arXiv subjects

D. Engels

Publications and source records attributed to D. Engels.

At least 55 records · Page 3Linked to original sources

arXiv2009

The transition from AGB to post-AGB evolution as observed by AKARI and Spitzer

The AKARI and Spitzer satellites provided an unique opportunity to observe a variety of stars, which are considered as departing from the Asymptotic Giant Branch (AGB) and have started their post-AGB evolution recently. Most of these stars are absent optically and are bright in the mid-IR wavelength range. Spectra of close to 200 objects have been obtained. For all of them the 1-60 micron spectral energy distribution has been constructed using photometric data from various surveys. We report here on the results of Spitzer observations of 88 IRAS selected post-AGB candidates and discuss them in comparison to the results of the AKARI observations of post-AGB candidates reported elsewhere in these proceedings. The dust compositions can be divided broadly in oxygen- and carbon-rich types, but a variety of intermediate types have been found. Among the oxygen-rich stars amorphous dust prevails, but a few sources show emission features from crystalline dust. The spectra from carbon-rich shells may be completely featureless, may show emission features from PAHs or a molecular absorption line from C2H2. We found also sources with a neon emission line at 12.8 micron. More than a third of all sources show a near-infrared excess at < 5 micron and almost all of them show evidence of C-rich dust in their shells. We postulate that the emerging post-AGB wind after the end of AGB evolution contains always carbon-rich dust irrespective of the chemistry of the former AGB star.

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arXiv2008

Water vapour masers in long-period variable stars. I. RX Boo and SV Peg

We wish to understand the reasons for the strong variability of H2O masers in circumstellar shells of late-type stars. We monitored RX Boo and SV Peg in the 22-GHz maser line of water vapour with single-dish telescopes. The monitoring period covered two decades for RX Boo (1987 -- 2007) and 12 years for SV Peg (1990 -- 1995, 2000 -- 2007). In addition, maps were obtained of RX Boo with the Very Large Array over several years. We find that most of the emission in the circumstellar shell of RX Boo is located in an incomplete ring with an inner radius of 91 mas (15 AU). A velocity gradient is found in a NW--SE direction. The maser region can be modelled as a shell with a thickness of 22 AU, which is only partially filled. The gas crossing time is 16.5 years. The ring-like structure and the velocity gradient remained stable for at least 11 years, while the maser line profiles varied strongly. This suggests that the spatial asymmetry is not accidental, so that either the mass loss process or the maser excitation conditions in RX Boo are not spherically symmetric. The strong variability of the maser spectral features is mainly due to incoherent intensity fluctuations of maser emission spots, which have lifetimes of the order of 1 year. We found no correlation between the optical and the maser variability in either star. The variability properties of the SV Peg masers do not differ substantially from those of RX Boo. There were fewer spectral features present, and the range of variations was narrower. The maser was active on the >10-Jy level only 1990 -- 1992 and 2006/2007. At other times the maser was either absent (<1 Jy) or barely detectable.

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arXiv2007

Lifetime of OH masers at the tip of the asymptotic giant branch

Context: A large fraction of otherwise similar asymptotic giant branch stars (AGB) do not show OH maser emission. As shown recently, a restricted lifetime may give a natural explanation as to why only part of any sample emits maser emission at a given epoch. Aims: We wish to probe the lifetime of 1612 MHz OH masers in circumstellar shells of AGB stars. Methods: We reobserved a sample of OH/IR stars discovered more than 28 years ago to determine the number of stars that may have since lost their masers. Results: We redetected all 114 OH masers. The minimum lifetime inferred is 2800 years (1 sigma). This maser lifetime applies to AGB stars with strong mass loss leading to very red infrared colors. The velocities and mean flux density levels have not changed since their discovery. As the minimum lifetime is of the same order as the wind crossing time, strong variations in the mass-loss process affecting the excitation conditions on timescales of ~3000 years or less are unlikely. Keywords: OH masers -- Stars: AGB and post-AGB -- circumstellar matter

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arXiv2007

HS1857+5144: A hot and young pre-cataclysmic variable

We report the discovery of a new white dwarf/M dwarf binary, HS1857+5144, identified in the Hamburg Quasar Survey (HQS). Time-resolved optical spectroscopy and photometry were carried out to determine the properties of this new cataclysmic variable progenitor pre-CV). The light curves of HS1857+5144 display a sinusoidal variation with a period of Porb=383.52 min and peak-to-peak amplitudes of 0.7 mag and 1.1 mag in the B-band and R-band, respectively. The large amplitude of the brightness variation results from a reflection effect on the heated inner hemisphere of the companion star, suggesting a very high temperature of the white dwarf. Our radial velocity study confirms the photometric period as the orbital period of the system. A model atmosphere fit to the spectrum of the white dwarf obtained at minimum light provides limits to its mass and temperature of Mwd=~0.6-1.0 Msun and Twd=~70000-100000 K, respectively. The detection of HeII 4686 absorption classifies the primary star of HS1857+5144 as a DAO white dwarf. Combining the results from our spectroscopy and photometry, we estimate the mass of the companion star and the binary inclination to be Msec=~0.15-0.30 Msun and i=~45-55 deg, respectively. We classify HS1857+5144 as one of the youngest pre-CV known to date. The cooling age of the white dwarf suggests that the present system has just emerged from a common envelope phase ~10^5 yr ago. HS1857+5144 will start mass transfer within or below the 2-3h period gap.

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arXiv2007

SW Sextantis stars: the dominant population of CVs with orbital periods between 3-4 hours

[Abridged] We present time-series optical photometry of five new CVs identified by the Hamburg Quasar Survey. The eclipses observed in HS 0129+2933, HS 0220+0603, and HS 0455+8315 provided very accurate orbital periods of 3.35129827(65), 3.58098501(34), and 3.56937674(26) h, respectively. HS 0805+3822 shows grazing eclipses and has a likely orbital period of 3.2169(2) h. Time-resolved optical spectroscopy of the new CVs (with the exception of HS 0805+3822) is also presented. Radial velocity studies provided an orbital period of 3.55 h for HS 1813+6122, which allowed us to identify the observed photometric signal at 3.39 h as a negative superhump wave. The spectroscopic behaviour clearly identifies these new CVs as new SW Sextantis stars. These new additions increase the number of known SW Sex stars to 35. Almost 40 per cent of the total SW Sex population do not show eclipses, invalidating the requirement of eclipses as a defining characteristic of the class and the models based on a high orbital inclination geometry alone. On the other hand, the predominance of orbital periods in the narrow 3-4.5 h range is becoming more pronounced. In fact, almost half the CVs which populate the 3-4.5 h period interval are definite members of the class. These statistics are confirmed by our results from the Hamburg Quasar Survey CVs. Remarkably, 54 per cent of the Hamburg nova-like variables have been identified as SW Sex stars with orbital periods in the 3-4.5 h range. The observation of this pile-up of systems close to the upper boundary of the period gap is difficult to reconcile with the standard theory of CV evolution, as the SW Sex stars are believed to have the highest mass transfer rates among CVs. Finally, we review the full range of common properties that the SW Sex stars exhibit.

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arXiv2006

Near-IR variability properties of a selected sample of AGB stars

We present the results of a near-infrared monitoring programme of a selected sample of stars, initially suspected to be Mira variables and OH/IR stars, covering more than a decade of observations. The objects monitored cover the typical range of IRAS colours shown by O-rich stars on the Asymptotic Giant Branch and show a surprisingly large diversity of variability properties. 16 objects are confirmed as large-amplitude variables. Periods between 360 and 1800 days and typical amplitudes from 1 to 2 magnitudes could be determined for nine of them. In three light curves we find a systematic decrease of the mean brightness, two light curves show pronounced asymmetry. One source, IRAS 07222-2005, shows infrared colours typical of Mira variables but pulsates with a much longer period (approx. 1200 days) than a normal Mira. Two objects are ither close to (IRAS 03293+6010) or probably in (IRAS 18299-1705) the post-AGB phase. In IRAS 16029-3041 we found a systematic increase of the H-K colour of approximately 1 magnitude, which we interpret as evidence of a recent episode of enhanced mass loss. IRAS 18576+0341, a heavily obscured Luminous Blue Variable was also monitored. The star showed a continued decrease of brightness over a period of 7 years (1995 - 2002).

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arXiv2006

Dwarf novae in the Hamburg Quasar Survey: Rarer than expected

We report the discovery of five new dwarf novae from the Hamburg Quasar Survey (HQS), and discuss the properties of the sample of dwarf novae in the HQS. The orbital periods of the new dwarf novae are ~105.1 min or ~109.9 min, 114.3+-2.7 min, 92.66+-0.17 min, 272.317+-0.001 min, 258.02+-0.56 min for HS0417+7445, HS1016+3412, HS1340+1524, HS1857+7127, and HS2214+2845, respectively. HS1857+7127 is found to be partially eclipsing. In HS2214+2845 the secondary star of spectral type M3+-1 is clearly detected, and we estimate the distance to the system to be d=390+-40 pc. We recorded one superoutburst of the SU UMa system HS0417+7445. HS1016+3412 and HS1340+1524 have rare outbursts, and their subtype is yet undetermined. HS1857+7127 frequently varies in brightness and may be a Z Cam-type dwarf nova. HS2214+2845 is a U Gem-type dwarf nova with a most likely cycle length of 71 d. To date, 14 new dwarf novae have been identified in the HQS. The ratio of short-period (<3 h) to long-period (>3 h) systems of this sample is 1.3, much smaller compared to the ratio of 2.7 found for all known dwarf novae. The HQS dwarf novae display typically infrequent or low-amplitude outburst activity, underlining the strength of spectroscopic selection in identifying new CVs independently of their variability. The spectroscopic properties of short-period CVs in the HQS suggest that most of them are still evolving towards the minimum period. Their total number agrees with the predictions of population models within an order of magnitude. However, the bulk of all CVs is predicted to have evolved past the minimum period, and those systems remain unidentified. Those post-bounce systems may have markedly weaker Hbeta emission lines compared to the average known short-period CVs, and probably undergo no or extremely rare outbursts.

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arXiv2005

An infrared study of galactic OH/IR stars. I. An optical/near-IR atlas of the Arecibo sample

In this paper we present optical and near-infrared finding charts, accurate astrometry (~1") and single-epoch near-infrared photometry for 371 IRAS sources, 96% of those included in the so-called Arecibo sample of OH/IR stars (Eder et al. 1988; Lewis et al. 1990a; Chengalur et al. 1993). The main photometric properties of the stars in the sample are presented and discussed as well as the problems found during the process of identification of the optical/near-infrared counterparts. In addition, we also identify suitable reference stars in each field to be used for differential photometry purposes in the future. We find that 39% of the sources (144 in number) have no optical counterpart, 8 of them being invisible even at near infrared wavelengths. The relative distribution of sources with and without optical counterpart in the IRAS two-colour diagram and their characteristic near infrared colours are interpreted as the consequence of the increasing thickness of their circumstellar shells. Among the objects not detected at near infrared wavelengths four non-variable sources are proposed to be heavily obscured post-AGB stars which have just very recently left the AGB. Eight additional objects with unusually bright and/or blue near-infrared colours are identified as candidate post-AGB stars and/or proto-planetary nebulae.

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arXiv2005

An infrared study of galactic OH/IR stars. II. The `GLMP sample' of red oxygen-rich AGB stars

We present optical and near-infrared finding charts taken from the DSS and 2MASS surveys of 94 IRAS sources selected from the GLMP catalogue (Garc\'ıa-Lario 1992), and accurate astrometry (~0.2") for most of them. Selection criteria were very red IRAS colours representative for OH/IR stars with optically thick circumstellar shells and the presence of variability according to the IRAS variability index (VAR>50). The main photometric properties of the stars in this `GLMP sample' are presented, discussed and compared with the correspondent properties of the `Arecibo sample' of OH/IR stars studied in Jiménez-Esteban et al. (2005a). We find that 37% of the sample (N=34) have no counterpart in the 2MASS, implying extremely high optical depths of their shells. Most of the sources identified in the 2MASS are faint (K>~8) and of very red colour in the near-infrared, as expected. The brightest 2MASS counterpart (K=5.3mag) was found for IRAS 18299--1705. Its blue colour H--K=1.3 suggests that IRAS 8299--1705 is a post-AGB star. A couple of GLMP sources have faint but relatively blue counterparts. They might be misidentifed field stars or stars hich experienced recently a drop of their mass loss rates. The `GLMP sample' in general is made of oxygen-rich AGB stars, which are highly obscured by their circumstellar shells. They belong to the same population as the reddest OH/IR stars in the `Arecibo sample'.

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arXiv2005

On the evolutionary connection between AGB stars and PNe

The `O-rich AGB sequence' is a sequence of colours describing the location of O-rich AGB stars in the IRAS two-colour diagram [12]--[25] vs [25]--[60]. We propose an evolutionary scenario for this sequence in which all stars, independent of their progenitor mass, start the AGB phase in the blue part of the `O-rich AGB sequence' and then evolve toward redder colors, although only the more massive stars would reach the very end of the `O-rich AGB sequence'. The sources located in the blue part of the sequence are mainly Mira variables, whose mean period is increasing with the IRAS colours. Most of them will evolve into O-rich Type II (and III) Planetary Nebulae. Part of the stars located in the red part of the sequence will change their chemical composition from O-rich to C-rich during their evolution in the AGB phase, and might evolve into C-rich Type II Planetary Nebulae. Hot bottom burning may prevent the conversion to carbon stars of the rest of sources located in the red part of the sequence and they will end up as N-rich Type I Planetary Nebulae.

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arXiv2005

Stellar evolution and the 'O-rich AGB sequence'

The `O-rich AGB sequence' is a well defined area occupied by oxygen-rich AGB stars (OH/IR) in the IRAS two-colour diagram [12]-[25] vs [25]-[60]. This sequence is interpreted in terms of evolutionary stage and/or progenitor mass with the aim to link observed classes of AGB stars and Planetary Nebulae (PNe).

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arXiv2005

HS 2134+0400 - new very metal-poor galaxy, a representative of void population?

We present the SAO 6m telescope spectroscopy of a blue compact galaxy (BCG) HS 2134+0400 discovered in frame of the dedicated Hamburg/SAO survey for Low Metallicity BCGs (HSS-LM). Its very low abundance of oxygen (12+log(O/H) = 7.44), as well as other heavy elements (S, N, Ne, Ar), assigns this dwarf galaxy to the group of BCGs with the lowest metal content. There are only eight that low metallicity among several thousand known BCGs in the nearby Universe. The abundance ratios for the heavy elements (S/O, Ne/O, N/O, and Ar/O) are well consistent with the typical values of other very metal-poor BCGs. The global environment of HS 2134+0400 is atypical of the majority of BCGs. The object falls within the Pegasus void, the large volume with the very low density of galaxies with the normal (M_B* = -19.6) or high luminosity. Since we found in voids a dozen more the very metal-poor galaxies, we discuss the hypothesis that such objects can be representative of a substantial fraction of the void dwarf galaxy population.

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arXiv2005

HS0139+0559, HS0229+8016, HS0506+7725 and HS0642+5049: Four new long-period cataclysmic variables

We present time-resolved optical spectroscopy and photometry of four relatively bright (V~14.0-15.5) long-period cataclysmic variables (CVs) discovered in the Hamburg Quasar Survey: HS0139+0559, HS0229+8016, HS0506+7725 and HS0642+5049. Their respective orbital periods, 243.69+-0.49min, 232.550+-0.049min, 212.7+-0.2min and 225.90+-0.23min are determined from radial velocity and photometric variability studies. HS0506+7725 is characterised by strong Balmer and He emission lines, short-period (~10-20min) flickering and weak X-ray emission in the ROSAT All Sky Survey. The detection of a deep low state (~18.5) identifies HS0506+7725 as a member of the VY Scl stars. HS0139+0559, HS0229+8016 and HS0642+5049 display thick-disc like spectra and no or only weak flickering activity. HS0139+0559 and HS0229+8016 exhibit clean quasi-sinusoidal radial velocity varations of their emission lines but no or very little orbital photometric variability. In contrast, we detect no radial velocity variation in HS0642+5049 but a noticeable orbital brightness variation. We identify all three systems either as UX UMa-type novalike variables or as Z Cam-type dwarf novae. Our identification of these four new systems underlines that the currently known sample of CVs is rather incomplete even for bright objects. The four new systems add to the clustering of orbital periods in the 3-4h range found in the sample of HQS selected CVs, and we discuss the large incidence of magnetic CVs and VY Scl/SW Sex stars found in this period range among the known population of CVs.

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arXiv2005

The Hamburg/SAO survey for emission-line galaxies. VI. The sixth list of 126 galaxies

We present the sixth list with results of the Hamburg/SAO Survey for Emission-Line Galaxies. The final list resulted from follow-up spectroscopy conducted with the 4.5m MMT telescope in 1996, and with 2.2m CAHA and 6m SAO telescopes in 2000 to 2003. The data of this snap-shot spectroscopy survey confirmed 134 emission-line objects out of 182 observed candidates and allowed their quantitative spectral classification and redshift determination. We classify 73 emission-line objects as definite or probable blue compact or HII galaxies (BCG), 8 as QSOs, 4 as Seyfert 1 and 2 galaxies. 30 low-excitation objects were classified as definite or probable starburst nuclei (SBN), 3 as dwarf amorphous nuclei starburst galaxies (DANS) and 2 as LINERs. Due to the low signal-to-noise ratio we could not classify 14 ELGs (NON). For another 9 galaxies we did not detect any significant emission lines. For 98 emission-line galaxies, the redshifts and/or line intensities are determined for the first time. For the remaining 28 previously-known ELGs we give either improved data the line intensities or some independent measurements. The detection rate of ELGs is ~70%. This paper completes the classification of strong-lined ELGs found in the zone of the Hamburg/SAO survey. Together with previously known BCG/HII galaxies in this zone, this sample of ~500 objects is the largest to date in a well bound region.

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arXiv2005

HS 0943+1404, a true intermediate polar

We have identified a new intermediate polar, HS 0943+1404, as part of our ongoing search for cataclysmic variables in the Hamburg Quasar Survey. The orbital and white dwarf spin periods determined from time-resolved photometry and spectroscopy are Porb ~= 250 min and Pspin = 69.171 +- 0.001 min, respectively. The combination of a large ratio Pspin/Porb ~= 0.3 and a long orbital period is very unusual compared to the other known intermediate polars. The magnetic moment of the white dwarf is estimated to be mu1 ~ 10^{34} G cm^3, which is in the typical range of polars. Our extensive photometry shows that HS 0943+1404 enters into deep (~ 3 mag) low states, which are also a characteristic feature of polars. We therefore suggest that the system is a true ``intermediate'' polar that will eventually synchronise, that is, a transitional object between intermediate polars and polars. The optical spectrum of HS 0943+1404 also exhibits a number of unusual emission lines, most noticeably NII 5680, which is likely to reflect enhanced nitrogen abundances in the envelope of the secondary.

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arXiv2004

HS2331+3905: The cataclysmic variable that has it all

We report detailed follow-up observations of the cataclysmic variable HS2331+3905, identified as an emission-line object in the Hamburg Quasar Survey. An orbital period of 81.08min is unambiguously determined from the detection of eclipses in the light curves of HS2331+3905. A second photometric period is consistently detected at P~83.38min, ~2.8% longer than the orbital period, which we tentatively relate to the presence of permanent superhumps. High time resolution photometry exhibits short-timescale variability on time scales of 5-6min which we interpret as non-radial white dwarf pulsations, as well as a coherent signal at 1.12min, which is likely to be the white dwarf spin period. A large-amplitude quasi-sinusoidal radial velocity modulation of the Balmer and Helium lines with a period ~3.5h is persistently detected throughout three seasons of time-resolved spectroscopy. However, this spectroscopic period, which is in no way related to the orbital period, is not strictly coherent but drifts in period and/or phase on time scales of a few days. Modeling the far-ultraviolet to infrared spectral energy distribution of HS2331+3905, we determine a white dwarf temperature of Teff=10500K (assuming Mwd=0.6Msun), close to the ZZ Ceti instability strip of single white dwarfs. The spectral model implies a distance of d=90+-15pc, and a low value for the distance is supported by the large proper motion of the system, mu=0.14"/yr. The non-detection of molecular bands and the low J, H, and K fluxes of HS2331+3905 make this object a very likely candidate for a brown-dwarf donor.

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arXiv2004

Detection of the white dwarf and the secondary star in the new SU UMa dwarf nova HS 2219+1824

We report the discovery of a new, non-eclipsing SU UMa-type dwarf nova, HS 2219+1824. Photometry obtained in quiescence (V ~ 17.5) reveals a double-humped light curve from which we derive an orbital period of ~ 86.2 min. Additional photometry obtained during a superoutburst reaching V ~ 12.0 clearly shows superhumps with a period of ~ 89.05 min. The optical spectrum contains double-peaked Balmer and HeI emission lines from the accretion disc as well as broad absorption troughs of Hbeta, Hgamma, and Hdelta from the white dwarf primary star. Modelling of the optical spectrum implies a white dwarf temperature of 13000 K <~ Twd <~ 17000 K, a distance of 180 pc <~ d <~ 230 pc, and suggests that the spectral type of the donor star is later than M5. Phase-resolved spectroscopy obtained during quiescence reveals a narrow Halpha emission line component which has a radial velocity amplitude and phase consistent with an origin on the secondary star, possibly on the irradiated hemisphere facing the white dwarf. This constitutes the first detection of line emission from the secondary star in a quiescent SU UMa star.

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arXiv2004

Time-resolved photometry and spectroscopy of the new deeply-eclipsing SW Sextantis star HS 0728+6738

We present time-resolved optical spectroscopy and photometry, and far-ultraviolet spectroscopy of HS 0728+6738, a cataclysmic variable discovered in the Hamburg Quasar Survey. We show that the system is a new eclipsing member of the SW Sex class of CVs with an orbital period of 3.21 hours. We derive an orbital inclination of ~85 +- 4 degrees from the average eclipse profile, making HS 0728+6738 the highest inclination SW Sex star known. The optical and far-ultraviolet emission lines are not or only weakly occulted during the eclipse, indicating the presence of line-emission sites either far outside the Roche lobe of the primary or, more likely, above the orbital plane of the binary. The photometric light curves exhibit fast variability with a period of \~7 min, which might be related to the spin of the white dwarf.

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