Engineering topics

Fitzpatrick, Edward L.

Publications and source records attributed to Fitzpatrick, Edward L..

16 records

NASA NTRS1995

Composition of interstellar clouds in the disk and halo. 3: HD 149881

The spectrum of HD 149881, a halo star 1300 pc above the Galactic plane, has been analyzed, based on high-resolution GHRS data from Hubble Space Telescope (HST) together with 21 cm emission observations and profiles of Na I and Ca II features. The results reveal 11 absorption components, of which nine are produced in H I regions, with log N(H(sup 0) in each region mostly approximately 19.5; one component clearly originates in an H II region. Detailed column densities show that (D(Zn)), the absolute value of the depletion D, for Zn, like that for S, does not much exceed 0.1 dex. With Zn used as a standard of reference, D(Fe) varies from -1.1 to -0.6 dex; (D(Cr)) and (D(Mn)) average less than (D(Fe)) by about 0.1 and 0.2 dex, respectively, with dispersions of about 0.1 dex in this difference. As in Papers I and II, the measured D(Si) is consistent with a 2 to 1 ratio of Fe to Si atoms in grains. The kinetic temperature is these H I regions varies widely among components. Maximum temperatures, found from the widths of 21 cm emission components, range from about 100 to 10,000 K, while actual temperatures, found by comparing the H I widths with the b-values for the ultraviolet features, are about equal or less than 1000 K for six of the H I components. In contrast, toward the halo star HD 93521, temperatures of about 6000 K were found for most of the components. With use of these HD 149881 temperatures, values of n(sub e) were obtained from the ionization equilibrium of Ca(+) and also of Na(0). These values range over an order of magnitude, from about 4.9 x 10(exp -2) to 1.3 x 10(exp -3)/cu cm, systematically smaller (but also less certain) than in HD 93521. For the low values of n(sub e) found in the coldest components, the electrons can be accounted for by ionization of C atoms, if n(sub H) approximately equals 10/cu cm. For the warmest component, an ionization probability of beta(H(sup 0)) approximately equals 3 x 10(exp -15)/s is required, with lower values for other H I components.

Spitzer, Lyman, Jr.↗

NASA NTRS1994

Composition of interstellar clouds in the disk and halo. 2: Gamma(sup 2) Velorum

High-resolution observations of gamma(sup 2) Vel with the Goddard High-Resolution Spectrograph (GHRS) echelle on the Hubble Space Telescope reveal the presence of seven narrow absorption components, with LSR velocities between -23 and +9 km s(exp -1). Three of these show column density ratios N(S(++))/N(S(+)) and N(P(++))/N(P+)) of about 1 or more, and can be identified as H II regions, while the other four are H I regions, consistent with the O I profile and with the overall H(sup 0) column density of 5.9 x 10(exp 19) cm(exp -2), given the usual assumptions that S is undepleted while O has a depletion D(O) = -0.3 dex. The depletions of Fe, Si, and Mn, which could be measure accurately for two of the four H I regions (components 6 and 7), differ somewhat from the values of D(sub ws) found for slowly moving warm clouds in HD 93521; in particular, for the component at 4.0 km s(exp -1) (No. 6), abosolute of D exceeds absolute of D(sub ws) by 0.1-0.4 dex, while for that at 9.3 km s(exp -1) (No. 7), absolute of D equals absolute of D(sub ws) on the average. The observed ratio of Fe + Mg atoms to Si atoms in the grains of component 6 is 2.04 +/-0.10, consistent with an olivine grain composition; the Fe/Mg ratio is 1.5 +/- 0.2. The electron density in component 6, determined from the C II(sup *) feature, is 0.075 +/- 0.013 cm (exp -3), about two-thirds of that found for clouds of this velocity in HD 93521. In the two conspicuous H II regions, components 3 and 4, n(sub e), determined from the Si II(sup *) feature, is about 1 cm(exp -3). From the column density of S(+) + S(++) in these two components, the total H II path length is about 40 pc. With the radius of a wind-blown bubble around gamma(sup 2) Vel set equal to 60 pc, the effective Stromgren radius is about 100 pc, requiring that T approx. equal to 50,000 K for the Wolf-Rayet component of the gamma(sup 2) Vel binary. Since zeta Pup is a comparable source of ionizing radiation, this temperature is an upper limit. The profiles of the strongest H2 absorption features, from Copernicus archives, indicate that the absorbing molecules have a mean velocity identical with that of the strongest H II component (No. 4). We have no explanation for the possible presence of these H2 molecules in a region of ionized H. Alternatively, the H2 profiles can be explained by molecules in the two adjacent (in velocity) H I regions, components 2 and 5, provided their H I gas has densities and temperatures typical of normal cold clouds. The GHRS data show absorption by highly ionized atoms Si(3+) and C(3+), N(4+) in broad features, in addition to the narrow-line absorption by Si(3+) and C(3+) observed in the dominant H II components, Nos. 3 and 4. The broad C(3+) and N(4+) features have widths corresponding to T in the range (4-8) x 10(exp 5) K, consistent with the broad O(5+) line shown in Copernicus data. Despite some observational uncertainties, the ratios of column densities in the broad C(3+), N(4+), and O(5+) features agree to +/- 0.1 dex with theoretical values for warm gas, heating and evaporating by thermal conduction from an adjacent hot region. Outward evaporation from an isolated cloud in a hot ambient gas cannot be distinguished, on the basis of these data, from inward evaporation of a warm shell, compressed by an expanding, hot stellar-wind bubble. For several halo stars, the C IV/O VI ratio has a quite different average value, perhaps consistent with cooling of infalling hot gas instead of conductive heating and evaporation.

Fitzpatrick, Edward L.↗

NASA NTRS1994

Model atmosphere analysis of selected luminous B stars

The general scientific goal of this program has been to determine whether the atmospheric structure of the B-type stars can be represented by the current generation of plane parallel, line-blanketed, LTE stellar atmosphere models sufficiently well to allow accurate effective temperatures and surface gravities to be deduced. The B stars cover a wide range of temperature and luminosity. For the hottest such stars (with T approximately 30,000 K) the applicability of the models may be compromised by departures from LTE in the stellar atmospheres ('non-LTE effects'). At the highest luminosities (the B 'super giants'), the models may be invalidated by departures from plane parallel geometry. Thus we seek to identify the temperature and luminosity range within which these effects are unimportant and where the models may be relied upon.

Fitzpatrick, Edward L.↗

NASA NTRS1993

Composition of interstellar clouds in the disk and halo. I - HD 93521

Interstellar column densities of Fe, S, Si, Mn, and Mg in their dominant ionization state, as well as of C(+) and S(2+), are presently derived from analyses of HST UV observations of the Galactic halo star HD 93521. Ratios of column densities for the dominant species yield approximations of the logarithmic depletion D(X) in the warm, primarily neutral H gas which produces each of the resolved components. Variations of D between components are highly correlated between elements; about two Fe atoms leave the grains for every Si atom, as if these atoms were primarily bound in the Fe2SiO4 molecules within the grain cores.

Spitzer, Lyman, Jr.↗

NASA NTRS1993

Evidence for processed material in the atmospheres of Large Magellanic Cloud B supergiants

We present a high-dispersion study of the optical spectra of 11 LMC B-type supergiants. These stars had been shown previously to have a wide range in nitrogen absorption line strengths, despite having very similar temperatures and luminosities. The current study shows that the relatively rare nitrogen-weak stars also have weaker photospheric helium lines and stronger photospheric oxygen lines than the more spectroscopically typical supergiants. This pattern suggests that the vast majority of LMC B-type supergiants have had their surfaces contaminated by material from their original hydrogen-burning cores - with a resultant enhancement of surface nitrogen and helium and a depletion in oxygen - while the spectroscopically nitrogen-weak stars are more likely to have retained their main-sequence surface abundances.

Fitzpatrick, Edward L.↗

NASA NTRS1992

Highly ionized atoms toward HD 93521

Results are reported from the HST High Resolution Spectrograph observations of absorption features of C IV and Si IV in the spectrum of the high-latitude O star HD 93521 (l = 183 deg; b = 62 deg). A comparison of Si IV and C IV profiles showed that the FWHM of both features is about 50 km/sec, in contrast to the 7 km/sec found for one of the several S II features. The line centers for C IV and Si IV are at v = -67 km/sec and -60 km/sec, respectively. As the interval velocity decreases from 90 to 50 km/sec, the Si IV/C IV ratio of the column density per unit velocity interval increases from about 0.2 to 0.4. The result is qualitatively consistent with a fountain model of Shapiro and Benjamin (1991) if the slower gas has cooled and recombined more than the faster gas.

Spitzer, Lyman, Jr.↗

NASA NTRS1991

A spectroscopic survey of B supergiants in the Large Magellanic Cloud

The results of a low-dispersion digital optical spectral survey of about 100 B-type supergiants in the Large Magellanic Cloud are presented. The MK spectral classification framework for B supergiants has been transferred to the metal-weak LMC stars, and recommended classification standards have been designated. Variations among the metal line strengths are examined. The most extreme variations are found for the nitrogen lines, for which a range of a factor of 3 or more may be seen in the equivalent widths within some spectral subclasses. It is suggested that these variations indicate a range of nitrogen surface abundances among the B supergiants, resulting from contamination of some of the stellar surfaces by processed material from the original H-burning core.

Fitzpatrick, Edward L.↗

NASA NTRS1991

DD 13 - A very young and heavily reddened early O star in the Large Magellanic Cloud

This paper investigates the Large Magellanic Cloud star DD 13, which is likely the major ionizing source of the nebula N159A. New optical spectroscopy and new estimates of the broadband photometric properties of DD 13 are obtained. A spectral type of O3-O6 V, E(B-V) = 0.64, and M(V) = -6.93 is found. The spectral type cannot be more precisely defined due to contamination of the spectral data by nebular emission, obliterating the important He I classification lines. These results, plus a published estimate of the Lyman continuum photon injection rate into N159A, suggest that DD 13 actually consists of about 2-4 young, early O stars still enshrouded by their natal dust cloud. The star DD 13 may be a younger example of the type of tight cluster represented by the LMC 'star' Sk-66 deg 41, recently revealed to be composed of six or more components.

Conti, Peter S.↗

NASA NTRS1990

Observations of Sk-69 deg 203 and the interstellar extinction towards SN 1987A

Optical and UV spectroscopic observations of the Large Magellanic Cloud (LMC) star Sk-69 deg 203 are discussed. The optical data reveal Sk-69 deg 203 to be a BO.7 Ia supergiant with a moderate nitrogen enhancement, and its UV spectrum is consistent with this classification. UV interstellar extinction curves were constructed for the star using, as flux standards, two lightly reddened LMS supergiants, which bracket Sk-69 deg 203's spectral type. The resultant extinction curves are consistent with the extinction law derived previously for the 30 Doradus region, and the results for Sk-69 deg 203 suggest that the general 30 Doradus extinction law is appropriate for dereddening the observed fluxes of SN 1987A. Published H I 21 observations place SN 1987A in a region with a strong E-W gradient in the total hydrogen content. Comparison with the H I column density implied by the reddening indicates that the supernova is imbedded approximately in the middle of the main H I complex.

Fitzpatrick, Edward L.↗

NASA NTRS1990

An analysis of the shapes of ultraviolet extinction curves. III - An atlas of ultraviolet extinction curves

The basic data needed to produce UV extinction curves for a sample of 78 stars, including spectral types, visual photometry, parameters used to produce fits to the UV extinction curves, distances to the program stars, and line-of-sight H I column densities, are presented. The observational and numerical techniques used to obtain the results are discussed in detail. The UV extinction curves and their analytic fits are shown graphically.

Fitzpatrick, Edward L.↗

NASA NTRS1988

Stellar winds in the Small Magellanic Cloud

According to the predictions of line-driven stellar wind theory, the mass loss from luminous stars should be proportional to some power of the metallicity, yet previous studies have not shown this effect clearly. The authors have observed 22 main-sequence and giant O-type stars n the SMC with the low-resolution mode of the IUE in order to study the stellar winds in a particularly metal-poor galaxy. These stars are compared with carefully selected Galactic counterparts for wind terminal velocities and line strengths of C IV and N v. While the wind velocities in the SMC stars are confirmed to be systematically lower than in Galactic stars, as theory predicts, the mass-loss rates which are inferred from these data are comparable to those in the Galactic stars. The possible reasons for this result are discussed, including systematic effects in the data, assumed metal abundance, different ionization structure, and different physical parameters for the SMC stars.

Garmany, Catharine D.↗

NASA NTRS1988

An analysis of the shapes of ultraviolet extinction curves. II - The far-UV extinction

In this paper the properties of interstellar extinction in the far-ultraviolet region are examined utilizing IUE extinction curves for a primary data sample of 45 reddened Milky Way OB stars. These results are combined with those derived for the 2175 A bump in a previous study. It is found that IUE extinction curves can be represented by linear combinations of a Lorentzian-like 2175 A bump profile, a well-determined FUV curvature term, and an underlying linear component. The parameters of the linear component are strongly correlated and therefore only five free parameters are required to fit all of the curves in our sample. Three parameters describe the 2175 A bump, one parameter describes the linear background, and one parameter describes the strength of the FUV curvature term. The shape of the FUV curvature is found to be identical, to within the observational errors, for all the curves in the sample - which spans virtually the entire range of extinction curve morphologies observed in the Milky Way.

Fitzpatrick, Edward L.↗

NASA NTRS1988

The relationship between the Eddington limit, the observed upper luminosity limit for massive stars, and the luminous blue variables

The observed upper luminosity limits in the Galaxy and the LMC are compared with the Eddington limit as estimated for plane-parallel LTE model atmospheres which include the full effects of metal line opacities in the ultraviolet. It is shown that the Humphreys-Davidson (HD) limit corresponds to the locus of extremely low effective gravities. This result suggests that stars approaching the HD limit will suffer high mass-loss rates because of the reduction of the effective gravity due to radiation pressure. These high mass-loss rates ultimtely lead to the core mass fraction reaching its critical value and the reversal of the stellar evolution tracks. It is shown that radiation pressure, as an agent for producing enhanced mass loss near the HD limit, can in a natural way explain the kink in the HD limit near T(eff) roughly 10,000 K and the upper luminosity limit for yellow and red supergiants. The high mass-loss rates of the luminous blue variables, their location in the HR diagram, and their evolutionary stage are also discussed.

Lamers, Henny J. G. L. M.↗

NASA NTRS1987

Anomalous dust in the open cluster Trumpler 37

New IUE data were obtained towards stars in the open cluster Trumpler 37. Extinction curves derived from these data show that the anomalous dust detected towards HD 204827 and HD 210072 is not unique to those lines of sight but occurs generally in that region of the sky. The dust is apparently associated with the Trumpler 37 cluster, which forms the center of Cepheus OB2. This is a complex region of star formation where the evolution of dust grains through shock waves could be considered likely. A supernova in a slightly older star-forming region of Cep OB2 located next to Trumpler 37 is a possible source of such a shock wave.

Clayton, Geoffrey C.↗

NASA NTRS1986

An average interstellar extinction curve for the Large Magellanic Cloud

LMC regions located more than 0.5 deg from the center of the 30 Doradus nebula have an average UV extinction curve that is about 2 units higher in the far UV than the average Galactic curve. Within the 30 Doradus region, the normalized far-UV extinction is much steeper than the average Galactic or average LMC curves, yet has been considered to be representative of the LMC in general in the past. The present results show typical LMC extinction properties to have much more in common with those in the Milky Way.

Fitzpatrick, Edward L.↗

NASA NTRS1986

The energy distributions of B supergiants in the Large Magellanic Cloud

It is shown that line-blanketed, LTE, plane-parallel model atmosphere calculations provide excellent fits to the ultraviolet-through-visual energy distributions of B supergiants in the Large Magellanic Cloud. The models were computed using Kurucz's (1979) ATLAS atmosphere program, but with lower gravities than were contained in Kurucz's published model grid. The ultraviolet continua of low gravity stars are found to be sensitive to changes in temperature and gravity. Measurements of Teff and log g for ten LMC B supergiants from model atmosphere fits to the energy distributions yield estimates of their radii, luminosities, and masses. Model atmosphere fits suggest that the late B supergiants have significantly lower masses than the earlier B types of the same luminosity, contrary to stellar evolution theory which predicts that B supergiants are in a post-core hydrogen burning phase and should evolve very quickly and at essentially constant mass.

Fitzpatrick, Edward L.↗