(Abridged) We present high-speed, three-colour photometry of seven short period (Porb <= 95 mins) eclipsing CVs from the Sloan Digital Sky Survey. We determine the system parameters via a parametrized model of the eclipse fitted to the observed lightcurve by chi2 minimization. Three out of seven of the systems possess brown dwarf donor stars and are believed to have evolved past the orbital period minimum. This is in line with predictions that 40-70 per cent of CVs should have evolved past the orbital period minimum. Therefore, the main result of our study is that the missing population of post-period minimum CVs has finally been identified. The donor star masses and radii are, however, inconsistent with model predictions; the donor stars are approximately 10 per cent larger than expected across the mass range studied here. One explanation for the discrepancy is enhanced angular momentum loss (e.g. from circumbinary discs), however the mass-transfer rates, as deduced from white dwarf effective temperatures, are not consistent with enhanced angular momentum loss. We show it is possible to explain the large donor radii without invoking enhanced angular momentum loss by a combination of geometrical deformation and the effects of starspots due to strong rotation and expected magnetic activity. Choosing unambiguously between these different solutions will require independent estimates of the mass-transfer rates in short period CVs.
The orbital evolution of cataclysmic variables in which the companion is illuminated by a fraction of the accretion luminosity consists of irradiation-driven limit cycles on thermal timescales, superimposed on a secular evolution toward shorter periods due to systemic angular momentum losses. We show that positive orbital period derivatives during bright phases are a natural consequence of the expansion of the companion during high mass transfer phases in the limit cycle. The irradiation instability may be enhanced by consequential angular momentum losses, CAML, accompanying the limit cycle. W…
I present brief status reports on three large observational projects that are designed to test our current understanding of the evolution of cataclysmic variables (CVs): The spectroscopic selection of new CVs in the Hamburg Quasar Survey, the search for pre-CVs based on Sloan colours and UK Schmidt/6dF multiobject spectroscopy, and the identification of CVs that descended from supersoft X-ray binaries using a HST/STIS far-ultraviolet spectroscopic survey.
Large differences between the properties of the known sample of cataclysmic variable stars (CVs) and the predictions of the theory of binary star evolution have long been recognised. However, because all existing CV samples suffer from strong selection effects, observational bias must be considered before it is possible to tell whether there is an inconsistency, which would imply a failure of the evolutionary model. We have modelled common selection effects and illustrate their influence on observed CV samples.
We show that the secondaries in short-period cataclysmic variables with orbital periods $P < 3$ hr are close to the solar-abundance main sequence defined by single field stars. In cataclysmic variables with $P > 3$ hr, the earliest spectral types at a given period correspond to main sequence stars, while the majority of secondaries have later spectral types. Possible causes are nuclear evolution prior to mass transfer and lack of thermal equilibrium due to mass transfer. A comparison with evolutionary sequences obtained with up--to--date stellar models implies unusually high transfer rates and…
We suggest a new mechanism to account for the loss of angular momentum in binaries with non-conservative mass exchange. It is shown that in some cases the loss of matter can result in increase of the orbital angular momentum of a binary. If included into consideration in evolutionary calculations, this mechanism appreciably extends the range of mass ratios of components for which mass exchange in binaries is stable. It becomes possible to explain the existence of some observed cataclysmic binaries with high donor/accretor mass ratio, which was prohibited in conservative evolution models.
Using VLTI/VINCI angular diameter measurements, we constrain the evolutionary status of three asteroseismic targets: the stars $δ$ Eri, $ξ$ Hya, $η$ Boo. Our predictions of the mean large frequency spacing of these stars are in agreement with published observational estimations. Looking without success for a companion of $δ$ Eri we doubt on its classification as an RS CVn star.
Using the Hipparcos data, absolute magnitudes and thus the evolutionary status for the group of lambda Bootis stars were derived. The origin of this small group of non-magnetic, chemically peculiar stars, still remains a matter of debate. Using new evolutionary tracks, an age determination could be provided to distinguish between the two competing theories - the diffusion/mass-loss and the accretion. The results establish the members of this group as objects which are very close to the Main Sequence. This is supported by Pre-Main Sequence evolutionary tracks as well as by observational results…
95% of all stars end their lives as white dwarfs. About 20% of the hot post-AGB stars are hydrogen deficient. Most of these are the result of a late helium-shell flash, but the evolutionary status of a fraction of about 10-20% of the hottest hydrogen-deficient stars, namely four O(He) stars, is as yet unexplained. They could be the long-searched hot successors of RCrB stars, which have not been identified up to now. If this turns out to be true, then a third post-AGB evolutionary sequence is revealed, which is probably the result of a double degenerate merging process. More generally, understa…
Despite its mean apparent magnitude of m(v)=6.39, the evolutionary status of HD190390 (HR7671), a luminous F-type supergiant at high galactic latitude, is still not very clear, but in most papers a post-AGB classification is assumed. New observational material has been obtained with four different instruments and is presented here. An extensive abundance analysis based on high resolution, high signal-to-noise NTT+EMMI spectra confirms the metal deficiency of this object ([Fe/H]=-1.6), together with a high lithium content (log(Li)=1.9). A variability analysis based on Geneva photometry over sev…
This paper describes a key submillimetre survey which we are currently conducting to address some of the outstanding questions in cosmology - how, at what epoch and over what period of time did massive galaxies form at high redshift? A summary of the technical feasibility of future submillimetre observations with new ground-based, airborne and satellite telescopes is also presented.
System parameters of the object LTT560 are determined in order to clarify its nature and evolutionary status. We apply time-series photometry to reveal orbital modulations of the light curve, time-series spectroscopy to measure radial velocities of features from both the primary and the secondary star, and flux-calibrated spectroscopy to derive temperatures of both components. We find that LTT560 is composed of a low temperature T~7500K DA white dwarf as the primary and an M5.5+-1 main-sequence star as the secondary component. The current orbital period is Porb=3.54(07)h. We derive a mass rati…
Current evolutionary models imply that most Cataclysmic Variables (CVs) have P_orb<2 hours and are Dwarf Nova (DN) systems that are quiescent most of the time. Observations of nearby quiescent DN find that the UV spectrum is dominated by the hot white dwarf (WD), indicating that it provides a significant fraction of the optical light in addition to the quiescent disk and main sequence companion. Hence, identifying a faint, quiescent CV in either the field or a globular cluster (GC) from broadband colors depends on our ability to predict the WD contribution in quiescence. We are undertaking a t…
We consider possible evolutionary models for SS 433. We assume that common-envelope evolution is avoided if radiation pressure is able to expel most of a super-Eddington accretion flow from a region smaller than the accretor's Roche lobe. This condition is satisfied, at least initially, for largely radiative donors with masses in the range 4-12 solar masses. For donors more massive than about 5 solar masses, moderate mass ratios q = M_2/M_1 > 1 are indicated, thus tending to favor black-hole accretors. For lower mass donors, evolutionary considerations do not distinguish between a neutron star…
The previously known, 6-yr spectroscopic binary HR 6046 has been speculated in the past to contain a compact object as the secondary. A recent study has re-determined the orbit with great accuracy, and shown that the companion is an evolved but otherwise normal star of nearly identical mass as the primary, which is also a giant. The binary motion was detected by the Hipparcos mission but was not properly accounted for in the published astrometric solution. Here we use the Hipparcos intermediate data in combination with the spectroscopic results to revise that solution and establish the orbital…
Well-determined physical parameters of 130 W UMa systems have been collected from the literature. Based on these data, the evolutionary status and dynamical evolution of W UMa systems are investigated. It is found that there is no evolutionary difference between W- and A-type systems in $M-J$ diagram which is consistent with the results derived from the analysis of observed spectral type, $M-R$ and $M-L$ diagrams of W UMa systems. $M-R$ and $M-L$ diagrams of W- and A-type systems indicate that a large amount of energy should be transferred from the more massive to the less massive component so…
We apply the K-correction to the black hole LMXB GX 339-4 which implies Mx > 6 Msun by only assuming that the companion is more massive than ~0.17 Msun, the lower limit allowed by applying a 'stripped-giant' model. This evolutionary model successfully reproduces the observed properties of the system. We obtain a maximum mass for the companion of M2 < 1.1 Msun and an upper limit to the mass ratio of q(=M2/Mx)< 0.125. The high X-ray activity displayed by the source suggests a relatively large mass transfer rate which, according to the model, results in M2 > 0.3 Msun and Mx > 7 Msun. We have also…
In this paper, we investigate the evolutionary status of B[e]~stars from the point of view of stellar evolution theory. We try to answer to the question of how massive hot supergiants --- i.e. evolved stars --- can be capable of producing a circumstellar disk. We find and discuss three possibilities: very massive evolved main sequence stars close to critical rotation due to their proximity to their Eddington-limit, blue supergiants which have just left the red supergiant branch, and single star merger remnants of a close binary system. While the latter process seems to be required to understan…
We present visible-wavelength spectropolarimetric measurements of the emission-line star MWC 349A and its close optical companion MWC 349B, conducted with the HPOL spectropolarimeter on both the 0.9 m telescope at the University of Wisconsin's Pine Bluff Observatory and the 3.5 m WIYN telescope on Kitt Peak. Our measurements allow us to estimate the interstellar polarization contribution and thus constrain the intrinsic polarization of MWC 349A, which we find to be consistent in position angle with the dusty disk seen perpendicular to the bipolar outflow. Our analysis reopens the possibility t…
We present new observational data for IRAS 09371+1212, the Frosty Leo Nebula, in the form of infrared spectra from 2.2-2.5 microns which reveal photopheric bands of CO. The ^12CO/^13CO band ratio determined is similar to those exhibited by evolved K giant stars, and supports the proposal that the object is highly evolved. The equivalent width of the CO bands implies, however, that the spectral type lies in the range G5III to K0III, somewhat earlier than K7III, as derived from colours and optical spectra. The smaller equivalent widths of the CO bands may reflect a low metal abundance which coul…
We present further evidence that the magnetic chemically peculiar stars (CP2) of the upper main sequence already occur at very early stages of the stellar evolution, significantly before they reach 30% of their life-time on the main sequence. This result is especially important for models dealing with dynamo theories, angular momentum loss during the pre- as well as main sequence and evolutionary calculations for CP2 stars. Results from the literature either derived for objects in the Hyades and the UMa cluster or from the Hipparcos mission contradict each other. A way out of this dilemma is t…
A sample of 97 galactic field Be stars were studied by taking into account the effects induced by the fast rotation on their fundamental parameters. All program stars were observed in the BCD spectrophotometric system in order to minimize the perturbations produced by the circumstellar environment on the spectral photospheric signatures. This is one of the first attempts at determining stellar masses and ages by simultaneously using model atmospheres and evolutionary tracks, both calculated for rotating objects. The stellar ages ($τ$) normalized to the respective inferred time that each rotati…
We present a study of the abundance of Be stars in open clusters as a function of the cluster age, using whenever possible ages determined through Stromgren uvby photometry. For the first time in studies of this kind we have considered separately classical and Herbig Be stars. The main results can be summarized as follows: 1.- Clusters associated to emitting nebulosities and undergoing stellar formation are rich in emission line objects, which most likely are all pre-main sequence objects. No bona fide classical Be star has yet been identified among them. 2.- Clusters younger than 10 Myr and w…
Although a large number of orbital periods of cataclysmic variable stars (CVs) have been measured, comparison of period and luminosity distributions with evolutionary theory is affected by strong selection effects. A test has been discovered which is independent of these selection effects and is based upon the kinematics of CVs (Kolb & Stehle, 1996). If the standard models of evolution are correct then long-period (P_orb > 5 hrs) CVs should be typically less than 1.5 Gyr old, and their line-of-sight velocity dispersion ($σ_γ$) should be small. We present results from a pilot study which indica…
We investigate the structure and evolution of a geometrically thin viscous Keplerian circumbinary (CB) disk, using detailed models of their radiative/convective vertical structure. We use a simplified description for the evolution of the cataclysmic binary and focus on cases where the circumbinary disk causes accelerated mass transfer (> 1e-8 Msun/yr). The inner edge of the disk is assumed to be determined by the tidal truncation radius and the mass input rate into the disk is assumed to be a small fraction (1e-5-0.01) of the mass transfer rate. Under the action of the viscous stresses in the …
The existence of faint blue stars far above the galactic plane, which have spectra that are similar to nearby Population I B stars, present several interesting questions. Among them are: Can a Population I B star travel from the disk to a position many kiloparsecs above the plane in a relatively short main sequence lifetime? Is it possible that single massive star formation is occurring far from the galactic plane? Are these objects something else masquerading as main sequence B stars? This paper (the first of two) analyzes the abundances of a sample of these stars and reveals several which ar…
The population synthesis of cataclysmic variables below the period is investigated. A grid of detailed binary evolutionary sequences has been calculated and included in the simulations to take account of additional angular momentum losses beyond that associated with gravitational radiation and mass loss, due to nova outbursts, from the system. As a specific example, we consider the effect of a circumbinary disk to gain insight into the ingredients necessary to reproduce the observed orbital period distribution. The resulting distributions show that the period minimum lies at about 80 minutes w…
We revealed that the dwarf nova 1RXS J232953.9+062814 is an SU UMa-type system with a superhump period of 66.774+/-0.010 min. The short period strongly indicates that the orbital period of this object is below the period minimum of cataclysmic variables. The superhump period is 4.04+/-0.02 % longer than the photometric period during quiescence (64.184+/-0.003 min), which is probably associated with the orbital period. Although the standard evolutionary scenario of cataclysmic variables predicts lower mass-transfer rates in systems with shorter orbital periods, we revealed firm evidence of a re…
We investigate why the spectral type of most cataclysmic variable (CV) secondaries is significantly later than that of a ZAMS star with the same mean density. Using improved stellar input physics, tested against observations of low-mass stars at the bottom of the main sequence, we calculate the secular evolution of CVs with low-mass donors. We consider sequences with different mass transfer rates and with a different degree of nuclear evolution of the donor prior to mass transfer. Systems near the upper edge of the gap ($P \sim 3 - 6$ h) can be reproduced by models with a wide range of mass tr…
The radio source FIRST J102347.6+003841 was presented as the first radio-selected cataclysmic. In the discovery paper, Bond et al. (2002) show a spectrum consistent with a magnetic AM Her-type system and a light curve with rapid, irregular flickering. In contrast, Woudt, Warner, and Pretorius (2004) found a smoothly-varying light curve with a period near 4.75 h and one minimum per orbit, indicating a dramatic change. We present time-resolved spectra showing a superficially normal, mid-G type photosphere, with no detectable emission lines. The absorption-line radial velocity varies sinusoidally…
We introduce our project that investigates the kinematic properties of early-type galaxies in 6 distant poor clusters at z~0.25. This study represents a continuation of our efforts to understand galaxy evolution in low-density environments. Higher-resolution MOSCA spectra have been obtained at the Calar Alto 3.5-m telescope with which we can measure absorption line strengths and velocity dispersions. In conjunction with our HST/F702W images of all the clusters, we are able to construct the Fundamental Plane of ellipticals and S0 galaxies in poor clusters at a look-back time of ~3 Gyrs. For gal…
This paper continues the analysis of faint high latitude B stars from Martin (2004). Here we analyze the kinematics of the stars and combine them with the abundance information from the first paper to classify each one. The sample contains 31 Population I runaways, fifteen old evolved stars (including five BHB stars, three post-HB stars, a pulsating helium dwarf, and six stars of ambiguous classification), one F-dwarf, and two stars which do not easily fit in one of the other categories. No star in the sample unambiguously shows the characteristics of a young massive star formed in situ in the…
We have used a model of magnetic accretion to investigate the accretion flows of magnetic cataclysmic variables. Numerical simulations demonstrate that four types of flow are possible: discs, streams, rings and propellers. The fundamental observable determining the accretion flow, for a given mass ratio, is the spin-to-orbital period ratio of the system. If IPs are accreting at their equilibrium spin rates, then for a mass ratio of 0.5, those with Pspin/Porb < 0.1 will be disc-like, those with 0.1 < Pspin/Porb < 0.6 will be stream-like, and those with Pspin/Porb ~ 0.6 will be ring-like. The sp…
The evolution of low mass (M < 2.5 Msun) binaries through the common envelope phase has been studied for systems in which one member is on its first ascent of the red giant branch. Three-dimensional hydrodynamical simulations have been carried out for a range of red giant masses (1 - 2 Msun) with degenerate helium cores (0.28 - 0.45 Msun) and companions (0.1 - 0.45 Msun) for initial orbital periods ranging from about 15 - 1000 days. The results suggest that these low mass binary systems can survive the common envelope phase provided that the helium degenerate core is more massive than about 0.…
We have observed a new cataclysmic variable (CV) SDSS J080434.20+510349.2 and study the origin of a long-term variability found in its light curve. Multi-longitude time-resolved photometric observations were carried out to analyze the uncommon behavior also found recently in two newly discovered CVs. This study of SDSS J080434.20+510349.2 mainly concerns the understanding of the nature of the observed double-humped light curve and its relation to a cyclic brightening occurring during quiescence. The observations were obtained early in 2007, when the object was at about V~17.1, 0.4 mag brighter…
We explore the evolution of the early-type galaxy population in the rich cluster Abell 2390 at z=0.23. For this purpose, we have obtained spectroscopic data of 51 elliptical and lenticular galaxies with MOSCA at the 3.5 m telescope on Calar Alto Observatory. As our investigation spans both a broad range in luminosity (-22.3<M_B<-19.3) and a wide field of view (10'x10'), the environmental dependence of different formation scenarios can be analysed in detail as a function of radius from the cluster center. In this paper, we present first results on the Faber-Jackson relation and, for a subsample…
V Hya, an evolved carbon star with a complex circumstellar envelope, has two variability periods, 530d and 6000d (17 years). We analyze recent light curve data and show that both variations have benn present for at least 100 years and have been regular over this time. The 530d period and its 1.5 - 2 magnitude amplitude show that V Hya is a Mira variable. We suggest that the star is in a binary system (as also suspected from the structure of the circumstellar envelope) and that the 17-year variation is due to extinction by circumstellar dust orbiting with the companion. The properties of the en…
We apply the angular momentum loss rates inferred from open cluster stars to the evolution of cataclysmic variables (CVs). We show that the angular momentum prescriptions used in earlier CV studies are inconsistent with the measured rotation data in open clusters. The timescale for angular momentum loss above the fully convective boundary is ~ 2 orders of magnitude longer than inferred from the older model, and the observed angular momentum loss properties show no evidence for a change in a behavior at the fully convective boundary. This provides evidence against the hypothesis that the period…
We numerically study the precessing disk model for superhump in the SU~UMa subclass of cataclysmic variables, using a two dimensional SPH code specifically designed for thin disk problems. Two disk simulations for a binary with mass ratio $q=\frac{3}{17}$ (similar to OY~Car) are performed, in order to investigate the Lubow (1991 a,b) tidal resonance instability mechanism. In the first calculation, a disk evolves under steady mass transfer from $L_1$. In the second simulation, mass is added in Keplerian orbit to the inner disk. The two disks follow similar evolutionary paths. However the $L_1$ …
A new scenario for evolution of contact binaries is presented and discussed. Arguments are given that W UMa-type systems are formed from detached binaries which lose angular momentum via magnetized wind. This takes typically several Gyr. Such an age is sufficient for the initially more massive component to deplete hydrogen in its core and to fill the Roche lobe. As a result, mass transfer occurs with a reversal of the mass ratio. After a fast mass exchange the present primaries of W UMa-type binaries land close to ZAMS whereas the secondaries develop small helium cores, which makes them consid…
The blue supergiant Sher25 in the massive Galactic cluster NGC3603 is surrounded by a striking emission line nebula. The nebula contains an equatorial ring and probable bi-polar outflows, and is similar in morphology, mass and kinematics to the structure visible around SN1987A. It has been suggested that both nebulae were ejected while Sher25 and the progenitor of SN1987A were in previous red supergiant phases. We present optical high-resolution spectra of Sher25 and a model photosphere and unified stellar wind analysis which determines atmospheric parameters, mass-loss rate and photospheric a…
Be stars are a class of rapidly rotating B stars with circumstellar disks that cause Balmer and other line emission. There are three possible reasons for the rapid rotation of Be stars: they may have been born as rapid rotators, spun up by binary mass transfer, or spun up during the main-sequence (MS) evolution of B stars. To test the various formation scenarios, we have conducted a photometric survey of 55 open clusters in the southern sky. Of these, five clusters are probably not physically associated groups and our results for two other clusters are not reliable, but we identify 52 definite…
[abridged] We present high resolution optical spectra of stars in Taurus-Auriga whose circumstellar environment suggests that they are less evolved than optically revealed T Tauri stars. Many of the stars are seen only via scattered light. These spectra are used to search for differences between stars which power Herbig-Haro flows and stars which do not, and to reassess the evolutionary state of so-called protostars (Class I stars) relative to optically revealed T Tauri stars (Class II stars). The stellar mass distribution of Class I stars is similar to that of Class II stars and includes 3 Cl…
Classical Novae (CNe) are the brightest manifestation of mass transfer onto a white dwarf in a cataclysmic variable (CV). As such, they are probes of the mass transfer rate, Mdot, and WD mass, Mwd, in these interacting binaries. Our calculations of the dependence of the CN ignition mass, Mign, on Mdot and Mwd yields the recurrence times of these explosions. We show that the observed CNe orbital period distribution is consistent with the interrupted magnetic braking evolutionary scenario, where at orbital periods Porb > 3 hr mass transfer is driven by angular momentum loss via a wind from the c…
We present an analysis of the submillimetre/X-ray properties of 19 X-ray absorbed, Compton-thin quasi-stellar objects (QSOs) selected to have luminosities and redshifts which represent the peak of cosmic QSO activity. i.e. ~L* objects at 1<z<3. Of these, we present new data for 11 objects not previously observed at submillimetre wavelengths and additional data for a further 3. The detection rate is 42 per cent, much higher than typically reported for samples of QSOs. Detection statistics show (at the 3-4 sigma level) that this sample of absorbed QSOs has a higher submillimetre output than a ma…
Splaver and coworkers have measured the masses of the white dwarf and the neutron star components of the PSR J1713+0747 binary system pair by Shapiro Delay. We attempt to find the original configuration of this system performing a set of binary evolution calculations to simultaneously account for the masses of both stars and the orbital period. We considered initial masses of 1.5 and 1.4 \msun for the normal (donor) and the neutron star, respectively. We assumed two metallicity values (Z = 0.010 and 0.020), and an initial orbital period near 3 days. We assume that the neutron star is only able…
The chemical compositions of 26 metal-poor stars that exhibit strong CH and/or C2 molecular bands are determined based on high-resolution spectroscopy. We define carbon-enhanced stars taking account of the carbon abundance ratio ([C/Fe]) and the evolutionary status. Twenty two stars in our sample satisfy our modified definition for Carbon-Enhanced Metal-Poor (CEMP) stars. In addition, we measure Na abundances for nine other carbon-enhanced stars for which abundances of other elements have been previously reported. Combining our new sample with the results of previous work, we investigate the a…
The results of UBV and H alpha imaging of a large sample of isolated dwarf irregular galaxies are interpreted in the context of composite stellar population models. The observed optical colors are best fit by composite stellar populations which have had approximately constant star formation rates for at least 10 Gyr. The galaxies span a range of central surface brightness, from 20.5 to 25.0 mag arcsec^{-2}; there is no correlation between surface brightness and star formation history. Although the current star formation rates are low, it is possible to reproduce the observed luminosities witho…
We recall the current status of the long-standing 3He problem, and its possible connection with chemical anomalies on the red giant branch. In this context, we collect in the literature all the available observations of the carbon isotopic ratio in field and cluster giant stars. Using the HIPPARCOS parallaxes, we get constraints on the evolutionary status of the field stars of the sample. This allows us to identify the stars that have passed the luminosity function bump and present 12C/13C ratios in disagreement with the standard predictions of stellar evolutionary models. We determine statist…
We present new results of a study of the sub-millimetre continuum emission from a sample of 9 radio galaxies and 4 radio-quiet quasars at redshifts $z = 0.75 - 4.26$. The observations were made at 800$μm$, using the single-element bolometer UKT14 on the James Clerk Maxwell Telescope (JCMT), reaching a typical r.m.s. sensitivity of $σ_{rms} \sim 4$ mJy and represent some of the deepest submillimetre extragalactic measurements made to date. Three detections were achieved, of which two are secure (4C41.17, Dunlop et al. (1994) and H1413$+$117) and one (53W002) is tentative, whilst comparable uppe…
$\sim$18 yr after the supernova explosion, the blast wave of SNR 1987A is entering the main body of the equatorial circumstellar material, which is causing a dramatic brightening of the remnant. We recently reported the observational evidence for this event from our {\it Chandra} data (Park et al. 2005b; P05 hereafter). We present here the temporal evolution of the X-ray emitting shock parameters and the detailed description of the spectral and image analysis of SNR 1987A, on which P05 was based. While the remnant becomes brighter, the softening of the overall X-ray spectrum continues and is e…
In order to test the prevailing paradigm of horizontal-branch (HB) stellar evolution, we use the large databases of measured RR Lyrae parameters for the globular cluster M3 (NGC 5272) recently provided by Bakos et al. and Corwin & Carney. We compare the observed distribution of fundamentalized periods against the predictions of synthetic HBs. The observed distribution shows a sharp peak at P_f ~ 0.55 d, which is primarily due to the RRab variables, whereas the model predictions instead indicate that the distribution should be more uniform in P_f, with a buildup of variables with shorter period…
We present deep B, R and Halpha imaging of 3 dwarf galaxies: NGC3377A, NGC4286, and IC3475. Based on previous broadband imaging and HI studies, these mixed-morphology galaxies were proposed by Sandage & Hoffman (1991) to be, respectively, a gas-rich low surface brightness Im dwarf, a nucleated dwarf that has lost most of its gas and is in transition from Im to dS0,N, and the prototypical example of a gas-poor ``huge low surface brightness'' early-type galaxy. From the combination of our broadband and Halpha imaging with the published information on the neutral gas content of these three galaxi…
We have undertaken a survey of N2H+ and N2D+ towards 31 low-mass starless cores using the IRAM 30m telescope. Our main objective has been to determine the abundance ratio of N2D+ and N2H+ towards the nuclei of these cores and thus to obtain estimates of the degree of deuterium enrichment, a symptom of advanced chemical evolution according to current models. We find that the N(N2D+)/N(N2H+) ratio is larger in more "centrally concentrated cores" with larger peak H2 and N2H+ column density than the sample mean. The deuterium enrichment in starless cores is presently ascribed to depletion of CO in…
We present the results of a systematic study of the evolution of low- and intermediate-mass X-ray binaries (LMXBs and IMXBs). Using a standard Henyey-type stellar-evolution code and a standard model for binary interactions, we have calculated 100 binary evolution sequences containing a neutron star and a normal-type companion star, where the initial mass of the secondary ranges from 0.6 to 7\Ms and the initial orbital period from $\sim 4 $hr to $\sim 100 $d. This grid of models samples the entire range of parameters one is likely to encounter for LMXBs and IMXBs. The sequences show an enormous…
Repeated spectroscopic observations made with the 6m telescope of yielded new data on the radial-velocity variability of the anomalous yellow supergiant QYSge. The strongest and most peculiar feature in its spectrum is the complex profile of NaI D lines, which contains a narrow and a very wide emission components. The wide emission component can be seen to extend from -170 to +120 km/s, and at its central part it is cut by an absorption feature, which, in turn, is split into two subcomponents by a narrow (16km/s at r=2.5) emission peak. An analysis of all the Vr values leads us to adopt for th…
Combined HST, X-ray, and ground-based optical studies show that clusters of galaxies are largely "in place" by $z \sim 1$, an epoch when the Universe was less than half its present age. High resolution images show that elliptical, S0, and spiral galaxies are present in clusters at redshifts up to $z \sim 1.3$. Analysis of the CMDs suggest that the cluster ellipticals formed their stars several Gyr earlier, near redshift 3. The morphology--density relation is well established at $z\sim1$, with star-forming spirals and irregulars residing mostly in the outer parts of the clusters and E/S0s conce…
We present the results of new rest-frame far-IR observations of the z = 4.25 radio galaxy, 8C1435+635, which not only confirm that it contains an enormous quantity of dust (as first inferred from its mm-wave detection by Ivison 1995) but also allow the first meaningful constraints to be placed on the mass of this dust and associated gas. The new measurements consist of: (i) clear detections of sub-mm continuum emission at 450 and 850um obtained with the new sub-mm bolometer array, SCUBA, on the JCMT, (ii) continuum upper limits at 350, 750 and 175um obtained with SCUBA and the PHT far-IR camer…
We present the results of spectrophotometry and V,R,I, Halpha CCD photometry of the blue compact dwarf (BCD) galaxy SBS 0940+544. Broad-band images taken with the 2.1m KPNO and 1.23m Calar Alto telescopes reveal a compact high-surface-brightness H II region with ongoing star formation, located at the northwestern tip of the elongated low surface brightness (LSB) main body of the BCD. High signal-to-noise 4.5m Multiple Mirror Telescope (MMT) and 10m Keck II telescope long-slit spectroscopy of SBS 0940+544 is used to derive element abundances of the ionized gas in the brightest H II region and t…
We investigate a spectroscopic sample of 48 early-type (E+S0) galaxies in the rich cluster Abell 2390 (z=0.23) and 48 E+S0 galaxies from a previously published survey of Abell 2218 (z=0.18). The A2390 spectra are based on Multi Object Spectroscopy with MOSCA at the Calar Alto 3.5-m, complemented by ground-based and HST F555W and F814W observations. Our investigation spans a broad range in luminosity (-20.5>M_r>-23.0) and a wide FOV of 1.5x1.5 Mpc^2. Since the A2218 and A2390 samples are very similar, we can combine them and analyse a total of 96 E+S0 galaxies at z~0.2. We construct the Faber-J…
The spectra of stars with the B[e] phenomenon are dominated by features that are related to physical conditions of circumstellar material around these objects and are not intrinsic to the stars. Because of this, they form a very heterogeneous group. This group contains objects with different evolutionary stages. Lamers et al. (1998) have suggested a new designation with five sub-groups, which indicate the evolutionary stage. They are: supergiants, pre-main sequence or Herbig Ae/Be, compact planetary nebulae, symbiotic and unclassified. The unclassified group has many objects that need a better…
