arXiv subjects

U. Heber

Publications and source records attributed to U. Heber.

198 records · Page 11Linked to original sources

arXiv2001

Early type stars at high galactic latitudes II. Four evolved B-type stars of unusual chemical composition

We present the result of differential spectral analyses of a further four apparently normal B-type stars. Abundance anomalies (e.g. He, C, N enrichment), slow rotation and/or high gravities suggest that the programme stars are evolved low-mass B-type stars. In order to trace their evolutionary status several scenarios are discussed. Post-AGB evolution can be ruled out. PG 0229+064 and PG 1400+389 could be horizontal branch (HB) stars, while HD 76431 and SB 939 have already evolved away from the extreme HB (EHB). The low helium abundance of HD 76431 is consistent with post-EHB evolution. The enrichment in helium, carbon and nitrogen of the remaining stars can be explained either by deep mixing of nuclearly processed material to the surface or by diffusion processes modified by magnetic fields and/or stellar winds. A kinematic study of their galactic orbits indicates that the stars belong to an old disk population.

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arXiv2001

Early type stars at high galactic latitudes I. Ten young massive B-type stars

We present the results of quantitative spectral analyses of ten apparently normal B-type stars. These stars were found to be young massive B-type stars at distances of z=2.6 to 7.6 kpc from the galactic plane based on their positions in the (Teff, log g) diagram, normal abundance patterns and/or large projected rotational velocities. We discuss formation scenarios (runaway star scenarios versus a scenario for star formation in the halo) by comparing times-of-flight and evolutionary time scales. For all stars (except SB 357 and HS 1914+7139) both the scales are similar, indicating that the stars could have formed in the galactic disk and been ejected from there soon aftertheir birth. Derived ejection velocities range from 130 km/s to 440 km/s and may be used to constrain models for ejection mechanisms. Using new proper motion measurements we show that PHL 346, which was considered the most likely candidate for a young B-type star born in the halo, can be explained as a runaway star from the galactic plane.

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arXiv2001

Binaries discovered by the SPY project I. HE 1047-0436: a subdwarf B + white dwarf system

In the course of our search for double degenerate binaries as potential progenitors of type Ia supernovae with the UVES spectrograph at the ESO VLT (ESO SN Ia Progenitor surveY - SPY) we discovered that the sdB star HE 1047-0436 is radial velocity variable. The orbital period of 1.213253d, a semi-amplitude of 94km/s, and a minimum mass of the invisible companion of 0.44Msol are derived from the analysis of the radial velocity curve. We use an upper limit on the projected rotational velocity of the sdB star to constrain the system inclination and the companion mass to M>0.71Msol, bringing the total mass of the system closer to the Chandrasekhar limit. However, the system will merge due to loss of angular momentum via gravitational wave radiation only after several Hubble times. Atmospheric parameters and metal abundances are also derived. The resulting values are typical for sdB stars.

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arXiv2001

The binary fraction of extreme horizontal branch stars

(Abbreviated) We have used precise radial velocity measurements of subdwarf-B stars from the Palomar-Green catalogue to look for binary extreme horizontal branch (EHB) stars. We identify 36 EHB stars in our sample and find that at least 21 of these stars are binaries. All but one or two of these are new identifications. The minimum binary fraction for EHB stars implied by our survey is 60+-8%. Our survey is sensitive to binaries with orbital periods P less than about 10d. For reasonable assumptions concerning the period distribution and the mass ratio distribution of the binaries, we find that the mean detection efficiency of our survey over this range of orbital periods is 87%. Allowing for this estimated detection efficiency, the fraction of EHB stars which are short-period binaries ($0.03 < P <10d, approximately) is 69+-9%. The value is not strongly dependent on the period distribution below P=10d or the mean companion mass for these short-period binaries. The orbital separation of the stars in these binaries is much less than the size of the red giant from which the EHB star has formed. This is strong evidence that binary star evolution is fundamental to the formation of the majority of EHB stars. If there are also binary EHB stars whose orbital periods are more than about 10d, the fraction of EHB stars whose evolution has been affected by the presence of a companion may be much higher.

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arXiv2000

The stellar content of the Hamburg/ESO survey. I. Automated selection of DA white dwarfs

We describe automatic procedures for the selection of DA white dwarfs in the Hamburg/ESO objective-prism survey (HES). For this purpose, and the selection of other stellar objects (e.g., metal-poor stars and carbon stars), a flexible, robust algorithm for detection of stellar absorption and emission lines in the digital spectra of the HES was developed. Broad band (U-B, B-V) and narrow band (Strömgren c_1) colours can be derived directly from HES spectra, with precisions of sigma(U-B)=0.092mag; sigma(B-V)=0.095mag; sigma(c_1)=0.15mag. We describe simulation techniques that allow to convert model or slit spectra to HES spectra. These simulated objective-prism spectra are used to determine quantitative selection criteria, and for the study of selection functions. We present an atlas of simulated HES spectra of DA and DB white dwarfs. Our current selection algorithm is tuned to yield maximum efficiency of the candidate sample (minimum contamination with non-DAs). DA candidates are selected in the B-V versus U-B and c_1 versus W_λ(Hbeta+Hgamma+Hdelta) parameter spaces. The contamination of the resulting sample with hot subdwarfs is expected to be as low as ~8%, while there is essentially no contamination with main sequence or horizontal branch stars. We estimate that with the present set of criteria, ~80% of DAs present in the HES database are recovered. A yet higher degree of internal completeness could be reached at the expense of higher contamination. However, the external completeness is limited by additional losses caused by proper motion effects and the epoch differences between direct and spectral plates used in the HES.

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arXiv2000

Spectral analysis of multi mode pulsating sdB stars II. Feige 48, KPD 2109+4401 and PG 1219+534

Three members of the new class of pulsating sdB stars (sdBV or EC 14026 stars) are analysed from Keck HIRES spectra using line blanketed NLTE and LTE model atmospheres. Atmospheric parameters (Teff, log g, He/H), metal abundances and rotational velocities are determined. As is typical for sdB stars, all programme stars are found to be helium deficient, with a He abundance ranging from 1/80 solar for Feige 48 to 1/3 solar for PG 1219+534, probably due to diffusion. Most metals are also depleted. The abundances of C, O, Ne, Mg, Al and Si in the high gravity programme stars KPD 2109+4401 and PG 1219+534 are considerably lower than in the lower gravity stars Feige 48 and PG 1605+072 which could be explained by an equilibrium between gravitational settling and radiative levitation. Surprisingly iron is solar to within error limits in all programme stars irrespective of their gravity, confirming predictions from diffusion calculations. The metal lines are very sharp and allow the microturbulent velocity to be constrained to be lower than 5km/s (KPD2109+4401, PG 1219+534). Also the projected rotational velocities have to be very low (vrot sini<10km/s). For Feige 48 the limits are even tighter (vmicro<=3km/s, vrot sini<=5km/s).

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arXiv2000

Hot HB stars in globular clusters - Physical parameters and consequences for theory. V. Radiative levitation versus helium mixing

Atmospheric parameters (effective temperature Teff, surface gravity log g), masses and helium abundances are derived for 42 hot horizontal branch (HB) stars in the globular cluster NGC 6752. For 19 stars we derive magnesium and iron abundances as well and find that iron is enriched by a factor of 50 on average with respect to the cluster abundance whereas the magnesium abundances are consistent with the cluster abundance. Radiation pressure may levitate heavy elements like iron to the surface of the star in a diffusive process. Taking into account the enrichment of heavy elements in our spectroscopic analyses we find that high iron abundances can explain part, but not all, of the problem of anomalously low gravities along the blue HB. The blue HB stars cooler than about 15,100 K and the sdB stars (Teff > 20,000 K) agree well with canonical theory when analysed with metal-rich ([M/H] = +0.5) model atmospheres, but the stars in between these two groups remain offset towards lower gravities and masses. Deep Mixing in the red giant progenitor phase is discussed as another mechanism that may influence the position of the blue HB stars in the (Teff, log g)-plane but not their masses.

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arXiv2000

The Schweizer-Middleditch star revisited

We have re-observed and re-analysed the optical spectrum of the Schweizer-Middleditch star, a hot subdwarf which lies along almost the same line-of-sight as the centre of the historic SN1006 supernova remnant (SNR). Although this object is itself unlikely to be the remnant of the star which exploded in 1006AD, Wellstein et al. (1999) have demonstrated that it could be the remnant of the donor star in a pre-supernova Type Ia interacting binary, if it possesses an unusually low mass. We show that, if it had a mass of 0.1-0.2 Msun, the SM star would lie at the same distance (~800pc) as the SNR as estimated by Willingale et al. (1995). However, most distance estimates to SN1006 are much larger than this, and there are other convincing arguments to suggest that the SM star lies behind this SNR. Assuming the canonical subdwarf mass of 0.5 Msun, we constrain the distance of the SM star as 1050 pc<d<2100 pc. This places the upper limit on the distance of SN1006 at 2.1 kpc.

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arXiv2000

First VLT spectra of white dwarfs in a globular cluster

We present the first spectra obtained with the Very Large Telescope for white dwarfs in a globular cluster. Estimates of atmospheric parameters are obtained and compared to evolutionary tracks. We discuss possible implications for the distance scale of globular clusters and white dwarf evolution and demonstrate how white dwarfs might be used to establish an independent distance scale to globular clusters.

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arXiv1999

Spectral analysis of four multi mode pulsating sdB stars

Four members of the new class of pulsating sdB stars are analysed from Keck HIRES spectra using NLTE and LTE model atmospheres. Atmospheric parameters (Teff, log g, log(He/H)), metal abundances and rotational velocities are determined. Balmer line fitting is found to be consistent with the helium ionization equilibrium for PG1605+072 but not so for PG1219+534 indicating that systematic errors in the model atmosphere analysis of the latter have been underestimated previously. All stars are found to be helium deficient probably due to diffusion. The metals are also depleted with the notable exception of iron which is solar to within error limits in all four stars, confirming predictions from diffusion calculations of Charpinet et al. (1997). While three of them are slow rotator's (vsini < 10km/s), PG1605+072 displays considerable rotation (vsini = 39km/s, P<8.7h) and is predicted to evolve into an unusually fast rotating white dwarf. This nicely confirms a prediction by Kawaler (1999) who deduced a rotation velocity of 130km/s from the power spectrum of the pulsations which implies a low inclination angle of the rotation axis.

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arXiv1999

ORFEUS II echelle spectra: deuterium and molecular hydrogen in the ISM towards BD +39 3226

In ORFEUS II spectra of the sdO star BD +39 3226 interstellar hydrogen and deuterium is detected. From Ly alpha profile fitting and a curve of growth analysis of the Lyman series of H I and D I we derive the column densities N(H)=1.20(+0.28/-0.22)*10^20 cm^(-2) and N(D)=1.45(+0.50/-0.38)*10^(15) cm^(-2). From the analysis of metal absorption lines in ORFEUS and IUE spectra we obtain column densities for 11 elements. In addition, we examine absorption lines of H_2 for rotational excitation states up to J=7. We find an H_2 ortho-to-para ratio of 2.5, the fractional abundance of molecular hydrogen has a low value of log f=-4.08 for a total amount of N(H_2)=4.8(+2.0/-1.6)*10^15 cm^(-2). The column densities of the excitation states reveal a moderate Boltzmann excitation temperature of 130 K and an equivalent excitation temperature for the excited upper states due to UV pumping of <1800 K.

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arXiv1999

Spectral analysis of the multi mode pulsating subluminous B star PG1605+072

PG 1605+072 has unique pulsational properties amongst the members of the new class of pulsating sdB (EC 14026) stars. It has the longest periods and the richest, most puzzling frequency spectrum (55 periods). We present a quantitative analysis of a Keck HIRES spectrum using NLTE and LTE model atmospheres. Atmospheric parameters (Teff, log g, log(He/H)), metal abundances, and the rotational velocity are determined. He, C, N and O are subsolar as well as the intermediate mass elements Mg and Si, whereas Ne and Fe are solar. This abundance pattern is caused by diffusion. PG 1605+072 displays considerable line broadening probably caused by rotation (vsini = 39km/s, P<8.7h) and, therefore, is predicted to evolve into an unusually fast rotating white dwarf. Unequal rotational splitting may explain its puzzling pulsation pattern. The solar Fe abundance and the rapid rotation nicely confirms recent predictions of diffusion/pulsation models.

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arXiv1999

Physical Parameters of Hot Horizontal-Branch Stars in NGC 6752: Deep Mixing and Radiative Levitation

Atmospheric parameters (Teff, log g and log(He/H)) are derived for 42 hot horizontal branch (HB) stars in the globular cluster NGC 6752. For 19 stars MgII and FeII lines are detected indicating an iron enrichment by a factor 50 on average with respect to the cluster abundance whereas the magnesium abundances are consistent with the cluster metallicity. This finding adds to the growing evidence that radiative levitation plays a significant role in determining the physical parameters of blue HB stars. Indeed, we find that iron enrichment can explain part, but not all, of the problem of anomalously low gravities along the blue HB. Thus the physical parameters of horizontal branch stars hotter than about 11,500K in NGC 6752, as derived in this paper, are best explained by a combination of helium mixing and radiative levitation effects.

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arXiv1999

BUSCA: A Telescope Instrumentation for Simultaneous Imaging in 4 Optical Bands

The "Bonn University Simultaneous CAmera" (BUSCA) is a CCD camera system which allows simultaneous direct imaging of the same sky area in four colors. The optics are designed for an f/8 beam and four 4Kx4K CCDs with 15m pixels covering a field of view of 12 arcmin x 12 arcmin at a 2m class telescope. In September 1998 BUSCA has seen "First Light". The instrument is based on three dichroic beam splitters which separate optical wavelength bands such (at 430nm, 540nm, 730nm) that standard astronomical intermediate-band filter systems can be used. The dichroics are made of plane-parallel glass plates mounted at an angle of 45 degrees. Astigmatism in the transmitted beams (f/8) is completely cancelled by identical plane-parallel glass plates of suitable orientation. BUSCA offers new perspectives in astronomical multicolor photometry: i) The broadband spectral properties (e.g. color indices) of astronomical objects in the optical can be determined with high reliability even in non-photometric atmospheric conditions. ii) Precious observing time is used very efficiently. iii) With the large field of view, extended objects like globular and open star clusters or galaxies are covered in a single exposure. iv) Each exposure results in a complete data set.

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arXiv1998

Abundances of UV bright stars in globular clusters. I. ROA 5701 in omega Centauri and Barnard 29 in M 13

Two UV brights stars in globular clusters, ROA 5701 (omega Cen) and Barnard 29 (M 13) are analysed from high-resolution UV and optical spectra. The main aim is the measurement of iron abundances from UV spectra obtained with the HST-GHRS. In addition atmospheric parameters and abundances for He, C, N, O, and Si are derived from optical spectra (ESO CASPEC) for ROA 5701 or taken from literature for Barnard 29. Both stars are found to be post-asymptotic giant branch stars. Surprisingly, their iron abundances lie significantly below the cluster abundance in both cases. Barnard 29 lies 0.5 dex below the iron abundance derived for giant stars in M 13 and the iron abundance of ROA 5701 is the lowest of any star in omega Cen analysed so far. Barnard 29 shows the same abundance pattern as the red giant stars in M 13, except for its stronger iron deficiency. The iron depletion could be explained by gas-dust separation in the AGB progenitor's atmosphere, if iron condensed into dust grains which were then removed from the atmosphere by a radiatively driven wind. The interpretation of the abundance pattern for ROA 5701 is hampered by the star-to-star abundance variations seen in omega Cen, but its abundance pattern appears to be consistent with the gas-dust separation scenario.

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arXiv1998

The UV bright star ZNG 1 in M 5

We report the results of H_alpha imaging and UV spectroscopy with the Hubble Space Telescope of the hot post-AGB star ZNG 1 in M 5 and its suspected planetary nebula.

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arXiv1998

PG1323-086 and PG1704+222 - two post-AGB stars at high galactic latitudes

Two high galactic latitude B-type stars, PG1323-086 and PG1704+222, are analysed from low and medium resolution optical spectra and Stroemgren photometry. A differential abundance analysis for He, C, N, O, Mg, Al, and Si reveals that the He abundance is close to solar while metal underabundances relative to the solar value of typically 1.3 and 1.2 dex for PG1323-086 and PG1704+222, respectively, are found. For both stars, carbon is even more depleted. The atmospheric parameters are consistent with evolutionary tracks for stars evolving from the asymptotic giant branch (AGB) with a stellar mass of 0.55 solar masses. The anomalous compositions are compatible with those of other high galactic latitude post-AGB stars. Hence, PG1323-086 and PG1704+222 are low mass post-AGB stars in an evolutionary stage between those of A- and F-type supergiants of low mass and central stars of planetary nebulae. From kinematic data and distances a population II membership is probable for both stars.

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arXiv1997

Masses and Gravities of Blue Horizontal Branch Stars Revisited

Previous spectroscopic analyses of Blue Horizontal Branch (BHB) stars in six globular clusters revealed too low masses in four clusters when compared to canonical evolutionary theory, while the masses of the BHB stars in NGC 6752 and M 5 are found to be consistent with theory. We recalculated BHB star masses using new cluster distances derived by Reid (1997a,b) from HIPPARCOS parallaxes of local subdwarfs by main sequence fitting. The new distances are larger than previous estimates resulting in larger masses for the BHB stars. Since the increase in distance is small for NGC 6752 and M 5, the agreement with predicted masses persists. For M 15 and M 92 the masses now come into good agreement with theoretical predictions, while for NGC 288 and NGC 6397 the mass deficit is reduced but the BHB star masses remain slightly too low. Previous spectroscopic analyses also highlighted the problem of too low gravities for some BHB stars. The gravities and absolute magnitudes of BHB stars are revisited in the light of new evolutionary Horizontal Branch models.

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