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I report the discovery that the 9th-magnitude Galactic-halo star BD+14$^circ$3061 is a member of the rare class of luminous metal-poor yellow post-AGB stars. Its Gaia DR2 parallax implies an absolute magnitude of $M_V=-3.44pm0.27$, and it is a very high-velocity star moving in a retrograde Galactic orbit. BD+14$^circ$3061 is a field analog of the half-dozen yellow PAGB stars known in Galactic globular clusters, which have closely similar absolute magnitudes. These objects are the visually brightest members of old stellar populations; their apparently narrow luminosity function makes them potentially useful as Population II standard candles. The spectral-energy distribution of BD+14$^circ$3061 out to $22,mu$m shows no evidence for circumstellar dust. The star is a low-amplitude semi-regular pulsating variable, with typical periods of 30-32 days. A radial-velocity study suggests that it is a spectroscopic binary with a period of 429.6 days, making it similar to known binary yellow PAGB stars such as HD 46703 and BD+39$^circ$4926.
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We report discovery of a luminous F-type post-asymptotic-giant-branch (PAGB) star in the Galactic globular cluster (GC) M79 (NGC 1904). At visual apparent and absolute magnitudes of V=12.20 and Mv=-3.46, this yellow PAGB star is by a small margin the visually brightest star known in any GC. It was identified using CCD observations in the uBVI photometric system, which is optimized to detect stars with large Balmer discontinuities, indicative of very low surface gravities. Follow-up observations with the SMARTS 1.3- and 1.5-m telescopes show that the star is not variable in light or radial velocity, and that its velocity is consistent with cluster membership. Near- and mid-infrared observations with 2MASS and WISE show no evidence for circumstellar dust. We argue that a sharp upper limit to the luminosity function exists for yellow PAGB stars in old populations, making them excellent candidates for Population II standard candles, which are four magnitudes brighter than RR Lyrae variables. Their luminosities are consistent with the stars being in a PAGB evolutionary phase, with core masses of ~0.53 Msun. We also detected four very hot stars lying above the horizontal branch (AGB-manque stars); along with the PAGB star, they are the brightest objects in M79 in the near ultraviolet. In an Appendix, we give periods and light curves for five variables in M79: three RR Lyrae stars, a Type II Cepheid, and a semiregular variable.
We aim at describing and understanding binary interaction processes in systems with very evolved companions. Here, we focus on understanding the origin and determining the properties of the high-velocity outflow observed in one such system. We present a quantitative analysis of BD+46$^{circ}$442, a post-AGB binary which shows active mass transfer that leads to the creation of a disk-driven outflow or jet. We obtained high-resolution optical spectra from the HERMES spectrograph, mounted on the 1.2m Flemish Mercator Telescope. By performing a time-series analysis of the Halpha profile, we dissected the different components of the system. We deduced the jet geometry by comparing the orbital phased data with our jet model. In order to image the accretion disk around the companion of BD+46$^{circ}$442, we applied the technique of Doppler tomography. The orbital phase-dependent variations in the Halpha profile can be related to an accretion disk around the companion, from which a high-velocity outflow or jet is launched. Our model shows that there is a clear correlation between the inclination angle and the jet opening angle. The latitudinally dependent velocity structure of our jet model shows a good correspondence to the data, with outflow velocities at least higher than 400km/s. We show that BD+46$^{circ}$442, is a result of a binary interaction channel. The origin of the fast outflow in this system can be attributed to a gaseous disk around the secondary component, which is most likely a main sequence star. Our analysis suggests the outflow to have a rather wide opening angle instead of being strongly collimated. Similar orbital phase-dependent Halpha profiles are commonly observed in post-AGB binaries. Post-AGB binaries provide ideal test bets to study jet formation and launching mechanisms over a wide range of orbital conditions.
We report the discovery of a luminous yellow post-asymptotic-giant-branch (PAGB) star in the globular cluster (GC) M19 (NGC 6273), identified during our uBVI survey of Galactic GCs. The uBVI photometric system is optimized to detect stars with large Balmer discontinuities, indicating very low surface gravities and high luminosities. The spectral-energy distribution (SED) of the star is consistent with an effective temperature of about 6250 K and a surface gravity of $log g=0.5$. We use Gaia data to show that the stars proper motion and radial velocity are consistent with cluster membership. One aim of our program is to test yellow PAGB stars as candidate Population II standard candles for determining extragalactic distances. We derive a visual absolute magnitude of $M_V=-3.39pm0.09$ for the M19 star. This is in close agreement with the $M_V$ values found for yellow PAGB stars in the GCs omega Cen, NGC 5986, and M79, indicating a very narrow luminosity function. These objects are four magnitudes brighter than RR Lyrae variables, and they can largely avoid the issues of interstellar extinction that are a problem for Population I distance indicators. We also identified a second luminous PAGB object in M19, this one a hotter UV-bright star. Its SED is consistent with an effective temperature of about 11,750 K and $log g=2.0$. The two objects have nearly identical bolometric luminosities, $log L/L_odot=3.24$ and 3.22, respectively.
Post-asymptotic giant branch (post-AGB) stars are known to be chemically diverse. In this paper we present the first observational evidence of a star that has failed the third dredge-up (TDU). J005252.87-722842.9 is a A-type ($T_{rm eff}$ = 8250 $pm$ 250K) luminous (8200 $pm$ 700 $rm L_{odot}$), metal-poor ($textrm{[Fe/H]}$ = $- 1.18 pm$ 0.10), low-mass (M$_{rm initial}$ $approx$ 1.5 $-$ 2.0 $rm M_{odot}$) post-AGB star in the Small Magellanic Cloud. Through a systematic abundance study, using high-resolution optical spectra from UVES, we found that this likely post-AGB object shows an intriguing photospheric composition with no confirmed carbon-enhancement (upper limit of [C/Fe] $<$ 0.50) nor enrichment of $s$-process elements. We derived an oxygen abundance of [O/Fe] = 0.29 $pm$ 0.1. For Fe and O, we took into account the effects of non-local thermodynamic equilibrium (NLTE). We could not derive an upper limit for the nitrogen abundance as there are no useful nitrogen lines within our spectral coverage. The chemical pattern displayed by this object has not been observed in single or binary post-AGBs. Based on its derived stellar parameters and inferred evolutionary state, single star nucleosynthesis models predict that this star should have undergone TDU episodes while on the AGB and be carbon-enriched. However, our observations are in contrast with these predictions. We identify two possible Galactic analogues which are likely to be post-AGB stars, but the lack of accurate distances (hence luminosities) to these objects does not allow us to confirm their post-AGB status. If they have low luminosities then they are likely to be dusty post-RGB stars. The discovery of J005252.87-722842.9 reveals a new stellar evolutionary channel whereby a star evolves without any third dredge-up episodes.
This paper is part of a larger project in which we systematically study the chemical abundances of Galactic and extragalactic post-asymptotic giant branch (post-AGB) stars. Lead (Pb) is the final product of the s-process nucleosynthesis and is predicted to have large overabundances with respect to other s-process elements in AGB stars of low metallicities. However, Pb abundance studies of enriched post-AGB stars in the Magellanic Clouds show the Pb overabundance is not observed. We used high-resolution UVES and HERMES spectra for detailed spectral abundance studies of our sample of 14 Galactic post-AGB stars. We do not find any clear evidence of Pb overabundances in our sample. Stars with T(eff) > 7500 K do not provide strong constraints on the Pb abundance. We conclude that the discrepancy between theory and observation increases towards lower metallicities. All objects, except IRAS 17279-1119, confirm the relation between neutron exposure [hs/ls] and third dredge-up efficiency [s/Fe], whereas no relation between metallicity and neutron exposure is detected within the metallicity range of our total sample (-1.4 < [Fe/H] < -0.2). The mild enrichment of IRAS 17279-1119 can probably be attributed to a cut-off of the AGB evolution due to binary interactions. To our knowledge, IRAS 17279-1119 is the first s-process enhanced Galactic post-AGB star known in a binary system and is a possible precursor of the extrinsic Ba dwarf stars. Lead-rich stars have yet to be found in post-AGB stars. extrinsic Ba dwarf stars. Lead-rich stars are yet to be found in post-AGB stars.
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