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An accurate value of the deuterium/hydrogen (D/H) ratio in the local interstellar medium (LISM) and a better understanding of the D/H variations with position in the Galactic disk can provide essential information on the primordial D/H ratio in the Galaxy at the time of the protosolar nebula, and the amount of astration and mixing in the Galaxy over time. Recent measurements have been obtained with UV spectrographs on FUSE, HST, and IMAPS using hot white dwarfs, OB stars, and late-type stars as background light sources against which to measure absorption by D and H in the interstellar medium along the lines of sight. Recent analyses of FUSE observations of seven white dwarfs and subdwarfs provide a weighted mean value of D/H = (1.52 +/- 0.08) 10(-5) (15.2 +/- 0.8 ppm), consistent with the value of (1.50 +/- 0.10) 10(-5) (15.0 +/- 1.0 ppm) obtained from analysis of lines of sight toward nearby late-type stars. Both numbers refer to the ISM within about 100 pc of the Sun, which samples warm clouds located within the Local Bubble. Outside of the Local Bubble at distances of 200 to 500 pc, analyses of far-UV spectra obtained with IMAPS indicate a much wider range of D/H ratios between 0.8 to 2.2 ppm, providing information on inhomogeneous astration in the Galactic disk., Comment: 12 pages, 3 figures

We present a survey of coronal forbidden lines detected in Far Ultraviolet Spectroscopic Explorer (FUSE) spectra of nearby stars. Two strong coronal features, Fe XVIII 974 A and Fe XIX 1118 A, are observed in 10 of the 26 stars in our sample. Various other coronal forbidden lines, observed in solar flares, also were sought but not detected. The Fe XVIII feature, formed at log T (K) = 6.8, appears to be free of blends, whereas the Fe XIX line can be corrupted by a C I multiplet. FUSE observations of these forbidden iron lines at spectral resolution R ~ 15,000 provides the opportunity to study dynamics of hot coronal plasmas. We find that the velocity centroid of the Fe XVIII feature deviates little from the stellar rest frame, confirming that the hot coronal plasma is confined. The observed line widths generally are consistent with thermal broadening at the high temperatures of formation and show little indication of additional turbulent broadening. The fastest rotating stars, 31 Com, alpha Aur Ab, and AB Dor, show evidence for excess broadening beyond the thermal component and the photospheric v sin i. The anomalously large widths in these fast rotating targets may be evidence for enhanced rotational broadening consistent with emission from coronal regions extending an additional delta R ~ 0.4-1.3 R_star above the stellar photosphere or represent the turbulent broadening caused by flows along magnetic loop structures. For the stars in which Fe XVIII is detected, there is an excellent correlation between the observed Rontgensatellit} (ROSAT) 0.2-2.0 keV soft X-ray flux and the coronal forbidden line flux. As a result, Fe XVIII is a powerful new diagnostic of coronal thermal conditions and dynamics that can be utilized to study high temperature plasma processes in late-type stars., Comment: 29 pages, 7 figures, AASTEX v5.0, accepted by ApJ

We use ultraviolet spectra of Capella from the Hubble Space Telescope (HST) and Far Ultraviolet Spectroscopic Explorer (FUSE) satellites to study interstellar absorption lines from the Local Interstellar Cloud (LIC). Measurements of these lines are used to empirically determine the ionization states of carbon, nitrogen, and silicon in the LIC, for comparison with the predictions of theoretical photoionization models. We find that the observed ionization states are consistent with previously published photoionization predictions. Total abundances are determined for the elements mentioned above, and others, for comparison with solar abundances. Magnesium, aluminum, silicon, and iron are all depleted by at least a factor of 10 toward Capella. The abundances of carbon, nitrogen, and oxygen are essentially solar, although the error bars are large enough to also allow depletions of about a factor of 2 for these elements., Comment: 31 pages, 10 figures; AASTEX v5.0 plus EPSF extensions in mkfig.sty; accepted by ApJ

We present the analysis of a 38.4 ks and a 23.6 ks observation of the core of the Pleiades open cluster. The Advanced CCD Imaging Spectrometer on board the Chandra X-ray Observatory detected 99 X-ray sources in a 17'X17' region, including 18 of 23 Pleiades members. Five candidate Pleiades members have also been detected, confirming their cluster membership. Fifty-seven sources have no optical or near-infrared counterparts to limiting magnitudes V=22.5 and J=14.5. The unidentified X-ray sources are probably background AGN and not stars. The Chandra field of view contains seven intermediate mass cluster members. Five of these, HII 980 (B6 + G), HII 956 (A7 + F6), HII 1284 (A9 + K), HII 1338 (F3 + F6), and HII 1122 (F4 + K), are detected in this study. All but HII 1284 have high X-ray luminosity and soft X-ray spectra. HII 1284 has X-ray properties comparable to non-flaring K-type stars. Since all five stars are visual or spectroscopic binaries with X-ray properties similar to F-G stars, the late-type binary companions are probably producing the observed coronal X-ray emission. Strengthening this conclusion is the nondetection by Chandra of two A stars, HII 1362 (A7, no known companion) and HII 1375 (A0 + A SB) with X-ray luminosity upper limits 27-54 times smaller than HII 980 and HII 956, the B6-A7 stars with cooler companions. Despite the low number statistics, the Chandra data appear to confirm the expectation that late-B and A stars are not strong intrinsic X-ray sources. The ACIS spectra and hardness ratios suggest a gradual increase in coronal temperature with decreasing mass from F4 to K. M stars appear to have somewhat cooler coronae than active K stars., Comment: 22 pages, 4 figures, 7 tables, to appear in ApJ

Collisions between the winds of solar-like stars and the local ISM result in a population of hot hydrogen gas surrounding these stars. Absorption from this hot H I can be detected in high resolution Lyman-alpha spectra of these stars from the Hubble Space Telescope. The amount of absorption can be used as a diagnostic for the stellar mass loss rate. We present new mass loss rate measurements derived in this fashion for four stars (Epsilon Eri, 61 Cyg A, 36 Oph AB, and 40 Eri A). Combining these measurements with others, we study how mass loss varies with stellar activity. We find that for the solar-like GK dwarfs, the mass loss per unit surface area is correlated with X-ray surface flux. Fitting a power law to this relation yields Mdot ~ Fx^(1.15+/-0.20). The active M dwarf Proxima Cen and the very active RS CVn system Lambda And appear to be inconsistent with this relation. Since activity is known to decrease with age, the above power law relation for solar-like stars suggests that mass loss decreases with time. We infer a power law relation of Mdot ~ t^(-2.00+/-0.52). This suggests that the solar wind may have been as much as 1000 times more massive in the distant past, which may have had important ramifications for the history of planetary atmospheres in our solar system, that of Mars in particular., Comment: 34 pages, 9 figures; AASTEX v5.0 plus EPSF extensions in mkfig.sty; accepted by ApJ

We observed the classical T Tauri star TW Hya with \textit{HST}/STIS using the E140M grating, from 1150--1700 \AA, with the E230M grating, from 2200--2900 \AA, and with \FUSE from 900--1180 \AA. Emission in 143 Lyman-band H$_2$ lines representing 19 progressions dominates the spectral region from 1250--1650 \AA. The total H$_2$ emission line flux is $1.94 \times 10^{-12}$ erg cm$^{-2}$ s$^{-1}$, which corresponds to $1.90\times10^{-4}$ $L_\odot$ at TW Hya's distance of 56 pc. A broad stellar \Lya line photoexcites the H$_2$ from excited rovibrational levels of the ground electronic state to excited electronic states. The \ion{C}{2} 1335 \AA doublet, \ion{C}{3} 1175 \AA\ multiplet, and \ion{C}{4} 1550 \AA doublet also electronically excite H$_2$. The velocity shift of the H$_2$ lines is consistent with the photospheric radial velocity of TW Hya, and the emission is not spatially extended beyond the $0\farcs05$ resolution of \textit{HST}. The H$_2$ lines have an intrinsic FWHM of $11.91\pm0.16$ \kms. One H$_2$ line is significantly weaker than predicted by this model because of \ion{C}{2} wind absorption. We also do not observe any H$_2$ absorption against the stellar \Lya profile. From these results, we conclude that the H$_2$ emission is more consistent with an origin in a disk rather than in an outflow or circumstellar shell. We also analyze the hot accretion-region lines (e.g., \ion{C}{4}, \ion{Si}{4}, \ion{O}{6}) of TW Hya, which are formed at the accretion shock, and discuss some reasons why Si lines appear significantly weaker than other TR region lines., Comment: accepted by ApJ, 42 pages -- 20 text, 11 figures

We review what is currently known about the structure of interstellar material in the solar neighborhood, emphasizing how observations from the Hubble Space Telescope (HST) have improved our understanding of how interstellar gas is distributed near the Sun. The nearby ISM is not uniform but shows variations in both temperature and metal abundances on distance scales of just a few parsecs. The observations also show that nearby gas does not have a single uniform velocity vector. Instead, different components are often seen in different directions for even very short lines of sight. However, interpretation of these components remains difficult. It is uncertain whether the components represent physically distinct clouds or perhaps are just symptomatic of velocity gradients within the cloud. Finally, since it is the local interstellar medium's interaction with the solar wind that is the primary application of ISM studies considered in these proceedings, we also review how the same HST data used to study the local ISM structure has also been used to study both the heliospheric interaction with the solar wind and also "astrospheric" interactions with the winds of other stars., Comment: 23 pages, 16 figures; LaTex plus Elsevier style file espcrc1.sty; to appear in conference proceedings for the COSPAR colloquium on The Interstellar Environment of the Heliosphere

We analyze high-resolution UV spectra of the Mg II h and k lines for 18 members of the Hyades Cluster to study inhomogeneity along these proximate lines of sight. The observations were taken by the Space Telescope Imaging Spectrograph (STIS) instrument on board the Hubble Space Telescope (HST). Three distinct velocity components are observed. All 18 lines of sight show absorption by the Local Interstellar Cloud (LIC), ten stars show absorption by an additional cloud, which we name the Hyades Cloud, and one star exhibits a third absorption component. The LIC absorption is observed at a lower radial velocity than predicted by the LIC velocity vector derived by Lallement & Bertin (1992) and Lallement et al. (1995), (v(predicted LIC) - v(observed LIC) = 2.9 +/- 0.7 km/s), which may indicate a compression or deceleration at the leading edge of the LIC. We propose an extention of the Hyades Cloud boundary based on previous HST observations of other stars in the general vicinity of the Hyades, as well as ground-based Ca II observations. We present our fits of the interstellar parameters for each absorption component. The availability of 18 similar lines of sight provides an excellent opportunity to study the inhomogeneity of the warm, partially ionized local interstellar medium (LISM). We find that these structures are roughly homogeneous. The measured Mg II column densities do not vary by more than a factor of 2 for angular separations of < 8 degrees, which at the outer edge of the LIC correspond to physical separations of < 0.6 pc., Comment: 35 pages, 11 figures, AASTEX v.5.0 plus EPSF extensions in mkfig.sty; accepted by ApJ

We study H I Lyman-alpha absorption observed by the Hubble Space Telescope toward the nearby binary system Alpha Cen (G2 V+K0 V) and its distant companion star Proxima Cen (M5.5 Ve). Absorption from heliospheric H I heated by the solar wind/ISM interaction is observed toward both Alpha Cen and Proxima Cen. Absorption from analogous "astrospheric" material surrounding the stars is detected toward Alpha Cen, but not Proxima Cen. The nondetection of astrospheric absorption toward Proxima Cen suggests that the stellar wind of Proxima Cen must be significantly weaker than that of the Alpha Cen system. We use hydrodynamic models of the astrospheres computed assuming different mass-loss rates to predict astrospheric Lyman-alpha absorption for comparison with the observations. The model that best matches the Alpha Cen data has a mass-loss rate of twice the solar rate, and the models suggest an upper limit of 0.2 solar for Proxima Cen. Finally, we note that the heliospheric absorption observed toward Proxima Cen in 2000 May is identical to the heliospheric absorption observed toward Alpha Cen in 1995 May, implying that the structure of the outer heliosphere does not change significantly during the solar activity cycle., Comment: 11 pages, 3 figures; AASTEX v5.0 plus EPSF extensions in mkfig.sty; accepted by ApJ Letters

We present results from a 36-ksec observation of the core of the Pleiades open cluster using ACIS-I on the Chandra X-ray Observatory. We have detected 57 sources, most of which do not have previously known optical counterparts. Follow-up photometry indicates that many of the detections are likely to be AGNs, in accordance with extragalactic source counts, but some of the sources may be previously undiscovered low-mass members of the Pleiades. We discuss our dataset and our findings about X-ray emission from early-type stars as well as very late type stars. In particular, the large X-ray fluxes, lack of variability, and hardness ratios of the four Pleiades B6 IV -- F4 V stars suggest a tentative conclusion that Pleiades stars in this spectral type range are intrinsic X-ray sources rather than previously unknown binaries in which the X-ray emission is from a late-type companion. Also the sensitivity of Chandra allowed us to detect nonflare X-ray emission from late-M stars., Comment: 25 pages, 7 figures. Accepted for publication in The Astronomical Journal

We analyze local ISM absorption observed in the Lyman-alpha and Mg II h & k lines of six nearby K dwarf stars, using UV spectra of these stars obtained with the Goddard High Resolution Spectrograph on the Hubble Space Telescope. For four of the six stars, we detect an absorption component with a velocity and column density consistent with the Local Interstellar Cloud (LIC). For HD 197890, there is no observed component at the expected LIC velocity, or at the projected velocity of the G cloud, which is a nearby cloud in the general direction of the Galactic Center. It also seems doubtful that either of the two components seen toward HD 82558 are LIC or G cloud absorption. The total H I column density toward HD 82558 (d=18.3 pc) is extremely high (log N(H)=19.05 +/- 0.15), representing the largest average H I density detected for any line of sight through the nearby LISM (n(H)=0.2 cm^-3). This is particularly remarkable considering that this star is only 39 degrees from the "interstellar tunnel" toward Epsilon CMa, where column densities are an order of magnitude lower than this toward stars which are an order of magnitude farther away., Comment: 23 pages, 4 figures; AASTEX v5.0 plus EPSF extensions in mkfig.sty; accepted by ApJ

We use high-resolution UV spectra taken by the Space Telescope Imaging Spectrograph instrument on board the Hubble Space Telescope to study the 5.5 pc line of sight to the K0 V star 36 Oph A. The one interstellar component detected for this line of sight has a velocity inconsistent with the local interstellar cloud (LIC) flow vector, but consistent with the flow vector of the so-called G cloud, a very nearby warm cloud in the Galactic Center direction. From our data, we measure the following values for the interstellar temperature, nonthermal velocity, H I column density, and D/H value: T=5900+/-500 K, xi=2.2+/-0.2$ km/s, log N(H)=17.85+/-0.15, and D/H=(1.5+/-0.5)e-5. The temperature of the G cloud is somewhat lower than that of the LIC, and Mg and Fe depletions are also lower, but the D/H value appears to be the same. Based on upper limits for the LIC absorption, we estimate the distance to the edge of the LIC to be d(edge)<0.19 pc, which the Sun will reach in t(edge)<7400 yrs. The H I Lyman-alpha absorption line has properties inconsistent with those of the other absorption lines, indicating the presence of one or more absorption components not seen in the other lines. We present evidence that hot neutral hydrogen local to both the Sun and the star is responsible for the excess Lyman-alpha absorption. This hot H I is created by the interaction between the ISM and the winds of the Sun and 36 Oph A. The observed line of sight lies only 12 degrees from the upwind direction of the LIC flow vector, where hydrodynamic models of the heliosphere suggest that heliospheric H I absorption should be particularly prominent., Comment: 21 pages, 6 figures; AASTEX v5.0 plus EPSF extensions in mkfig.sty; to appear in ApJ

We have used the VLA to conduct a deep search for 3.6 cm radio emission from nearby very low mass stars and brown dwarfs. The Gudel-Benz relation is used to predict radio luminosities for some very low mass stars and candidate brown dwarfs with measured X-ray fluxes. The predicted radio fluxes are quite small, whereas the measured radio flux from the brown dwarf candidate Rho Oph GY 31 is relatively strong. In light of our new observations, this object remains an anomaly. We present upper limits for our measured radio fluxes at 3.6 cm for our targets., Comment: 10 pages, no figures. Accepted for publication in AJ

We present Extreme Ultraviolet Explorer spectroscopy and photometry of the nearby F8 V star HD 35850 (HR 1817). The EUVE spectra reveal 28 emission lines from Fe IX and Fe XV to Fe XXIV. The Fe XXI 102, 129 A ratio yields an upper limit for the coronal electron density, log n < 11.6 per cc. The EUVE SW spectrum shows a small but clearly detectable continuum. The line-to-continuum ratio indicates approximately solar Fe abundances, 0.8 < Z < 1.6. The resulting emission-measure distribution is characterized by two temperature components at log T of 6.8 and 7.4. The EUVE spectra have been compared with non-simultaneous ASCA SIS spectra of HD 35850. The SIS spectrum shows the same temperature distribution as the EUVE DEM analysis. However, the SIS spectral firs suggest sub-solar abundances, 0.34 < Z < 0.81. Although some of the discrepancy may be the result of incomplete X-ray line lists, we cannot explain the disagreement between the EUVE line-to-continuum ratio and the ASCA-derived Fe abundance. Given its youth (t ~ 100 Myr), its rapid rotation (v sin i ~ 50 km/s), and its high X-ray activity (Lx ~ 1.5E+30 ergs/s), HD 35850 may represent an activity extremum for single, main-sequence F-type stars. The variability and EM distribution can be reconstructed using the continuous flaring model of Guedel provided that the flare distribution has a power-law index of 1.8. Similar results obtained for other young solar analogs suggest that continuous flaring is a viable coronal heating mechanism on rapidly rotating, late-type, main-sequence stars., Comment: 32 pages incl. 14 figures and 3 tables. To appear in the 1999 April 10 issue of The Astrophysical Journal

We report the discovery of large-amplitude, periodic X-ray emission from the O7 V star theta^1 Orionis C, the central star of the Orion Nebula. Ten ROSAT HRI snapshots of the Trapezium cluster taken over the course of 21 days show that the count rate of theta^1 Ori C varies from 0.26 to 0.41 counts/s with a clear 15-day period. The soft X-ray variations have the same phase and period as H-alpha and He II 4686 variations reported by Stahl et al., and are in anti-phase with the C IV and Si IV ultraviolet absorption features. We consider five mechanisms which might explain the amplitude, phase, and periodicity of the X-ray variations: (1) colliding-wind emission with an unseen binary companion, (2) coronal emission from an unseen late-type pre-main--sequence star, (3) periodic density fluctuations, (4) absorption of magnetospheric X-rays in a corotating wind, and (5) magnetosphere eclipses. The ROSAT data rule out the first three scenarios, but cannot rule out either of the latter two which require the presence of an extended magnetosphere, consistent with the suggestion of Stahl et al. that theta^1 Ori C is an oblique magnetic rotator. As such, theta^1 Ori C may be the best example of a high-mass analog to the chemically peculiar, magnetic Bp stars., Comment: 12 pages, Latex, to appear in The Astrophysical Journal Letters, supplemental material available at http://casa.colorado.edu/~marc/theta_letter.html

Using new and archival spectra from the Goddard High Resolution Spectrograph, we have searched for evidence of chromospheric and transition region emission in six stars of mid- to late-A spectral type. Two of the stars, alpha Aql (A7 IV-V) and alpha Cep (A7 IV-V), show emission in the C~II 1335 A doublet, confirming the presence of hot plasma with temperatures comparable to that of the solar transition region. Using radiative equilibrium photospheric models, we estimate the net surface fluxes in the C II emission line to be 9.4 X 10^4 erg/cm/cm/s for alpha Aql and 6.5 X 10^4 erg/cm/cm/s for alpha Cep. These are comparable to fluxes observed in early to mid-F-type dwarfs, indicating that significant upper atmospheric heating is present in at least some stars as hot as ~8000 K (B-V=0.22). We find no evidence for the blue-shifted emission reported by Simon et al (1994). We estimate the basal flux level to be about 30% of that seen in early~F stars, and that the bulk of the emission is not basal in origin. We conclude that the basal flux level drops rapidly for B-V < 0.3, but that magnetic activity may persist to B-V as small as 0.22., Comment: uuencoded, compressed, tarred postscript. 8 figures included. Latex file astars.tex (AAS/WGAS macros version 3.0) or postscript file (astars.ps) and individual postscript figures (astars_fig_*.ps) are available by at ftp://sbast3.ess.sunysb.edu/pub/fred/ . Astrophysical Journal, in press (1 July 1995).