Pluto's tenuous nitrogen atmosphere was first detected by the imprint left on the light curve of a star that was occulted by the planet in 1985 (ref. 1), and studied more extensively during a second occultation event in 1988 (refs 2-6). These events are, however, quite rare and Pluto's atmosphere remains poorly understood, as in particular the planet has not yet been visited by a spacecraft. Here we report data from the first occultations by Pluto since 1988. We find that, during the intervening 14 years, there seems to have been a doubling of the atmospheric pressure, a probable seasonal effect on Pluto.
Context. Planetary nebula distance scales often suffer from model-dependent solutions. Model-independent trigonometric parallaxes have been rare. Space-based trigonometric parallaxes are now available for a larger sample using the second Data Release of Gaia. Aims. We aim to derive a high-quality approach for selection criteria of trigonometric parallaxes for planetary nebulae and discuss possible caveats and restrictions in the use of this Data Release. Methods. A few hundred sources from previous distance scale surveys were manually cross-identified with data from the second Gaia Data Release (DR2) because coordinate-based matching does not work reliably. The data were compared with the results of previous distance scales and to the results of a recent similar study that used the first Data Release Gaia DR1. Results. While the few available previous ground-based trigonometric parallaxes as well as those obtained with the Hubble Space Telescope perfectly match the new data sets, older statistical distance scales, reaching larger distances, do show small systematic differences. When we restrict the comparison to the central stars for which the photometric colors of Gaia show a negligible contamination by the surrounding nebula, the difference is negligible for statistical distances based on radio flux, while those derived from Hα surface brightness still show minor differences. The DR2 study significantly improves the previous recalibration of the statistical distance scales using DR1/TGAS.
Context. To add to the growing collection of well-studied double periodic variables (DPVs) we have carried out the first spectroscopic and photometric analysis of the eclipsing binary DQ Velorum to obtain its main physical stellar and orbital parameters. Aims. Combining spectroscopic and photometric observations that cover several orbital cycles allows us to estimate the stellar properties of the binary components and the orbital parameters. We also searched for circumstellar material around the more massive star. Methods. We separated DQ Velorum composite spectra and measured radial velocities with an iterative method for double spectroscopic binaries. We obtained the radial velocity curves and calculated the spectroscopic mass ratio. We compared our single-lined spectra with a grid of synthetic spectra and estimated the temperature of the stars. We modeled the V-band light curve with a fitting method based on the simplex algorithm, which includes an accretion disc. To constrain the main stellar parameters we fixed the mass ratio and donor temperature to the values obtained by our spectroscopic analysis. Results. We obtain a spectroscopic mass ratio q = 0.31 ± 0.03 together with donor and gainer masses M d = 2.2 ± 0.2 M , M g = 7.3 ± 0.3 M , the radii R d = 8.4 ± 0.2 R , R g = 3.6 ± 0.2 R and temperatures T d = 9400 ± 100 K, T g = 18 500 ± 500 K for the stellar components. We find that DQ Vel is a semi-detached system consisting of a B3V gainer and an A1III donor star plus an extended accretion disc around the gainer. The disc is filling 89% of the gainer Roche lobe with a temperature of 6580 ± 300 K at the outer radius. It has a concave shape that is thicker at its edge (d e = 0.6 ± 0.1 R ) than at its centre (d c = 0.3 ± 0.1 R ). We find a significant sub-orbital frequency of 0.19 d −1 in the residuals of the V-band light curve, which we interpret as a pulsation of an slowly pulsating B-type (SPB) of a gainer star. We also estimate the distance to the binary (d ∼ 3.1 kpc) using the absolute radii, apparent magnitudes, and effective temperatures of the components found in our study.
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