STIS Spectra Research Articles

High‐Ionization Forbidden Lines in the UV Spectrum of AG Draconis

High-ionization forbidden lines from Ca VII, Fe VII, Mg V, Mg VI, Mg VII, and Si VII are found in recent Hubble Space Telescope STIS ultraviolet spectra of the symbiotic star AG Draconis. These species have ionization potentials between 99 and 205 eV, which are unexpected due to the high density (≈1010 cm-3) of the AG Dra nebula. The identification of the Mg VII λλ2510, 2629 lines is the first in astrophysical or laboratory spectra, and revised rest wavelengths are suggested from the STIS spectra. Plasma diagnostics from Mg V-VII are applied, but do not provide a consistent constraint on temperature or density. A density ≥108 cm-3 is confirmed, however. The lines show double-peaked profiles with widths ≈100-160 km s-1, suggestive of an origin in an accretion disk. However, the line widths, if identified with motion in a Keplerian disk, indicate radii much smaller than sizes inferred from the line fluxes themselves. The source of these high-ionization forbidden lines remains unidentified.

An Atlas of Hubble Space Telescope STIS Spectra of Seyfert Galaxies

We present a compilation of spectra of 101 Seyfert galaxies obtained with the Space Telescope Imaging Spectrograph (HST-STIS), covering the UV and/or optical spectral range. Information on all the available spectra have been collected in a Mastertable, which is a very useful tool for anyone interested in a quick glance at the existent STIS spectra for Seyfert galaxies in the HST archive, and it can be recovered electronically at the URL address www.if.ufrgs.br/~pat/atlas.htm. Nuclear spectra of the galaxies have been extracted in windows of 0.2 arcsec for an optimized sampling (as this is the slit width in most cases), and combined in order to improve the signal-to-noise ratio and provide the widest possible wavelength coverage. These combined spectra are also available electronically.

Interstellar abundances in the neutral and ionized gas of NGC 604

Aims. We present FUSE spectra of the giant H ii region NGC 604 in the spiral galaxy M 33. Chemical abundances are tentatively derived from far-UV absorption lines and compared to those derived from optical emission lines. Methods. Absorption lines from neutral hydrogen and heavy elements were observed against the continuum provided by the young massive stars embedded in the H ii region. We derived the column densities of H i, N i, O i, Si ii, P ii, Ar i, and Fe ii, fitting the line profiles with either a single component or several components. We used CLOUDY to correct for contamination from the ionized gas. Archival HST/ STIS spectra across NGC 604 allowed us to investigate how inhomogeneities affect the final H i column density. Results. Kinematics show that the neutral gas is physically related to the H ii region. The STIS spectra reveal H i column density fluctuations up to 1 dex across NCG 604. Nevertheless, we find that the H i column density determined from the global STIS spectrum does not differ significantly from the average over the individual sightlines. Our net results using the column densities derived with FUSE , assuming a single component, show that N, O, Si, and Ar are apparently underabundant in the neutral phase by a factor of ${\sim} $10 or more with respect to the ionized phase, while Fe is the same. However, we discuss the possibility that the absorption lines are made of individual unresolved components, and find that only P ii, Ar i, and Fe ii lines should not be affected by the presence of hidden saturated components, while N i, O i, and Si ii might be much more affected. Conclusions. If N, O, and Si are actually underabundant in the neutral gas of NGC 604 with respect to the ionized gas, this would confirm earlier results obtained for the blue compact dwarfs, and their interpretations. However, a deeper analysis focused on P, Ar, and Fe mitigates the above conclusion and indicates that the neutral gas and ionized gas could have similar abundances.

Kinematics of Interstellar Gas in Nearby UV‐selected Galaxies Measured withHSTSTIS Spectroscopy

We measure Doppler shifts of interstellar absorption lines in HST STIS spectra of individual star clusters in nearby UV-selected galaxies. Values for systemic velocities, which are needed to quantify outflow speeds, are taken from the literature and verified with stellar lines. We detect outflowing gas in 8 of 17 galaxies via low-ionization lines (e.g., C II, Si II, Al II), which trace cold and/or warm gas. The starbursts in our sample are intermediate in luminosity (and mass) to dwarf galaxies and luminous infrared galaxies (LIRGs), and we confirm that their outflow speeds (ranging from -100 to nearly -520 km s-1, with an accuracy of ~80 km s-1) are intermediate to those previously measured in dwarf starbursts and LIRGs. We do not detect the outflow in high-ionization lines (such as C IV or Si IV); higher quality data will be needed to empirically establish how velocities vary with the ionization state of the outflow. We do verify that the low-ionization UV lines and optical Na I doublet give roughly consistent outflow velocities, solidifying an important link between studies of galactic winds at low and high redshift. To obtain a higher signal-to-noise ratio (S/N), we create a local average composite spectrum and compare it to the high-z Lyman break composite spectrum. It is surprising that the low-ionization lines show similar outflow velocities in the two samples. We attribute this to a combination of weighting toward higher luminosities in the local composite, as well as both samples being, on average, brighter than the turnover luminosity in the v-SFR relation.

New results on the distribution of thermal pressures in the diffuse ISM

AbstractThe ground electronic state of neutral atomic carbon has three fine-structure levels. In the interstellar medium, the relative populations of the upper two levels are established by collisional excitations (and de-excitations) balanced against spontaneous radiative decay. Consequently, the fractions of C I in the upper two levels indicate acceptable combinations of local temperature and density, which in turn indicate the approximate thermal pressures of the medium. We can measure the values of these fractions and how they vary from one location to the next by observing the multiplets of C I seen in absorption in the ultraviolet spectra of hot stars.We have identified 102 stars for which the HST MAST archive has E140H STIS spectra that are suitable for measuring the absorption features of C I at velocity resolutions of 3 kms−1(or better). A special analysis method developed by Jenkins & Tripp (2001) permits determinations of the amounts of C I in each of the three levels as a function of radial velocity over a wide dynamic range in column density, since several multiplets of vastly different strengths can be considered simultaneously.The C I data reveal that the much of the diffuse, cold, neutral medium has pressures that are distributed in an approximately log-normal fashion, spread over a range 1000 < p/k < 104 cm−3 K (FWHM), but with low level tails outside this range. The dispersion of pressures increases slightly for gases that have radial velocities outside the expected range for quiescent material along each line of sight. This link to the kinematics of the gas is consistent with the picture that pressure fluctuations are driven by the dynamics of a turbulent medium. If the gas is a single medium that is being driven by turbulent forces, its barytropic index (slope of log p vs. log n) is more than 0.9, which is inconsistent with the value 0.72 for material that is expected to be in thermal equilibrium. Slightly less than one part in a thousand of the gas is at pressures of order or greater than ~105cm−3 K and seems to nearly always accompany the gas at normal pressures.

The Origins of Fluorescent H2Emission From T Tauri Stars

We survey fluorescent H2 emission in HST STIS spectra of the classical T Tauri stars (CTTSs) TW Hya, DF Tau, RU Lupi, T Tau, and DG Tau, and the weak-lined T Tauri star (WTTS) V836 Tau. From each of those sources we detectbetween41and209narrowH2emissionlines,mostofwhicharepumpedbystrongLyemission.H2emission is not detected from the WTTS V410 Tau. The fluorescent H2 emission appears to be common to circumstellar environments around all CTTSs, but high spectral and spatial resolution STIS observations reveal diverse phe- nomenon. Blueshifted H2 emission detected from RU Lupi, T Tau, and DG Tau is consistent with an origin in an outflow. The H2 emission from TW Hya, DF Tau, and V836 Tau is centered at the radial velocity of the star and is consistent with an origin in a warm disk surface. The H 2 lines from RU Lupi, DF Tau, and T Tau also have excess blueshiftedH2emissionthatextendstoasmuchas � 100kms � 1 .ThestrengthofthisblueshiftedcomponentfromDF Tau and T Tau depends on the upper level of the transition. In all cases, the small aperture and attenuation of H2 emission by stellar winds restricts the H2 emission to be formed close to the star. In the observation of RU Lupi, the Lyemission and the H2 emission that is blueshifted by 15 km s � 1 are extended to the SW by � 0B07, although the faster H2 gas that extends to � 100 km s � 1 is not spatially extended. We also find a small reservoir of H 2 emission from TW Hya and DF Tau consistent with an excitation temperature of � 2:5 ; 10 4 K. Subject headingg accretion, accretion disks — circumstellar matter — line: identification — stars: pre-main-sequence — ultraviolet: stars

Quasars near the line of sight towards Q 0302-003 and the transverse proximity effect

We report the discovery of the faint (V=21.7) quasar QSO03027-0010 at z=2.808 in the vicinity of Q0302-003, one of the few quasars observed with STIS to study intergalactic HeII absorption. Together with another newly discovered QSO at z=2.29, there are now 6 QSOs known near the line of sight towards Q0302-003, of which 4 are located within the redshift region 2.76<=z<=3.28 covered by the STIS spectrum. We correlated the opacity variations in the HI and HeII Lyman forest spectra with the locations of known quasars. There is no significant proximity effect in the HI Lyman alpha forest for any of the QSOs, except for the well-known line of sight effect for Q0302-003 itself. By comparing the absorption properties in HI and HeII, we estimated the fluctuating hardness of the extragalactic UV radiation field along this line of sight. We find that close to each foreground quasar, the ionizing background is considerably harder than on average. In particular, our newly discovered QSO03027-0010 shows such a hardness increase despite being associated with an overdensity in the HI Lyman forest. We argue that the spectral hardness is a sensitive physical measure to reveal the influence of QSOs onto the UV background even over scales of several Mpc, and that it breaks the density degeneracy hampering the traditional transverse proximity effect analysis. We infer from our sample that there is no need for significantly anisotropic UV radiation from the QSOs. From the transverse proximity effect detected in the sample we obtain minimum quasar lifetimes in the range ~10-30 Myr.

FUSEandHSTSTIS Far‐Ultraviolet Observations of AM Herculis in an Extended Low State

We have obtained FUSE and HST STIS time- resolved spectroscopy of the Polar AM Herculis during a deep low state. The spectra are entirely dominated by the emission of the white dwarf. Both the far- ultraviolet ( FUV) flux and the spectral shape vary substantially over the orbital period, with maximum flux occurring at the same phase as during the high state. The variations are due to the presence of a hot spot on the white dwarf, which we model quantitatively. The white dwarf parameters can be determined from a spectral fit to the faint- phase data, when the hot spot is self- eclipsed. Adopting the distance of 79(-6)(+8) pc determined by Thorstensen, we find an effective temperature of 19; 800 +/- 700 K and a mass of MWD 0: 78 + 0: 12 - 0: 17 M circle dot. The hot spot has a lower temperature than during the high state, similar to 34,000 - 40,000 K, but covers a similar area, similar to 10% of the white dwarf surface. Low-state FUSE and STIS spectra taken during four different epochs in 2002 - 2003 show no variation of the FUV flux level or spectral shape, implying that the white dwarf temperature and the hot spot temperature, size, and location do not depend on the amount of time the system has spent in the low state. Possible explanations are ongoing accretion at a low level or deep heating; both alternatives have some weaknesses, which we discuss. No photospheric metal absorption lines are detected in the FUSE and STIS spectra, suggesting that the average metal abundances in the white dwarf atmosphere are lower than similar to 10(-3) times their solar values.

Fundamental Properties of O‐Type Stars

We present a comprehensive analysis of high-resolution, far-ultraviolet HST STIS, FUSE, and optical spectra of 18 O stars in the Small Magellanic Cloud. Our analysis is based on the OSTAR2002 grid of NLTE metal-line-blanketed model atmospheres calculated with our code TLUSTY. We systematically explore and present the sensitivity of various UV and optical lines to different stellar parameters. We have obtained consistent fits of the UV and the optical spectrum to derive the effective temperature, surface gravity, surface composition, and microturbulent velocity of each star. Stellar radii, masses, and luminosities follow directly. For stars of the same spectral subtype, we find a general good agreement between effective temperature determinations obtained with TLUSTY, CMFGEN, and FASTWIND models, which are all lower than the standard T(sub eff) calibration of O stars. We propose a new calibration between the spectral type and effective temperature based on our results from UV metal lines, as well as optical hydrogen and helium lines. The lower effective temperatures translate into ionizing luminosities that are smaller by a factor of 3 compared to luminosities inferred from previous standard calibrations. The chemical composition analysis reveals that the surface of about 80% of the program stars is moderately to strongly enriched in nitrogen, while showing the original helium, carbon, and oxygen abundances. Our results support the new stellar evolution models that predict that the surface of fast rotating stars becomes nitrogen-rich during the main-sequence phase because of rotationally induced mixing. Enrichment factors are, however, larger than predicted by stellar evolution models. Most stars exhibit the mass discrepancy problem, which we interpret as a result of fast rotation that lowers the measured effective gravity. Nitrogen enrichment and low spectroscopic masses are therefore two manifestations of fast rotation. Our study thus emphasizes the importance of rotation in our understanding of the properties of massive stars and provides a framework for investigating populations of low-metallicity massive stars at low and high redshifts.

The First Direct Spectroscopic Detection of a White Dwarf Primary in an AM CVn System

We report the results of a synthetic spectral analysis of HST STIS spectra of the AM CVn-type cataclysmic variable CP Eri obtained when the system was in quiescence. The FUV spectrum is best fitted by a helium-dominated, hybrid composition (DBAZ) white dwarf with Teff ~ 17,000 ± 1000 K, log g ~ 8, He/H abundance ratio by number ~1000, metallicity Z ~ 0.05 × solar, and V sin i ~ 400 ± 100 km s-1. This is the first directly detected primary white dwarf in any AM CVn system, and the surface abundance and rotation rate for the white dwarf primary are the first to be reported for AM CVn systems. The model-predicted distance is ~1000 pc. The spectral fits using pure He photospheres or He-rich accretion disks were significantly less successful. Based on the analysis of our FUV spectra, CP Eri appears to contain a hybrid composition DBAZ white dwarf with a metallicity that sets it apart from the other two AM CVn stars that have been observed in quiescence and are metal-poor. The implications of this analysis for evolutionary channels leading to AM CVn systems are discussed.

Hubble Space TelescopeSTIS Spectroscopy of the Lyα Emission Line in the Central Dominant Galaxies in A426, A1795, and A2597: Constraints on Clouds in the Intracluster Medium

We report on HST STIS spectra of the Lyα emission in the central dominant galaxies in three rich clusters of galaxies. We find evidence for a population of clouds in the intracluster medium. We detect 10 Lyα absorption systems toward the nucleus of NGC 1275 with columns of N(H ) ~ 1012-1014 cm-2. These columns would not have been detected in the 21 cm line but are easily detected in the Lyα line. Most of the absorption features are located in the broad wings of the emission line. The detected absorption features are most consistent with associated nuclear absorption systems. There is very little nuclear absorption at the systemic velocity in NGC 1275 [feature 8 contains N(H ) ~ 3 × 1012 cm-2]. This implies that the large columns detected in the 21 cm line toward the parsec-scale radio source avoid the line of sight to the nucleus. This gas may be located in a circumnuclear disk or torus. We detect at least one and possibly two absorption features toward the extended Lyα in A426. We do not detect Lyα absorption toward the extended Lyα emission in A1795 and A2597 with upper limits N(H ) ~ 1013 cm-2 for optically thin absorbers. Our data constrain the covering factor of any high column density gas [N(H ) > 1015 cm-2] in the ICM to be less than 25%. Our results suggest that the lack of observed intermediate-temperature gas is not explained by obscuration. In addition, the low columns of gas on ~100 kpc scales in the ICM suggest that (1) the rate at which cold gas accumulates in the ICM on these scales is very low and (2) the dense nebulae in the central ~10 kpc must have cooled or been deposited in situ.

Nuclear Properties of Nearby Spiral Galaxies fromHubble Space TelescopeNICMOS Imaging and STIS Spectroscopy

We investigate the central regions of 23 spiral galaxies using archival NICMOS imaging and STIS spectroscopy. The sample is taken from our program to determine the masses of central massive black holes (MBH) in 54 nearby spiral galaxies. Stars are likely to contribute significantly to any dynamical central mass concentration that we find in our MBH program and this paper is part of a series to investigate the nuclear properties of these galaxies. We use the Nuker law to fit surface brightness profiles, derived from the NICMOS images, to look for nuclear star clusters and find possible extended sources in 3 of the 23 galaxies studied (13 per cent). The fact that this fraction is lower than that inferred from optical Hubble Space Telescope studies is probably due to the greater spatial resolution of those studies. Using R-H and J-H colors and equivalent widths of H-alpha emission (from the STIS spectra) we investigate the nature of the stellar population with evolutionary models. Under the assumption of hot stars ionizing the gas, as opposed to a weak AGN, we find that there are young stellar populations (~10-20 Myr) however these data do not allow us to determine what percentage of the total nuclear stellar population they form. Also, in an attempt to find any unknown AGN we use [N II] and [S II] line flux ratios (relative to H-alpha) and find tentative evidence for weak AGN in NGC 1300 and NGC 4536.

The Connection between the Narrow‐Line Region and the UV Absorbers in Seyfert Galaxies

We present evidence that the outflowing UV absorbers in Seyfert 1 galaxies arise primarily in their inner narrow (emission) line regions (NLRs), based on similarities in their locations, kinematics, and physical conditions. 1) Hubble Space Telescope observations show that nearly all Seyfert galaxies have bright, central knots of [O III] emission in their NLRs with radii of tens of parsecs. These sizes are consistent with most previous estimates of the distances of UV (and X-ray) absorbers from their central continuum sources, and a recently obtained reliable distance of ~25 pc for a UV absorber in the Seyfert 1 galaxy NGC 3783. 2) The nuclear emission-line knots in a sample of 10 Seyfert galaxies have velocity widths of 300 - 1100 km/s (half-width at zero intensity), similar to the radial velocities of most UV absorbers. The highest radial velocity for a Seyfert UV absorber to date is only -2100 km/s, which is much lower than typical broad-line region (BLR) velocities. There is also mounting evidence that the NLR clouds are outflowing from the nucleus, like the UV absorbers. 3) If our hypothesis is correct, then the NLR must have a component with a high global covering factor (Cg) of the continuum source and BLR, to match that found from previous surveys of UV absorbers (Cg = 0.5 - 1.0). Using STIS spectra of NGC 4151, obtained when the continuum and BLR fluxes were low, we find evidence for optically thin gas in its nuclear emission-line knot. We are able to match the line ratios from this gas with photoionization models that include a component with Cg ~ 1 and an ionization parameter and hydrogen column density that are typical of UV absorbers.

Far‐Ultraviolet Spectroscopy of Magnetic Cataclysmic Variables

We have obtained Hubble Space Telescope (HST) STIS data for a total of 11 polars as part of a program aimed at compiling a homogeneous database of high-quality far-ultraviolet (FUV) spectra for a large number of cataclysmic variables (CVs). Of the 11 polars, eight were found in a state of low accretion activity (V347 Pav, VV Pup, V834 Cen, BL Hyi, MR Ser, ST LMi, RX J1554.2+2721, and V895 Cen) and three in a state of high activity ( CD Ind, AN UMa, and UW Pic). The STIS spectra of the low-state polars unambiguously reveal the photospheric emission of their white dwarf (WD) primaries. We have used pure hydrogen WD models to fit the FUV spectra of the low-state systems (except RX J1554.2+2721, which is a high-field polar) in order to measure the WD effective temperatures. In all cases, the fits could be improved by adding a second component, which is presumably due to residual accretion onto the magnetic pole of the WD. The WD temperatures obtained range from 10,800 to 14,300 K for log g = 8.0. Our analysis more than doubles the number of polars with accurate WD effective temperatures. Comparing the WD temperatures of polars with those of nonmagnetic CVs, we find that at any given orbital period the WDs in polars are colder than those in nonmagnetic CVs. The temperatures of polars below the period gap are consistent with gravitational radiation as the only active angular momentum loss mechanism. The differences in WD effective temperatures between polars and nonmagnetic CVs are significantly larger above the period gap, suggesting that magnetic braking in polars might be reduced by the strong field of the primary. We derive distance estimates to the low-state systems from the flux scaling factors of our WD model fits. Combining these distance measurements with those from the literature, we establish a lower limit on the space density of polars of 1.3 x 10(-6) pc(-3).

Fe VII lines in the spectrum of RR Telescopii

Thirteen transitions within the ground 3d^2 configuration of Fe VII are identified in ultraviolet and optical spectra of the symbiotic star RR Telescopii obtained with the STIS instrument of the Hubble Space Telescope. The line fluxes are compared with theoretical data computed with the recent atomic data of K.A. Berrington et al., and high resolution optical spectra from VLT/UVES are used to identify blends. Seven branching ratios are measured, with three in good agreement with theory and one affected by blending. The lambda5277/lambda4943 branching ratio is discrepant by > 3 sigma, indicating errors in the atomic data for the lambda5277 line. A least-squares minimization scheme is used to simultaneously derive the temperature, T, and density, N_e, of the RR Tel nebula, and the interstellar extinction, E(B-V), towards RR Tel from the complete set of emission lines. The derived values are: log T/K = 4.50 +/- 0.23, log N_e/cm^-3=7.25 +/- 0.05, and E(B-V)<0.27. The extinction is not well-constrained by the Fe VII lines, but is consistent with the more accurate value E(B-V)=0.109^{+0.052}_{-0.059} derived here from the Ne V lambda2974/lambda1574 ratio in the STIS spectrum. Large differences between the K.A. Berrington et al. electron excitation data and the earlier F.P. Keenan & P.H. Norrington data-set are demonstrated, and the latter is shown to give worse agreement with observations.

A damped Ly$\alpha$ system along two lines of sight at $z=0.93$

HE0512–3329 is a gravitationally-lensed double QSO with damped Ly $\alpha$ systems observed at $z=0.931$ in front of both QSO images (DLA A and B). We have obtained spatially resolved HST STIS and optical VLT UVES spectra of both QSOs in order to study differences in the metal abundances across the lines of sight. We detect substantial differences, of roughly 0.5 dex, in both [Mn/H] and [Fe/H], on a transverse scale of 5 $h^{-1}$ kpc. Differential dust depletion appears as the most plausible explanation.

Composite Accretion Disk and White Dwarf Photosphere Analyses of theFUSEandHubble Space TelescopeObservations of EY Cygni

We explore the origin of FUSE and Hubble Space Telescope STIS far-UV spectra of the dwarf nova EY Cygni during its quiescence using combined high-gravity photosphere and accretion disk models, as well as model accretion belts. The best-fitting single-temperature white dwarf model to the FUSE plus STIS spectrum of EY Cyg has Teff = 24,000 K and log g = 9.0 with an Si abundance of 0.1 times solar and C abundance of 0.2 times solar, but the distance is only 301 pc. The best-fitting composite model consists of a white dwarf with Teff = 22,000 K and log g = 9, plus an accretion belt with Tbelt = 36,000 K covering 27% of the white dwarf surface with Vbelt sin i = 2000 km s-1. The accretion belt contributes 63% of the far-UV light and the cooler white dwarf latitudes contribute 37%. This fit yields a distance of 351 pc, which is within 100 pc of our adopted distance of 450 pc. EY Cyg has very weak C IV emission and very strong N V emission, which is atypical of the majority of dwarf novae in quiescence. We also conducted a morphological study of the surroundings of EY Cyg using direct imaging in narrow nebular filters from ground-based telescopes. We report the possible detection of nebular material associated with EY Cyg. Possible origins of the apparently large N V/C IV emission ratio are discussed in the context of nova explosions, contamination of the secondary star, and accretion of nova abundance-enriched matter back to the white dwarf via the accretion disk or as a descendant of a precursor binary that survived thermal-timescale mass transfer. The scenario involving pollution of the secondary by past novae may be supported by the possible presence of a nova remnant–like nebula around EY Cyg.

The nearest extreme starburst: bubbles, young star clusters and outflow in the merger NGC 3256

We have studied the extreme starburst in the infrared merger NGC 3256. We detect four galactic bubbles (using ESO New Technology Telescope and Hubble Space Telescope WFPC2 Hα images). These shells would be associated with previous supernova explosions. The first analysis of the spatial distribution of young star cluster candidates shows that more than 90 per cent of them are located in a complex starburst structure, including some of the bubble walls, three nuclei and three blue asymmetrical spiral arms. We have made a kinematic study of the ionized gas in the core of the main optical nucleus, performed with HST STIS spectra. The shape of the rotation curve and the emission-line profile can be explained by the presence in the core of young star clusters with outflow. Any low-luminosity active galactic nucleus associated with this core would have a mass less than 107 M⊙. It is also probable that the compact X-ray and radio emission of ULX(7)N – the source coincident with the main optical nucleus – is the result of a few recent supernova remnants.

A Comparison of the Outer Atmosphere of the “Flat Activity” Star τ Ceti (G8 V) with the Sun (G2 V) and α Centauri A (G2 V)

Driven by the desire to understand the roles of acoustic and magnetic mechanisms in heating the outer atmospheres of Sun-like stars, we compare solar UV spectra with archival STIS spectra from the Hubble Space Telescope of α Cen A (G2 V) and new STIS spectra of the extremely inactive dwarf star τ Cet (G8 V, V = 3.5). The activity of τ Cet shows occasional rotational modulations but no long-term cyclic variation. It may be in a phase analogous to the solar Maunder minimum. Solar disk center intensities from both the HRTS instrument and the SUMER instrument on SOHO were converted to Sun-as-a-star fluxes by using center-to-limb data from Dammasch and colleagues. The derived solar flux spectrum represents conditions near the minimum of the solar magnetic activity cycle. We find that the τ Cet line profiles differ systematically from those of the Sun and α Cen A. While lines formed in the middle chromospheres appear similar, the entire emission from the upper chromosphere to the middle transition region of τ Cet has lower flux densities by factors of ≈2, the line widths are significantly narrower, and, uniquely, the transition region lines are not significantly redshifted. The soft X-ray surface flux of τ Cet, measured between 0.1 and 2.4 keV, is ≈9 × 103 ergs cm-2 s-1, several times smaller than the median solar value. We also find that the UV spectrum of α Cen serves as a proxy for a Sun-as-a-star spectrum when the Sun is in an intermediate phase of its activity cycle but that its coronal emission may be somewhat smaller. We discuss the implications of these results for magnetic fields and heating mechanisms in the stars and speculate that τ Cet's UV spectrum might represent the solar spectrum during a grand minimum phase.

The Nature of SN 1961V

The nature of SN 1961V has been uncertain. Its peculiar optical light curve and slow expansion velocity are similar to those of superoutbursts of luminous blue variables (LBVs), but its nonthermal radio spectral index and declining radio luminosity are consistent with decades-old supernovae (SNe). We have obtained Hubble Space Telescope STIS images and spectra of the stars in the vicinity of SN 1961V and find object 7 identified by Filippenko and coworkers to be closest to the optical and radio positions of SN 1961V. Object 7 is the only point source detected in our STIS spectra, and only its Hα emission is detected; it cannot be the SN or its remnant because of the absence of forbidden lines. While the Hα line profile of object 7 is remarkably similar to that of η Carinae, the blue color (similar to an A2 Ib supergiant) and lack of appreciable variability are unlike known postoutburst LBVs. We have also obtained Very Long Baseline Array observations of SN 1961V at 18 cm. The nondetection of SN 1961V places a lower limit on the size of the radio-emitting region, 7.6 mas or 0.34 pc, which implies an average expansion velocity in excess of 4400 km s-1, much higher than the optical expansion velocity measured in 1961. We conclude the following: (1) An SN occurred in the vicinity of SN 1961V a few decades ago. (2) If the SN 1961V light maximum originates from a giant eruption of a massive star, object 7 is the most probable candidate for the survivor, but its blue color and lack of significant variability are different from a postoutburst η Car. (3) The radio SN and object 7 could be physically associated with each other through a binary system. (4) Object 7 needs to be monitored to determine its nature and relationship to SN 1961V.