Showing papers by "European Southern Observatory published in 1997"

Transient optical emission from the error box of the gamma-ray burst of 28 February 1997

Abstract: For almost a quarter of a century1, the origin of γ-ray bursts— brief, energetic bursts of high-energy photons—has remained unknown. The detection of a counterpart at another wavelength has long been thought to be a key to understanding the nature of these bursts (see, for example, ref. 2), but intensive searches have not revealed such a counterpart. The distribution and properties of the bursts3 are explained naturally if they lie at cosmological distances (a few Gpc)4, but there is a countervailing view that they are relatively local objects5, perhaps distributed in a very large halo around our Galaxy. Here we report the detection of a transient and fading optical source in the error box associated with the burst GRB970228, less than 21 hours after the burst6,7. The optical transient appears to be associated with a faint galaxy7,8, suggesting that the burst occurred in that galaxy and thus that γ-ray bursts in general lie at cosmological distance.

The MACHO Project Large Magellanic Cloud Microlensing Results from the First Two Years and the Nature of the Galactic Dark Halo

Abstract: The MACHO Project is a search for dark matter in the form of massive compact halo objects (MACHOs). Photometric monitoring of millions of stars in the Large Magellanic Cloud (LMC), Small Magellanic Cloud (SMC), and Galactic bulge is used to search for gravitational microlensing events caused by these otherwise invisible objects. Analysis of the first 2.1 yr of photometry of 8.5 million stars in the LMC reveals eight candidate microlensing events. This is substantially more than the number expected (~1.1) from lensing by known stellar populations. The timescales (t) of the events range from 34 to 145 days. We estimate the total microlensing optical depth toward the LMC from events with 2 < < 200 days to be τ2002=2.9+ 1.4−0.9×10−7 based upon our eight event sample. This exceeds the optical depth, τbackgnd = 0.5 × 10-7, expected from known stars, and the difference is to be compared with the optical depth predicted for a "standard" halo composed entirely of MACHOs: τhalo = 4.7 × 10-7. To compare with Galactic halo models, we perform likelihood analyses on the full eight-event sample and a six-event subsample (which allows for two events to be caused by a nonhalo "background"). This gives a fairly model-independent estimate of the halo mass in MACHOs within 50 kpc of 2.0+ 1.2−0.7×1011 M☉, which is about half of the "standard halo" value. We also find a most probable MACHO mass of 0.5+ 0.3−0.2 M☉, although this value is strongly model dependent. In addition, the absence of short duration events places stringent upper limits on the contribution of low-mass MACHOs: objects from 10-4 M☉ to 0.03 M☉ contribute 20% of the "standard" dark halo.

The Supermassive Black Hole of M87 and the Kinematics of Its Associated Gaseous Disk

Abstract: We have carried out the first HST long-slit observations of a gaseous disk around a candidate super-massive black hole. The results of this study on the kinematics of the gaseous disk in M 87 are a considerable improvement in both spatial resolution and accuracy over previous observations and requires a projected mass of MBH (sin i)2=(2.0±0.5)×109 M⊙ (MBH=3.2×109 M⊙ for a disk inclination i=52°) concentrated within a sphere whose radius is less than 0″.05 (3.5 pc) to explain the observed rotation curve. The kinematics of the ionized gas is well described by a thin disk in Keplerian motion. A lower limit to the mass-to-light ratio of this region is M/LV⋍110, significantly strengthening the claim that this mass is due to the presence of a central black hole in M 87.

Internal dynamics of globular clusters

Abstract: Galactic globular clusters, which are ancient building blocks of our Galaxy, represent a very interesting family of stellar systems in which some fundamental dynamical processes have taken place on time scales shorter than the age of the universe. In contrast with galaxies, these clusters represent unique laboratories for learning about two-body relaxation, mass segregation from equipartition of energy, stellar collisions, stellar mergers, and core collapse. In the present review, we summarize the tremendous developments, as much theoretical as observational, that have taken place during the last two decades, and which have led to a quantum jump in our understanding of these beautiful dynamical systems.

Discovery of Extended Blue Horizontal Branches in Two Metal-rich Globular Clusters*

Abstract: We have used WFPC2 to construct B, V color-magnitude diagrams of four metal-rich globular clusters, NGC 104 (47 Tuc), NGC 5927, NGC 6388, and NGC 6441 All four clusters have well populated red horizontal branches (RHB), as expected for their metallicity However, NGC 6388 and 6441 also exhibit a prominent blue horizontal-branch (BHB) extension, including stars reaching as faint in V as the turnoff luminosity This discovery demonstrates directly for the first time that a major population of hot horizontal-branch (HB) stars can exist in old, metal-rich systems This may have important implications for the interpretation of the integrated spectra of elliptical galaxies The cause of the phenomenon remains uncertain We examine the possibility that NGC 6388 and 6441 are older than the other clusters, but a simple difference in age may not be sufficient to produce the observed distributions along the HB The high central densities in NGC 6388 and 6441 suggest that the existence of the BHB tails might be caused by stellar interactions in the dense cores of these clusters, which we calculate to have two of the highest collision rates among globular clusters in the Galaxy Tidal collisions might act in various ways to enhance loss of envelope mass and therefore populate the blue side of the HB However, the relative frequency of tidal collisions does not seem large enough (compared to that of the clusters with pure RHBs) to account for such a drastic difference in HB morphology While a combination of an age difference and dynamical interactions may help, prima facie the lack of a radial gradient in the BHB/RHB star ratio seems to argue against dynamical effects playing a role

Iron as a Tracer in Galaxy Clusters and Groups

Abstract: Available X-ray data are collected and organized concerning the iron and gas content of galaxy clusters and groups, together with the optical luminosity, mass, and iron abundance of cluster galaxies. Moving from such a restricted number of cluster parameters, several astrophysical inferences are drawn. These include the evidence for rich clusters having evolved without much baryon exchange with their surroundings and having experienced very similar star formation histories. Groups are more gas-poor as compared with clusters and appear instead to have shed a major fraction of their original cosmic share of baryons, which indicates that galaxy clusters cannot have formed by assembling groups that are similar to the present-day ones. It is argued that this favors low-Ω universes, in which the growth of rich clusters is virtually complete at high redshifts. It is also argued that elemental abundance ratios in clusters are nearly solar, which is consistent with a similar proportion of supernovae of Type Ia and Type II having enriched both the solar neighborhood and clusters as a whole. Much of the iron in clusters appears to reside in the intracluster medium rather than inside galaxies, the precise ratio being a function of the Hubble constant. It appears that the baryon to star conversion in clusters has been nearly as efficient as that currently observed for the universe as a whole. Yet the metallicity of the clusters is ~5 times higher than the global metallicity found for the nearby universe. It is concluded that the intergalactic medium should have a metallicity ~1/3 of the solar value if stellar nucleosynthesis has proceeded in stars within field galaxies with the same efficiency as in stars within clusters of galaxies.

Rotational Velocities and Chromospheric/Coronal Activity of Low-Mass Stars in the Young Open Clusters IC 2391 and IC 2602

Abstract: We have obtained high-resolution, moderate signal-to-noise ratio spectra for approximately 80 candidate low-mass members of the nearby, very young open clusters IC 2391 and IC 2602. Most of the stars observed are confirmed as cluster members based on a combination of photometric and spectroscopic criteria. We provide radial velocities, rotational velocities, and H? equivalent widths for these stars. From comparison to theoretical pre-main-sequence (PMS) evolutionary isochrones from D'Antona and Mazzitelli, we derive an estimated age of the two clusters of ~25 Myr. By contrast, the usually quoted upper main-sequence turnoff age for the clusters is ~35 Myr. We do not believe that this provides evidence for noncoeval star formation within these clusters, but rather that the best age estimate for them given the uncertainties is ~30 ? 5 Myr. In principle, the scatter of stars about the PMS isochrone provides a measure of the age spread among the low-mass stars in these clusters; however, with the data presently available, we are able to derive only a relatively uninteresting upper limit for an age spread of order 20 Myr. We compare the rotational velocity distribution for IC 2391/2602 to that observed for the Pleiades. For the G dwarfs in the IC clusters, we resolve rotation in all but one of the probable cluster members, and thus except for inclination effects, our data provide the complete distribution of rotational velocities for solar mass stars on their arrival on the ZAMS. The projected rotational velocities (v sin i) of the G dwarfs in the two IC clusters span the range from ~8 to ~200 km s-1. Comparison of the distribution of rotational velocities for the G dwarfs of the Pleiades and the IC clusters indicates that both the slow and the rapid rotators lose of order half their angular momentum during the first ~35 Myr on the main sequence if they rotate as solid bodies. The low-mass stars in these two clusters exhibit a similar correlation between rotation and coronal activity as is found in several other young open clusters. That is, there is a large spread in coronal activity for stars with v sin i < 25 km s-1, where we assume there is an intrinsic link between increasing rotation and increasing activity superimposed upon which are a variety of observational and physical mechanisms that act to smear out this relation; above v sin i ~ 25 km s-1, all of the low-mass stars have log (LX/Lbol) ~ -3.0, the canonical saturation limit. Our measurements of the H? equivalent widths are consistent with a similar relationship holding for chromospheric activity. One and possibly two of our spectra for M dwarf members of the IC clusters show broad wings for the H? profile, which we attribute to a flare event or to microflares. Since spectra of a small sample of late-type M dwarfs in the Pleiades also showed similarly broad H? wings, this suggests that flare frequencies for very young M dwarfs may be quite high.

The mean magnetic field modulus of Ap stars

Abstract: We present new measurements of the mean magnetic field modulus of a sample of Ap stars with spectral lines resolved into magnetically split components. We report the discovery of 16 new stars having this property. This brings the total number of such stars known to 42. We have performed more than 750 measurements of the mean field modulus of 40 of these 42 stars, between May 1988 and August 1995. The best of them have an estimated accuracy of . The availability of such a large number of measurements allows us to discuss for the first time the distribution of the field modulus intensities. A most intriguing result is the apparent existence of a sharp cutoff at the low end of this distribution, since no star with a field modulus (averaged over the rotation period) smaller than 2.8 kG has been found in this study. For more than one third of the studied stars, enough field determinations well distributed throughout the stellar rotation cycle have been achieved to allow us to characterize at least to some extent the variations of the field modulus. These variations are often significantly anharmonic, and it is not unusual for their extrema not to coincide in phase with the extrema of the longitudinal field (for the few stars for which enough data exist about the latter). This, together with considerations on the distribution of the relative amplitude of variation of the studied stars, supports the recently emerging evidence for markedly non-dipolar geometry and fine structure of the magnetic fields of most Ap stars. New or improved determinations of the rotation periods of 9 Ap stars have been achieved from the analysis of the variations of their mean magnetic field modulus. Tentative values of the period have been derived for 5 additional stars, and lower limits have been established for 10 stars. The shortest definite rotation period of an Ap star with magnetically resolved lines is 34, while those stars that rotate slowest appear to have periods in excess of 70 or 75 years. As a result of this study, the number of known Ap stars with rotation periods longer than 30 days is almost doubled. We briefly rediscuss the slow-rotation tail of the period distribution of Ap stars. This study also yielded the discovery of radial velocity variations in 8 stars. There seems to be a deficiency of binaries with short orbital periods among Ap stars with magnetically resolved lines.

Dust Filaments and Star Formation in OMC-2 and OMC-3

Abstract: We have mapped the 1300 μm dust emission of a 6' × 16' field in the OMC-2 and -3 region and detected an extended filamentary structure with at least 11 embedded condensations in OMC-2 and 10 in OMC-3. Six have been observed previously at 1300 μm, and two were also detected by IRAS. We observed eight of the new sources at six wave bands from 350 to 2000 μm and derived upper limits for their far-infrared (FIR) emission from the IRAS raw data. The millimeter/submillimeter emission originates from cold dust (Td ~ 20 K), and the individual components have gas masses of 5 M☉ < Mgas < 36 M☉. The ratio Lbol/Lsmm is below 70 for six sources associated with OMC-3, indicating that they are class 0-type objects. Condensations in the OMC-2 region have Lbol/Lsmm ≤ 360, suggesting an evolutionary effect from north to south. We report the discovery of a highly collimated bipolar CO outflow, most likely driven by the source OMC-3 MMS 8. The flow is over 5' (0.7 pc) long and less than 1' (0.15 pc) wide and is oriented nearly east-west. Less prominent outflows are associated with OMC-2 FIR 2/3 and OMC-3 MMS 6. The 1300 μm continuum emission is confined to a ridge less than 1' wide while the emission in J = 2-113CO, C18O, and CS is between 5' and 10' wide. The continuum emission is displaced toward the eastern side of the molecular ridge that contains it. Most 1300 μm sources lie in or close to line emission peaks. However, not all line emission peaks contain prominent 1300 μm continuum sources. The curved filamentary structure of the large-scale dust and molecular emission is likely the result of compression by the superbubble centered ~25-70 pc further north, plus the impact of energy from the younger 1c subgroup of the Orion OB association, which lies 10-50 pc in front of our mapped region, and by the very recent expansion of the NGC 1977 and M42 H II regions.

Kelu-1 : A Free-floating Brown Dwarf in the Solar Neighborhood

Abstract: We present the discovery of a field brown dwarf, Kelu-1. The object has a peculiar optical spectrum that, in spite of its extremely red color, shows no sign of the TiO and VO absorption bands typical of cool, low-mass stars. The optical spectrum does show the Li I λ6708 absorption line and Hα in emission. The infrared spectrum of Kelu-1 has strong H2O absorption and also shows the effect of pressure-induced H2 opacity. Kelu-1 was discovered through its large proper motion of 035 yr-1, with position angle θ=265°. A distance of ~10 pc is estimated astrometrically for the object, which implies absolute magnitudes of MV~22 and MK~12. Comparison of the observed energy distribution of Kelu-1 to synthetic spectra generated by atmosphere models of Allard et al., and structural and evolutionary models of Chabrier & Baraffe, implies that the object has Teff=1900±100 K and a mass below 0.075 M/M.

The FIRST Radio-loud Broad Absorption Line QSO and Evidence for a Hidden Population of Quasars

Abstract: We have discovered two low-ionization broad absorption line (BAL) quasars in programs to obtain optical spectra for radio-selected quasar candidates from the VLA Faint Images of the Radio Sky at Twenty Centimeters (FIRST) Survey. Both belong to the extremely rare class of BAL QSOs that exhibit narrow absorption lines from metastable excited levels of Fe II and Fe III. Until now, there was just a single object in this class, 0059-2735. In addition, one of our new objects is the first known radio-loud BAL QSO. The properties of these three unusual objects suggest a trend of increasing radio luminosity with the amount of absorption to the quasar, and are perhaps transition objects between radio-loud and radio-quiet quasars. The two new objects are from a radio-selected sample comprising less than 200 quasars; one is heavily attenuated at optical wavelengths in the observed frame. These objects would be easily overlooked by most optical QSO searches; their abundance in the radio sample suggests that they may be representatives of a largely undetected component of the quasar population, perhaps as numerous as ordinary low-ionization BAL QSOs, which constitute 1%-2% of all QSOs.

Hubble Space Telescope Faint Object Spectroscope Spectroscopy of Localized Chemical Enrichment from Massive Stars in NGC 5253

Abstract: Optical and ultraviolet spectroscopy obtained with the Hubble Space Telescope Faint Object Spectrograph at three locations in the central H II complex of the amorphous starburst galaxy NGC 5253 confirms an apparent area of enhanced nitrogen abundance seen in ground-based studies. At two positions, N appears enhanced by a factor of 3 [log (N/O) = -0.85] compared to a third location 50 pc away where the measured N abundance is typical of metal-poor galaxies [log (N/O) = -1.30]. No other elemental species shows spatial abundance fluctuations, including C as measured from the C III] ??1907, 1909 ? emission lines. Extinction to the emission-line regions is quite low (AV = 0.25), and we rule out reddening uncertainties as the cause of the apparent N enhancement. Comparison of the observed ionic fractions to predictions of photoionization models shows that ionization uncertainties cannot be responsible for the anomalously high N/O ratio. These findings require the existence of a N production mechanism that is decoupled from C and O production. Although the high N/He enrichment ratio is consistent with that expected from non-Type I planetary nebulae (PNs), the localized nature of the enrichment, the large number of PNs (150-500), and the long timescales (109 yr) required make this explanation highly unlikely. Given the proximity of the N overabundance to a very young, heavily obscured star cluster, we attribute the N excess to recent pollution from massive stars. Plausible N enrichment scenarios involve O star winds, He-deficient W-R star winds, and/or ejection events from luminous blue variables. If the N enrichment is due to localized pollution from the winds of W-R stars observed in the burst, then an accompanying He enrichment of 40% ought to be observed, but we are able to rule out He enhancements at the 2 ? level at both locations showing N enhancement. We propose that if the N-rich wind or ejecta is incorporated into self-gravitating clumps of molecular gas, and if the 106 yr old clusters of massive stars in starburst galaxies are precursors of globular clusters, then N-overabundant halo and globular cluster stars in the Galaxy may owe their chemical peculiarity to similar N-enrichment episodes early in the history of the Milky Way.

Steps toward Determination of the Size and Structure of the Broad-Line Region in Active Galactic Nuclei. XI. Intensive Monitoring of the Ultraviolet Spectrum of NGC 7469

Abstract: From 1996 June 10 to July 29, the International Ultraviolet Explorer monitored the Seyfert 1 galaxy NGC 7469 continuously in an attempt to measure time delays between the continuum and emission-line fluxes. From the time delays, one can estimate the size of the region dominating the production of the UV emission lines in this source. We find the strong UV emission lines to respond to continuum variations with time delays of about 23-31 for Lyα, 27 for C IV λ1549, 19-24 for N V λ1240, 17-18 for Si IV λ1400, and 07-10 for He II λ1640. The most remarkable result, however, is the detection of apparent time delays between the different UV continuum bands. With respect to the UV continuum flux at 1315 A, the flux at 1485 A, 1740 A, and 1825 A lags with time delays of 021, 035, and 028, respectively. Determination of the significance of this detection is somewhat problematic since it depends on accurate estimation of the uncertainties in the lag measurements, which are difficult to assess. We attempt to estimate the uncertainties in the time delays through Monte Carlo simulations, and these yield estimates of ~007 for the 1 σ uncertainties in the interband continuum time delays. Possible explanations for the delays include the existence of a continuum-flux reprocessing region close to the central source and/or a contamination of the continuum flux with a very broad time-delayed emission feature such as the Balmer continuum or merged Fe II multiplets.

MACHO Alert 95-30: First Real-Time Observation of Extended Source Effects in Gravitational Microlensing

Abstract: We present analysis of MACHO Alert 95-30, a dramatic gravitational microlensing event toward the Galactic bulge whose peak magnification departs significantly from the standard point-source microlensing model Alert 95-30 was observed in real time by the Global Microlensing Alert Network (GMAN), which obtained densely sampled photometric and spectroscopic data throughout the event We interpret the light-curve fine structure as indicating transit of the lens across the extended face of the source star This signifies resolution of a star several kiloparsecs distant We find a lens angular impact parameter θmin/θsource = 0715 ± 0003 This information, along with the radius and distance of the source, provides an additional constraint on the lensing system Spectroscopic and photometric data indicate the source is a M4 III star of radius 61 ± 12 R☉, located on the far side of the bulge at ~9 kpc We derive a lens angular velocity, relative to the source, of 215 ± 29 km s-1 kpc-1, where the error is dominated by uncertainty in the angular size of the source star Likelihood analysis yields a median lens mass of 067 -->+ 253−046 M☉, located with 80% probability in the Galactic bulge at a distance of 693 -->+ 156−225 kpc If the lens is a main-sequence star, we can include constraints on the lens luminosity This modifies our estimates to Mlens=053 -->+ 052−035 M☉ and Dlens=657 -->+ 099−225 kpc Spectra taken during the event show that the absorption-line equivalent widths of Hα and the TiO bands near 6700 A vary, as predicted for microlensing of an extended source This is most likely due to center-to-limb variation in the stellar spectral lines The observed spectral changes further support our microlensing interpretation These data demonstrate the feasibility of using microlensing limb crossings as a tool to probe stellar atmospheres directly

Spectropolarimetry of magnetic stars - VI. Longitudinal field, crossover and quadratic field: New measurements

Abstract: New determinations of the mean longitudinal magnetic field, of the crossover, and of the mean quadratic magnetic field of Ap stars are presented. They are based on spectra recorded simultaneously in both circular polarizations at ESO with the CASPEC spectrograph fed by the 3.6 m telescope. This paper discusses 95 observations of 44 stars. A major result of this study is the discovery that HD 137509 has a predominantly quadrupolar magnetic field, a strucuture previously found in only a couple of stars. Improvement or revision of the determination of the rotation period has been achieved for 3 stars. The stars studied in this work include 14 rapidly oscillating Ap stars (for 6 of which no previous attempt to detect a magnetic field had ever been made) and 21 Ap stars with spectral lines resolved into their magnetically split components when observed at high enough dispersion in unpolarized light (for 9 of these stars, no determination of the longitudinal field had been performed before). The observations discussed in this paper have been performed between 1989 and 1994, a period during which CASPEC and its Zeeman analyzer have progressively undergone various configuration changes. The results reported here demonstrate that the polarimetric performance of the instrument has remained unaltered through these modifications. Thanks to the latter, the achieved resolving power was increased, which resulted in improved magnetic measurement accuracies.

Discovery of a Highly Collimated Molecular Outflow in the Southern Bok Globule BHR 71

Abstract: We report observations of the southern Bok globule BHR 71 in the J = 1 → 0 and J = 2 → 1 lines of 12CO, the J = 1 → 0 lines of 13CO and C18O, and (J, K) = (1, 1) and (2, 2) inversion lines of NH3 made with angular resolution of ~20'' to ~9'. We also report 1.3 mm continuum observations made with SEST with ~20'' resolution. The low angular resolution molecular observations indicate that the globule has a diameter of ~0.5 pc, a kinetic temperature of 11 K, a total mass of ~40 M☉, and an average molecular density of ~9 × 103 cm-3. The high angular resolution observations reveal the presence, near the center of the globule, of a highly collimated bipolar outflow with lobes extending by ~0.3 pc in opposite directions from a strong 1.3 mm continuum source. The morphology and velocity structure of the flow is found to be well described by a biconical outflow that is inclined from the line of sight at an angle of ~84°, has a semi-opening angle of 15°, and in which the gas moves outward with an approximate constant radial velocity (with respect to the cone apex) of ~28 km s-1. The outflow appears to be driven by a very young stellar object with Lbol ~ 9 L☉, whose characteristics at infrared and millimeter wavelengths are similar to those of the so-called Class 0 sources.

The Iron Discrepancy in Elliptical Galaxies after ASCA

Abstract: We present estimates for the iron content of the stellar and diffused components of elliptical galaxies, as derived respectively from integrated optical spectra and from ASCA X-ray observations. A macroscopic discrepancy emerges between the expected iron abundances in the hot interstellar medium (ISM) and what is indicated by the X-ray observations, especially when allowance is made for the current iron enrichment by Type Ia supernovae. This strong discrepancy, that in some extreme instances may be as large as a factor of ~20, calls into question our current understanding of supernova enrichment and chemical evolution of galaxies. We discuss several astrophysical implications of the inferred low iron abundances in the ISM, including the chemical evolution of galaxies and clusters of galaxies, the evolution of gas flows in elliptical galaxies, and the heating of the intracluster medium. Some of the consequences appear hard to accept, and in the attempt to avoid some of the difficulties we explore ways of hiding or diluting iron in the ISM of ellipticals. None of these possibilities appears astrophysically plausible, and we alternatively raise the question of the reliability of iron L line diagnostic tools that are currently used to infer abundances from X-ray spectra. Various thin-plasma emission models are shown to give iron abundances that may differ significantly, especially at low temperatures (kT 1 keV), when the iron L complex is dominated by iron ions with still many bound electrons. From a collection of ASCA and other X-ray observatory data, it is shown that current thin-plasma codes tend to give very low iron abundances when the temperature of the objects is below ~1 keV. Such objects include various types of binary stars, supernova remnants, starburst galaxies, and AGNs, with the case of galaxy groups being especially well documented. We conclude that, besides rethinking the chemical evolution of galaxies, one should also consider the possibility that existing thin-plasma models may incorporate inaccurate atomic physics for the ions responsible for the iron L complex.

Where Do the Disks of Spiral Galaxies End

Abstract: In spiral galaxies, the H I surface density declines with increasing radius to a point at which it is seen to truncate dramatically in the best observed cases. If the ambient radiation field is sufficiently strong, there exists a maximum radius beyond which the cold gas is unable to support itself against ionization. We have now succeeded in detecting ionized gas beyond the observed H I disk in spirals. Here we report on our findings for the Sculptor galaxy NGC 253. The H I disks in Sculptor galaxies extend to only about 1.2R25, although we have detected ionized gas to the limits of our survey, out to 1.4R25. This has important ramifications for spiral galaxies in that it now becomes possible to trace the gravitational potential beyond where the H I disk ends. The detections confirm that the rotation curve continues to rise in NGC 253, as it appears to do for other Sculptor galaxies, from the H I measurements, but there is a hint that the rotation curve may fall abruptly not far beyond the edge of the H I disk. If this is correct, it suggests that the dark halo of NGC 253 may be truncated near the H I edge and provides further support for the link between dark matter and H I. The line ratios are anomalous with [N II] ?6548 to H? ratios close to unity. While metallicities at these large radii are uncertain, such enhanced ratios, compared to solar-abundance H II regions ([N II] ?6548/H? = 0.05-0.2), are likely to require selective heating of the electron population without further ionization of N+. We discuss the most likely sources of ionization and heating, and the possible role of refractory element depletion (e.g., Ca, Si, and Fe) onto dust grains.

The Tully-Fisher Relation and H0

Abstract: The use of the Tully-Fisher (TF) relation for the determination of H0 relies on the availability of an adequate template TF relation and of reliable primary distances. Here we use a TF template relation with the best available kinematical zero point, obtained from a sample of 24 clusters of galaxies extending to cz ~ 9000 km s-1, and the most recent set of Cepheid distances for galaxies fit for TF use. The combination of these two ingredients yields H0 = 69 ± 5 km s-1 Mpc-1. The approach is significantly more accurate than the more common application with single cluster (e.g., Virgo, Coma) samples.

The morphology of H II galaxies

Abstract: We present CCD images of a sample of 39 HII galaxies taken at the Danish 154m telescope on La Silla The images are used to analyse the morphology of these emission line dwarfs, and the structural properties of the knots of star formation and of the underlying galaxy The sizes of the starbursts are measured We propose a morphological classification based on the presence or absence of signs of tails, extensions, or distorted outer isophotes This criterion segregates the objects into two broad morphological types with different physical properties: the more disturbed and extended (type I) HII galaxies having larger luminosities and velocity dispersions than the more compact and regular (type II) objects The relative position of HII galaxies and of a sample of dwarf elliptical galaxies in the [R -- sigma] diagram support the hypothesis of a possible evolutionary link between the two types of galaxy

Dynamical Correlations for Globular Clusters in M31

Abstract: We present internal velocity dispersion measurements for a set of 21 globular clusters in the Andromeda galaxy (M31). We combine them with structural and photometric cluster parameters measured earlier with the Hubble Space Telescope and from the ground to explore correlations of cluster properties and to compare them with the equivalent correlations for the Galactic globular clusters. We find that the M31 globulars follow the same correlations between velocity dispersion and luminosity, central, and average surface brightness, as do their Galactic counterparts. This suggests a common physical origin for these correlations. They may be produced by the same astrophysical conditions and processes operating at the epoch of globular cluster formation in both galaxies. The very existence of these excellent correlations, and their quantitative form as scaling laws, represent challenges and constraints for theories of globular cluster formation. Preliminary estimates of the cluster M/L ratios show correlations with the cluster metallicity, in the sense of more metal-rich clusters having lower M/L, particularly in the near-infrared. At a given metallicity, there is no detectable systematic difference between the M31 globulars and their Galactic counterparts, which suggests a great similarity of their stellar populations. The observed scatter around these trends is comparable to the expected errors, which implies a small intrinsic scatter and thus which leaves little room for possible variations in the cluster age or stellar IMF at a given metallicity.

Supernova Remnants Associated with Molecular Clouds in the Large Magellanic Cloud

Abstract: We used the Swedish-ESO Submillimeter Telescope (SEST) to search for CO emission associated with three supernova remnants (SNRs) in the Large Magellanic Cloud: N49, N132D, and N23. Observations were carried out in the J = 2 → 1 rotational transition of CO (230.5 GHz) where the half-power beamwidth of the SEST is 23''. Molecular clouds were discovered near N49 and N132D; no CO emission was discovered in the region we mapped near N23. The N49 cloud has a peak line temperature of 0.75 K, spatial scale of ~7 pc and virial mass of ~3 × 104 M☉. The N132D cloud is brighter with a peak temperature of 5 K; it is also larger ~22 pc and considerably more massive ~2 × 105 M☉. The velocities derived for the clouds near N49 and N132D, +286.0 and +264.0 km s-1, agree well with the previously known velocities of the associated SNRs: +286 km s-1 and +268 km s-1, respectively. ROSAT X-ray images show that the ambient density into which the remnants are expanding appears to be significantly increased in the direction of the clouds. Taken together these observations indicate a physical association between the remnants and their respective, presumably natal, molecular clouds. The association of N49 and N132D with dense regions of molecular material means that both were likely products of short-lived progenitors that exploded as core-collapse supernovae.

Stellar Populations of the Dwarf Irregular Galaxy WLM

Abstract: We have obtained V-band photometry for 8000 stars, and I-band photometry for 16000 stars in the field of the dwarf irregular galaxy WLM, a member of the Local Group. From the VI color-magnitude diagram we infer metallicities and ages for the stellar populations in the main body and in the halo of WLM. The youngest stars in the galaxy have an age of {approximately}10{sup 7} Gyr. For the oldest stars we derive an age of {gt}10{sup 10} Gyr and a metallicity of [Fe/H]={minus}1.45{plus_minus}0.2. We construct a deep luminosity function, obtaining an accurate distance modulus m{minus}M=24.75{plus_minus}0.1 for this galaxy based on the I-magnitude of the RGB tip and adopting E(V{minus}I)=0.03. We establish the presence of an extended, slightly flattened halo consisting of Population II stars. Cluster D of Ables & Ables (1977, ApJS, 34, 245) is confirmed as the only globular cluster present in the main body of this galaxy. There are no other clusters brighter than I=19.5. The B{minus}V color gradient observed by Ables & Ables (1977) is shown to be caused by a stellar-population transition, and not a metallicity gradient or dust extinction. {copyright} {ital 1997 American Astronomical Society.}

The abundance gradient of NGC1365: evidence for a recently formed bar in an archetypal barred spiral galaxy?

Abstract: Emission-line optical spectrophotometry for 55 H ii regions in the prominent southern barred spiral galaxy NGC 1365 is presented. Nebular diagnostic diagrams such as (N ii)/(O ii) and (S ii)/(O ii) versus ((O ii) + (O iii))/Hshow that the H ii regions of the barred galaxy have the same range of physical conditions as found in non-barred late-type galaxies. Extinction is moderately high across the disc and there is evidence for a slight trend of extinction with galactocentric distance; the logarithmic extinction at Hfalls from about c(H�) = 1.2 in the centre to 0.6 - 0.8 in the outer regions. The global O/H distribution has a moderate gradient of � 0.5 dex �0 1 (� 0.02 dex kpc 1 ) consistent with the known trend between the slope of the abundance gradient and the strength of the bar. A break is seen in the O/H gradient just beyond the -4/1 resonance, the gradient being moderately steep at � 0.8 dex � 1 0 (-0.05 dex/kpc) inside this resonance, and flat beyond �/�0 � 0.55. The abundance distribution is compared with another barred spiral galaxy, NGC 3359, and with that of two well- sampled normal spiral galaxies, NGC 2997 and M 101. The possibility that the bar formed recently in NGC 1365 is considered. The difficulties encountered in doing spectrophotometry with fibre optics are discussed and shown not to be insurmountable.

Discovery of a bow shock around Vela X-1

Abstract: We report the discovery of a symmetric bow shock around the well-known high-mass X-ray binary (HMXB) Vela X-1. Wind bow shocks are a ubiquitous phenomenon around OB-runaway stars, but now such a structure is found around an HMXB. The presence of a bow shock indicates that the system has a high (supersonic) velocity with respect to the interstellar medium. From the symmetry of the bow shock, the direction of motion and, moreover, the origin and age of the system can be derived. Our observation supports Blaauw's scenario for the formation of an OB-runaway star by the supernova explosion of the binary companion.

Infrared Response of H2 to X-Rays in Dense Clouds

Abstract: The excitation by X-rays and cosmic rays of molecular hydrogen in interstellar clouds is analyzed. We carried out detailed calculations of entry efficiencies in rovibrational levels of H2 following impact with fast electrons produced by X-ray ionization of the gas. The competing effect of collisional excitation, and quenching by the ambient gas is examined in detail. Up to date values for H-H2 collisional rate coefficients are adopted, and some derivations of H2-H2 rovibrational rate coefficients from existing literature data are proposed. Several models as a function of temperature, density, and ionization rate are presented. We found that H2 infrared emission in X-ray dominated regions (XDR) is potentially observable for temperatures and ionization rates lower than certain critical values (typically T < 1000 K and zeta/n(sub H) < 10(exp -15) cc/s where zeta is the ionization rate). At higher temperatures, collisional excitation by the ambient gas dominates the population of low vibrational levels, and at higher values of zeta/n(sub H) the abundance of H2 is negligible. If such conditions are satisfied, the resulting infrared emission spectrum can be used as a diagnostic of nearby X-ray sources such as in cooling flows in galaxy clusters, quasars, Seyfert galaxies and supernova remnants. The intensity ratio of the 2-1S(1) and 1-0S(1) lines measured for the Seyfert galaxy NGC 1275 is consistent with X-ray pumping.

SN 1983V in NGC 1365 and the Nature of Stripped Envelope Core-Collapse Supernovae

Abstract: We present CCD photometry and low-resolution spectra of the spectroscopic Type Ic supernova SN 1983V in NGC 1365. Photometry in the B band spans nearly 250 days after maximum light, while the spectra cover the photospheric epoch, from 9 days before up to 38 days after B maximum. We also compile and discuss infrared photometry that has been published elsewhere. The photometric evolution of SN 1983V is analyzed in comparison with other Type Ic supernovae and with SN 1983N and SN 1993J. The spectroscopic evolution is described, line identifications are proposed, and evidence for H, He, and Si is discussed. The photometric evolution of SN 1983V suggests that it is a close relative of SN 1983N and SN 1993J, events for which it is supposed that the external layers comprising most of the He/H and H shells, respectively, were lost before the explosion. The mass-to-energy ratios of SN 1983V and SN 1993J are substantially different if estimated from the expansion velocities. However, the similarity of the B light curves suggests that if the masses and mass-to-energy ratios are not similar, then the mass of SN 1983V has to be large enough to compensate for the higher mass-to-energy ratio. The latter possibility contradicts the standard scenarios that describe these events and implies a precise fine-tuning of the masses and mass-to-energy ratios of the two supernovae. This contradiction, however, could be resolved if a significant fraction of the inner ejecta containing 56Ni does not follow the decrease in density expected for a spherically symmetric, homologous expansion.

The Boomerang Nebula - The Coldest Region of the Universe

Abstract: In this letter, we report such observations of the Boomerang Nebula which show it to be a unique object, consisiting of an ultra-cold and extremely massive molecular envolope, expanding at very high speed. The extreeme physical characteristics of the Boomerang Nebula reported here have never been seen before in any AGB or post-AGB object, and should spur new theoretical and obesrvational efforts to understand the nature of the mass-loss processes occurring during later stellar evolution.