becklin, E. E.↗
NASA NTRS2014

Spectral Energy Distribution Fitting of Hetdex Pilot Survey Ly-alpha Emitters in Cosmos and Goods-N

We use broadband photometry extending from the rest-frame UV to the near-IR to fit the individual spectral energy distributions of 63 bright (L(Ly-alpha) greater than 10(exp 43) erg s(exp −1) Ly-alpha emitting galaxies (LAEs) in the redshift range 1.9 less than z less than 3.6. We find that these LAEs are quite heterogeneous, with stellar masses that span over three orders of magnitude, from 7.5 greater than logM/solar mass less than 10.5. Moreover, although most LAEs have small amounts of extinction, some high-mass objects have stellar reddenings as large as E(B − V ) is approximately 0.4. Interestingly, in dusty objects the optical depths for Ly-alpha and the UV continuum are always similar, indicating that Lyα photons are not undergoing many scatters before escaping their galaxy. In contrast, the ratio of optical depths in low-reddening systems can vary widely, illustrating the diverse nature of the systems. Finally, we show that in the star-formation-rate-log-mass diagram, our LAEs fall above the "main-sequence" defined by z is approximately 3 continuum selected star-forming galaxies. In this respect, they are similar to submillimeter-selected galaxies, although most LAEs have much lower mass.

Emitters↗
NASA NTRS1992

BV RI CCD photometry of 361,281 objects in the field of M 31

Deep BV RI CCD photometry was performed on a 1 sq deg region of M 31. A catalog of photometry and astrometry of a total of 361,281 stars is presented, with typical completion limits of BV RI = (22.3, 22.2, 22.2, 20.9). Photometric accuracy is about 2 percent at V = 19. This catalog allows detailed studies of stellar populations and reddening. The data are currently being used to assist in finding the optical counterparts of Einstein and ROSAT X-ray sources.

Magnier, E. A.↗
NASA NTRS2012

Dust Attenuation and H(alpha) Star Formation Rates of Z Approx. 0.5 Galaxies

Using deep narrow-band and broad-band imaging, we identify 401 z approximately 0.40 and 249 z approximately 0.49 H-alpha line-emitting galaxies in the Subaru Deep Field. Compared to other H-alpha surveys at similar redshifts, our samples are unique since they probe lower H-alpha luminosities, are augmented with multi-wavelength (rest-frame 1000AA--1.5 microns) coverage, and a large fraction (20%) of our samples has already been spectroscopically confirmed. Our spectra allow us to measure the Balmer decrement for nearly 60 galaxies with H-beta detected above 5-sigma. The Balmer decrements indicate an average extinction of A(H-alpha)=0.7(uparrow){+1.4}_{-0.7} mag. We find that the Balmer decrement systematically increases with higher H-alpha luminosities and with larger stellar masses, in agreement with previous studies with sparser samples. We find that the SFRs estimated from modeling the spectral energy distribution (SED) is reliable---we derived an "intrinsic" H-alpha luminosity which is then reddened assuming the color excess from SED modeling. The SED-predicted H-alpha luminosity agrees with H-alpha narrow-band measurements over 3 dex (rms of 0.25 dex). We then use the SED SFRs to test different statistically-based dust corrections for H-alpha and find that adopting one magnitude of extinction is inappropriate: galaxies with lower luminosities are less reddened. We find that the luminosity-dependent dust correction of Hopkins et al. yields consistent results over 3 dex (rms of 0.3 dex). Our comparisons are only possible by assuming that stellar reddening is roughly half of nebular reddening. The strong correspondence argue that with SED modeling, we can derive reliable intrinsic SFRs even in the absence of H-alpha measurements at z approximately 0.5.

Ly, Chun↗
NASA NTRS1981

Anomalous extinction in the planetary nebula Abell 30

The stellar continuous energy distribution from 8000 to 1250 A in Abell 30 is quite peculiar. The observed flux can be understood if a hot central star (approximately 200,000 K) is immersed in an absorbing dust cloud which has anomalous absorption with its peak near 2470 A. The dust nebula is a strong infrared source, maintained by conversion of the stellar UV flux. The internal reddening is E(B - V) = 0.30 mag and reaches 2.5 mag at 2470 A; it is best matched not by graphite but by laboratory observations of carbon smoke. Intense P Cygni lines show that a strong stellar wind exists. The central star may be an O subdwarf, approaching more nearly a helium-carbon degenerate DO.

Greenstein, J. L.↗
NASA NTRS1988

Near-infrared stellar-occultation predictions for Uranus and Neptune - 1987-1990

Stellar-occultation predictions are presented for Uranus and Neptune covering the period 1987-1990, based on automated scans of R and I plates taken with the UK Schmidt Telescope in Australia. These scans have revealed many late-type or highly reddened stars suitable for stellar-occultation observations at 2.2 microns that were missed in previous searches of yellow-sensitive astrographic plates (Mink and Klemola, 1985). Comparisons involving several hundred stars measured by Mink and Klemola (1985) indicate that the internal random errors in the stellar positions estimated here are less than about 0.2 arcsec, comparable to the precision reached using conventional astrographic methods. In addition, JHK photometry was obtained for many of the Neptune stars, which is used to establish an empirical (I-K) vs (R-I) calibration for the remaining occultation candidates, and thus provide estimates of their K magnitudes.

Nicholson, Philip D.↗
NASA NTRS1975

Ultraviolet stellar astronomy

The author has identified the following significant results. During all three Skylab missions, prism-on observations were obtained in 188 starfields and prism-off observations in 31 starfields. In general, the fields are concentrated in the Milky Way where the frequency of hot stars is highest. These fields cover an area approximately 3660 degrees and include roughly 24 percent of a band 30 deg wide centered on the plane of the Milky Way. A census of stars in the prism-on fields shows that nearly 6,000 stars have measurable flux data at a wavelength of 2600A, that 1,600 have measurable data at 2000A, and that 400 show useful data at 1500A. Obvious absorption or emission features shortward of 2000A are visible in approximately 120 stars. This represents a bonanza of data useful for statistical studies of stellar classification and of interstellar reddening as well as for studies of various types of peculiar stars.

Henize, K. G.↗
NASA NTRS2014

A Magnified View of the Kinematics and Morphology of RCSGA 032727-132609: Zooming in on a Merger at z = 1.7

We present a detailed analysis of multi-wavelength Hubble Space Telescope/Wide Field Camera 3 (WFC3) imaging and Keck/OSIRIS near-infrared adaptive optics-assisted integral field spectroscopy for a highly magnified lensed galaxy at z = 1.70. This young starburst is representative of ultraviolet-selected star-forming galaxies (SFGs) at z approx. 2 and contains multiple individual star-forming regions. Due to the lensing magnification, we can resolve spatial scales down to 100 pc in the source plane of the galaxy. The velocity field shows disturbed kinematics suggestive of an ongoing interaction and there is a clear signature of a tidal tail. We constrain the age, reddening, star formation rate, and stellar mass of the star-forming clumps from spectral energy distribution (SED) modeling of the WFC3 photometry and measure their H(alpha) luminosity, metallicity, and outflow properties from the OSIRIS data.With strong star-formation-driven outflows in four clumps, RCSGA0327 is the first high-redshift SFG at stellar mass <10(exp 10) Stellar Mass with spatially resolved stellar winds. We compare the H(alpha) luminosities, sizes, and dispersions of the star-forming regions with other high-z clumps as well as local giant H(II) regions and find no evidence for increased clump star formation surface densities in interacting systems, unlike in the local universe. Spatially resolved SED modeling unveils an established stellar population at the location of the largest clump and a second mass concentration near the edge of the system that is not detected in H(alpha) emission. This suggests a picture of an equal-mass mixed major merger, which has not triggered a new burst of star formation or caused a tidal tail in the gas-poor component.

Magnified↗
NASA NTRS2003

Global Properties of Local Star Forming Galaxies (ADP 2000)

We performed an archival study of the Hopkins Ultraviolet Telescope (HUT) Astro-2 database. Nineteen spectra of star-forming regions and starburst galaxies were retrieved, reprocessed, and analyzed. The spectra cover the wavelength region 912- 1800 A, providing access to the domain of peak luminosity from a young stellar population. We created an atlas of galaxy spectra documenting the continuum and line properties with an emphasis on the relatively unexplored spectral region below 1200 A. The dust obscuration law was derived from a comparison of the HUT spectra with synthetic population models. The law is similar to the commonly adopted starburst reddening curve at longer wavelengths and approaches the Milky Way law near the Lyman break. A simple power-law parameterization is given, which allows users to express the reddening law in terms of the stellar or nebular color excess at ultraviolet or optical wavelengths. We studied the effect of time-dependent dust obscuration on synthetic ultraviolet line profiles of a young stellar population. If the youngest and most massive stars are more obscured than the older, less massive stars, the C IV 1550 and other stellar wind lines are significantly diluted with respect to a simple foreground screen model for the dust. We propose to use stellar wind lines as a probe of the dust-obscuration model instead of the previously employed nebular emission lines. Since purely stellar diagnostics are utilized, uncertain assumptions on the nebular properties are unnecessary. Photoionization models demonstrate that the C IV 1550 emission is typically dominated by stellar winds and nebular contamination is negligible. A first comparison with the galaxy sample observed with the Hopkins Ultraviolet Telescope favors a dust geometry affecting ionizing and nonionizing stars equally. We point out the need for higher quality data for a more rigorous comparison. The Hubble Space Telescope is capable of obtaining such data in the future.

Leitherer, Claus↗
NASA NTRS1990

On the disappearance of O I in some high-luminosity cool stars

Far UV spectra and flux measurements of cool giant and supergiant stars, focusing on the O I (UV 2) and S I/Si II fluxes and their dependence on stellar parameters are presented. The de-reddened O I surface fluxes vary by factors of several hundred over the 11 stars in the sample and, at a given luminosity, decrease with decreasing effective temperature. No flux at all is detected in the three M supergiants observed to date. In contrast, the S I/Si II blend near 1810 angstrom is clearly detected in all the stars. Its strength generally increases with luminosity, but has little effective temperature dependence. Possible explanations for the behavior of O I include changes in the intensities of Lyman alpha and beta which excite the lines of S I and O I, respectively, as well as scattering and/or absorption of the O I flux in a dense, circumstellar gas shell.

Carpenter, K. G.↗
NASA NTRS1993

Probing the optical depth of spiral galaxies using multi-waveband observations

The questions of the optical depth and extinction of galaxies obviously have far-reaching implications; from determining the true stellar content and baryonic mass of galaxies to the origin of the major component of their far infrared radiation. We have investigated whether the reddening of stars can be used to determine the amount and extent of obscuring dust. We find that the amount of reddening is very sensitive to the assumed geometry. The usually assumed case of a screen of dust between the observer and the star produces, for a given optical depth, the maximum amount of reddening. However, for external galaxies such a geometry is clearly wrong. More realistic geometries where the dust and stars are uniformly mixed (a 'slab') or where the dust is sandwiched by unobscured stars (a 'sandwich') produce significantly less reddening. For the slab geometry of the reddening rises to a maximum value as the optical depth is increased, and for a sandwich geometry the reddening will actually decrease as the optical depth becomes larger. The reddening vectors produced by these realistic geometries are compared to multi-waveband observations of individual regions of a sample of eight galaxies. We find evidence for significant optical depths in the central regions of some of these galaxies, but optical depths of less than unity at approx. 2 scale lengths from the centers. The optical depths derived are in good agreement with other tests such as the measured hydrogen column densities and the far infrared to blue light ratios. However, whereas these two latter approaches are insensitive to the relative distributions of the stars and the dust, the amount of reddening seen in these eight galaxies suggests that the dust to stellar scale height ratio xi is greater than or equal to 0.5. We find typical blue band extinctions of A(sub B) = 0.75-1.0, a factor of 3-4 higher than the RC2 value.

Evans, Rhodri↗
NASA NTRS1982

Picture gallery - A structured presentation of OAO-2 photometric data supported by OAO-2 spectrophotometric data and UBV, ANS and TD1 observations

Graphs are presented for the stellar fluxes of 531 stars in the 5500-1330 A wavelength range, which have been divided into 52 categories on the basis of spectral types. The merging of medium band interference filter photometry, UBV photometry, ANS photometry and TD1 fluxes, as well as the ordering of the objects, should prove helpful in studies of interstellar reddening, luminosity effects, bandwidth effects, and comparisons with model stellar atmospheres. The agreement between the various UV photometric systems for early-type stars is generally better than 0.10 mag. A list of stars whose photometric properties indicate stellar or interstellar anomalies is also provided.

Koornneef, J.↗
NASA NTRS2000

The Dust Content and Opacity of Actively Star-Forming Galaxies

We present far-infrared (FIR) photometry at 150 and 205 micron(s) of eight low-redshift starburst galaxies obtained with the Infrared Space Observatory (ISO) ISOPHOT. Five of the eight galaxies are detected in both wave bands, and these data are used, in conjunction with IRAS archival photometry, to model the dust emission at lambda approximately greater than 40 microns. The FIR spectral energy distributions (SEDs) are best fitted by a combination of two modified Planck functions, with T approx. 40 - 55 K (warm dust) and T approx. 20-23 K (cool dust) and with a dust emissivity index epsilon = 2. The cool dust can be a major contributor to the FIR emission of starburst galaxies, representing up to 60% of the total flux. This component is heated not only by the general interstellar radiation field, but also by the starburst itself. The cool dust mass is up to approx. 150 times larger than the warm dust mass, bringing the gas-to-dust ratios of the starbursts in our sample close to Milky Way values, once resealed for the appropriate metallicity. The ratio between the total dust FIR emission in the range 1-1000 microns and the IRAS FIR emission in the range 40 - 120 microns is approx. 1.75, with small variations from galaxy to galaxy. This ratio is about 40% larger than previously inferred from data at millimeter wavelengths. Although the galaxies in our sample are generally classified as "UV bright," for four of them the UV energy emerging shortward of 0.2 microns is less than 15% of the FIR energy. On average, about 30% of the bolometric flux is coming out in the UV-to-near-IR wavelength range; the rest is emitted in the FIR. Energy balance calculations show that the FIR emission predicted by the dust reddening of the UV-to-near-IR stellar emission is within a factor of approx. 2 of the observed value in individual galaxies and within 20% when averaged over a large sample. If our sample of local starbursts is representative of high-redshift (z approx. greater than 1), UV - bright star-forming galaxies, these galaxies' FIR emission will be generally undetected in submillimeter surveys, unless: (1) their bolometric luminosity is comparable to or larger than that of ultraluminous FIR galaxies and (2) their FIR SED contains a cool dust component.

Calzetti, Daniela↗
NASA NTRS1994

The metallicity of M4: Accurate spectroscopic fundamental parameters for four giants

High-quality spectra, covering the wavelength range 5480 to 7080 A, have been obtained for four giant stars in the intermediate-metallicity CN-bimodal globular cluster M4 (NGC 6121). We have employed a model atmosphere analysis that is entirely independent from cluster parameters, such as distance, age, and reddening, in order to derive accurate values for the stellar parameters effective temperature, surface gravity, and microturbulence, and for the abundance of iron relative to the Sun, (Fe/H), and of calcium, Ca/H, for each of the four stars. Detailed radiative transfer and statistical equilibrium calculations carried out for iron and calcium suggest that departures from local thermodynamic equilibrium are not significant for the purposes of our analysis. The spectroscopically derived effective temperatures for our program stars are hotter by about 200 K than existing photometric calibrations suggest. We conclude that this is due partly to the uncertain reddening of M4 and to the existing photometric temperature calibration for red giants being too cool by about 100 K. Comparison of our spectroscopic and existing photometric temperatures supports the prognosis of a significant east-west gradient in the reddening across M4. Our derived iron abundances are slightly higher than previous high-resolution studies suggested; the differences are most probably due to the different temperature scale and choice of microturbulent velocities adopted by earlier workers. The resulting value for the metallicity of M4 is (Fe/H )(sub M4) = -1.05 + or - 0.15. Based on this result, we suggest that metallicities derived in previous high-dispersion globular cluster abundance analyses could be too low by 0.2 to 0.3 dex. Our calcium abundances suggest an enhancement of calcium, an alpha element, over iron, relative to the Sun, in M4 of (Ca/H) = 0.23.

Drake, J. J.↗
NASA NTRS1986

Unipolar bubbles in star-forming regions

Near-infrared images of two star-forming regins, AFGL 2591 and S140, reveal nearly circular loops with the young stellar object at one edge. The loops are aligned with the direction of high-velocity outflow and are interpreted as the outline of a bubble or cavity of dimensions about 10,000 AU in the dense molecular cloud material. In both cases, the bubble appears on only one side of the young stellar object. For AFGL 2591, it corresponds to the blueshifted CO lobe, while for S140 IRS 1 it corresponds to the redshifted CO lobe. In 1982, Koenigl predicted this shape and geometry bubble for a stellar wind impinging on a molecular cloud with density gradient of roughly 1/z-squared. A similar density gradient is implied by reddening measurements of the bubble in AFGL 2591. These images show directly the interaction of the stellar wind from a newly forming star with its environment on about 1000 AU (arcsec) scales.

Forrest, W. J.↗
NASA NTRS2019

Mapping the Interstellar Reddening and Extinction toward Baade's Window Using Minimum Light Colors of ab-type RR Lyrae Stars: Revelations from the De-reddened Color–Magnitude Diagrams

We have obtained repeated images of six fields toward the Galactic bulge in five passbands (u, g, r, i, z) with the DECam imager on the Blanco 4 m telescope at CTIO. From more than 1.6 billion individual photometric measurements in the field centered on Baade’s window, we have detected 4877 putative variable stars. A total of 474 of these have been confirmed as fundamental mode RR Lyrae stars, whose colors at minimum light yield line-of- sight reddening determinations, as well as a reddenning law toward the Galactic Bulge, which differs significantly from the standard R(V) = 3.1 formulation. Assuming that the stellar mix is invariant over the 3 square-degree field, we are able to derive a line-of-sight reddening map with sub-arcminute resolution, enabling us to obtain de-reddened and extinction corrected color–magnitude diagrams (CMDs) of this bulge field using up to 2.5 million well-measured stars. The corrected CMDs show unprecedented detail and expose sparsely populated sequences: for example, delineation of the very wide red giant branch, structure within the red giant clump, the full extent of the horizontal branch, and a surprising bright feature that is likely due to stars with ages younger than 1 Gyr. We use the RR Lyrae stars to trace the spatial structure of the ancient stars and find an exponential decline in density with Galactocentric distance. We discuss ways in which our data products can be used to explore the age and metallicity properties of the bulge, and how our larger list of all variables is useful for learning to interpret future LSST alerts.

Abhijit Saha↗