ALBERT

Description: Compared to mass transfer in cataclysmic variables, the nature of accretion in symbiotic binaries in which red giants transfer material to white dwarfs (WDs) has been difficult to uncover. The accretion flows in a symbiotic binary are most clearly observable, however, when there is no quasi-steady shell burning on the WD to hide them. RT Cru is the prototype of such non-burning symbiotics, with its hard (δ-type) X-ray emission providing a view of its innermost accretion structures. In the past 20 yr, RT Cru has experienced two similar optical brightening events, separated by ~4000 days and with amplitudes of ΔV ~ 1.5 mag. After Swift became operative, the Burst Alert Telescope (BAT) detector revealed a hard X-ray brightening event almost in coincidence with the second optical peak. Spectral and timing analyses of multi-wavelength observations that we describe here, from NuSTAR, Suzaku, Swift/X-Ray Telescope (XRT) + BAT + UltraViolet Optical Telescope (UVOT) (photometry) and optical photometry and spectroscopy, indicate that accretion proceeds through a disk that reaches down to the WD surface. The scenario in which a massive, magnetic WD accretes from a magnetically truncated accretion disk is not supported. For example, none of our data show the minute-time-scale periodic modulations (with tight upper limits from X-ray data) expected from a spinning, magnetic WD. Moreover, the similarity of the UV and X-ray fluxes, as well as the approximate constancy of the hardness ratio within the BAT band, indicate that the boundary layer of the accretion disk remained optically thin to its own radiation throughout the brightening event, during which the rate of accretion onto the WD increased to 6.7 × 10−9M⊙ yr−1 (d/2 kpc)2. For the first time from a WD symbiotic, the NuSTAR spectrum showed a Compton reflection hump at E 〉 10 keV, due to hard X-rays from the boundary layer reflecting off of the surface of the WD; the reflection amplitude was 0.77 ± 0.21. The best fit spectral model, including reflection, gave a maximum post-shock temperature of kT = 53 ± 4 keV, which implies a WD mass of 1.25 ± 0.02 M⊙. Although the long-term optical variability in RT Cru is reminiscent of dwarf-novae-type outbursts, the hard X-ray behavior does not correspond to that observed in well-known dwarf nova. An alternative explanation for the brightening events could be that they are due to an enhancement of the accretion rate as the WD travels through the red giant wind in a wide orbit, with a period of about ~4000 days. In either case, the constancy of the hard X-ray spectrum while the accretion rate rose suggests that the accretion-rate threshold between a mostly optically thin and thick boundary layer, in this object, may be higher than previously thought.

Description: Aims. In an effort to improve spectroscopic methods of stellar parameters determination, we implemented non-solar-scaled opacities in a simultaneous derivation of fundamental parameters and abundances. We wanted to compare the results with the usual solar-scaled method using a sample of solar-type and evolved stars. Methods. We carried out a high-precision determination of stellar parameters and abundances by applying non-solar-scaled opacities and model atmospheres. Our sample is composed of 20 stars, including main sequence and evolved objects. The stellar parameters were determined by imposing ionization and excitation equilibrium of Fe lines, with an updated version of the FUNDPAR program, together with plane-parallel ATLAS12 model atmospheres and the MOOG code. Opacities for an arbitrary composition and vmicro were calculated through the opacity sampling (OS) method. We used solar-scaled models in the first step, and then continued the process, but scaled to the abundance values found in the previous step (i.e. non-solar-scaled). The process finishes when the stellar parameters of one step are the same as in the previous step, i.e. we use a doubly iterated method. Results. We obtained a small difference in stellar parameters derived with non-solar-scaled opacities compared to classical solar-scaled models. The differences in Teff, log g, and [Fe/H] amount to 26 K, 0.05 dex, and 0.020 dex for the stars in our sample. These differences can be considered the first estimation of the error due to the use of classical solar-scaled opacities to derive stellar parameters with solar-type and evolved stars. We note that some chemical species could also show an individual variation greater than those of the [Fe/H] (up to ~0.03 dex) and varying from one species to another, obtaining a chemical pattern difference between the two methods. This means that condensation temperature Tc trends could also present a variation. We include an example showing that using non-solar-scaled opacities, the solution found with the classical solar-scaled method indeed cannot always verify the excitation and ionization balance conditions required for a model atmosphere. We discuss in the text the significance of the differences obtained when using solar-scaled versus non-solar-scaled methods. Conclusions. We consider that the use of the non-solar-scaled opacities is not mandatory in every statistical study with large samples of stars. However, for those high-precision works whose results depend on the mutual comparison of different chemical species (such as the analysis of condensation temperature Tc trends), we consider its application to be worthwhile. To date, this is probably one of the most precise spectroscopic methods for stellar parameter derivation.

Description: A sudden increase in the rate at which material reaches the most internal part of an accretion disk, i.e., the boundary layer, can change its structure dramatically. We have witnessed such a change for the first time in the symbiotic recurrent nova T CrB. Our analysis of XMM-Newton, Swift Burst Alert Telescope (BAT)/X-Ray Telescope (XRT)/UltraViolet Optical Telescope (UVOT), and the American Association of Variable Stars Observers (AAVSO) V- and B-band data indicates that during an optical brightening event that started in early 2014 (ΔV ≈ 1.5) the following occurred: (i) the hard X-ray emission as seen with BAT almost vanished; (ii) the XRT X-ray flux decreased significantly, while the optical flux remained high; (iii) the UV flux increased by at least a factor of 40 over the quiescent value; and (iv) the X-ray spectrum became much softer and a bright, new blackbody-like component appeared. We suggest that the optical brightening event, which could be a similar event to that observed about 8 years before the most recent thermonuclear outburst in 1946, is due to a disk instability.

Description: Context. The detection of chromospheric activity cycles in solar-analogue and twin stars can be used to place the solar cycle in a wider context. However, relatively few of these stars with activity cycles have been detected. It is well known that the cores of the Ca II H&K lines are modulated by stellar activity. The behaviour of the Balmer and other optical lines with stellar activity is not yet completely understood. Aims. We search for variations in the Ca II H&K, Balmer, and Fe II lines modulated by stellar activity. In particular, we apply a novel strategy to detect possible shape variations in the Hα line. Methods. We analysed activity signatures in HD 38858 using HARPS and CASLEO spectra obtained between 2003 and 2017. We calculated the Mount Wilson index (SMW), log(R′HK), and the statistical moments of the Ca II H&K, Balmer, and other optical lines. We searched for periodicities using the generalized Lomb-Scargle periodogram. Results. We detect a long-term activity cycle of 10.8 yr in Ca II H&K and Hα in the solar-analogue star HD 38858. In contrast, this cycle is marginally detected in the Fe II lines. We also detect a noticeable variation in radial velocity that seems to be produced by stellar activity. Conclusions. HD 38858 is the second solar-analogue star where we find a clear activity cycle that is replicated in the Balmer lines. Spectral indexes based on the shape of Hα line seem to be more reliable than the fluxes in the same line for detecting activity variations. The cyclic modulation we detected gives place to a variation in radial velocity that previously has been associated with a super-Earth planet. Finally, due to the similarity of HD 38858 with the Sun, we recommend to continue monitoring this star.

Description: Aims. We explore for the first time the probable chemical signature of planet formation in the remarkable binary system HD 106515. Star A hosts a massive long-period planet with ~9 MJup detected by radial velocity, while there is no planet detected at the B star. We also refine stellar and planetary parameters by using non-solar-scaled opacities when modelling the stars. Methods. We carried out a simultaneous determination of stellar parameters and abundances by applying for the first time non-solar-scaled opacities in this binary system, in order to reach the highest possible precision. We used a line-by-line strictly differential approach, using the Sun and then the A star as reference. Stellar parameters were determined by imposing an ionization and excitation balance of Fe lines, with an updated version of the FUNDPAR program, ATLAS12 model atmospheres, and the MOOG code. Opacities for an arbitrary composition were calculated through the opacity sampling method. The chemical patterns were compared with solar-twins condensation temperature Tc trends from the literature and also mutually between both stars. We take the opportunity to compare and discuss the results of the classical solar-scaled method and the high-precision procedure applied here. Results. Stars A and B in the binary system HD 106515 do not seem to be depleted in refractory elements, which is different when comparing the Sun with solar twins. The terrestrial planet formation would have been less efficient in the stars of this binary system. Together with HD 80606/7, this is the second binary system that does not seem to present a (terrestrial) signature of planet formation, when both systems host an eccentric giant planet. This is in agreement with numerical simulations, where the early dynamical evolution of eccentric giant planets clears out most of the possible terrestrial planets in the inner zone. We refined the stellar mass, radius, and age for both stars and found a notable difference of ~78% in R⋆ compared to previous works. We also refined the planet mass to mp sini = 9.08 ± 0.20 MJup, which differs by ~6% compared with the literature. In addition, we showed that the non-solar-scaled solution is not compatible with the classical solar-scaled method, and some abundance differences are comparable to non-local thermodynamic equilibrium (NLTE) or galactic chemical evolution (GCE) effects especially when using the Sun as reference. Therefore, we encourage the use of non-solar-scaled opacities in high-precision studies such as the detection of Tc trends.

Description: Context. There is very little information to be found in the literature regarding the detection of planets orbiting chemically peculiar stars. Aims. Our aim is to determine the detailed chemical composition of the remarkable planet host star KELT-17. This object hosts a hot-Jupiter planet with 1.31 MJup detected by transits, and it is one of the more massive and rapidly rotating planet hosts seen to date. We set out to derive a complete chemical pattern for this star, in order to compare it with those of chemically peculiar stars. Methods. We carried out a detailed abundance determination in the planet host star KELT-17 via spectral synthesis. Stellar parameters were estimated iteratively by fitting Balmer line profiles and imposing the Fe ionization balance using the SYNTHE program together with plane-parallel ATLAS12 model atmospheres. Specific opacities for an arbitrary composition and microturbulence velocity vmicro were calculated through the opacity sampling (OS) method. The abundances were determined iteratively by fitting synthetic spectra to metallic lines of 16 different chemical species using SYNTHE. The complete chemical pattern of KELT-17 was compared to the recently published average pattern of Am stars. We estimated the stellar radius using two methods: a) comparing the synthetic spectral energy distribution with the available photometric data and the Gaia parallax, and b) using a Bayesian estimation of stellar parameters using stellar isochrones. Results. We found over-abundances of Ti, Cr, Mn, Fe, Ni, Zn, Sr, Y, Zr, and Ba, together with subsolar values of Ca and Sc. Notably, the chemical pattern agrees with those recently published for Am stars, making KELT-17 the first exoplanet host whose complete chemical pattern is unambiguously identified with this class. The stellar radius derived by two different methods agrees to each other and with those previously obtained in the literature.

Description: The Southern Photometric Local Universe Survey (S-PLUS) is imaging ∼9300 deg2 of the celestial sphere in 12 optical bands using a dedicated 0.8 m robotic telescope, the T80-South, at the Cerro Tololo Inter-american Observatory, Chile. The telescope is equipped with a 9.2k × 9.2k e2v detector with 10 $ m {mu m}$ pixels, resulting in a field of view of 2 deg2 with a plate scale of 0.55 arcsec pixel−1. The survey consists of four main subfields, which include two non-contiguous fields at high Galactic latitudes (|b| 〉 30°, 8000 deg2) and two areas of the Galactic Disc and Bulge (for an additional 1300 deg2). S-PLUS uses the Javalambre 12-band magnitude system, which includes the 5 ugriz broad-band filters and 7 narrow-band filters centred on prominent stellar spectral features: the Balmer jump/[OII], Ca H + K, H δ, G band, Mg b triplet, H α, and the Ca triplet. S-PLUS delivers accurate photometric redshifts (δz/(1 + z) = 0.02 or better) for galaxies with r 〈 19.7 AB mag and z 〈 0.4, thus producing a 3D map of the local Universe over a volume of more than $1, (mathrm{Gpc}/h)^3$. The final S-PLUS catalogue will also enable the study of star formation and stellar populations in and around the Milky Way and nearby galaxies, as well as searches for quasars, variable sources, and low-metallicity stars. In this paper we introduce the main characteristics of the survey, illustrated with science verification data highlighting the unique capabilities of S-PLUS. We also present the first public data release of ∼336 deg2 of the Stripe 82 area, in 12 bands, to a limiting magnitude of r = 21, available at datalab.noao.edu/splus.