Properties of central stars of planetary nebulae with distances in Gaia DR2

Abstract

Context. We have compiled a catalogue of central stars of planetary nebulae (CSPN) with reliable distances and positions obtained from Gaia Data Release 2 (DR2) astrometry. Distances derived from parallaxes allow us to analyse the galactic distribution and estimate other parameters such as sizes, kinematical ages, bolometric magnitudes, and luminosities. Aims. Our objective is to analyse the information regarding distances together with other available literature data about photometric properties, nebular kinematics, and stellar effective temperatures to throw new light on this rapid and rather unknown evolutionary phase. We seek to understand how Gaia distances compare with other indirect methods commonly used and, in particular, with those derived from non-local thermodynamic equilibrium (non-LTE) models; how many planetary nebulae (PNe) populate the Galaxy; and how are they spatially distributed. We also aim to comprehend their intrinsic luminosities, range of physical sizes of the nebulae; how to derive the values for their kinematical ages; and whether those ages are compatible with those derived from evolutionary models. Methods. We considered all PNe listed in catalogues from different authors and in Hong Kong/AAO/Strasbourg/H α (HASH) database. By X-matching their positions with Gaia DR2 astrometry we were able to identify 1571 objects in Gaia second archive, for which we assumed distances calculated upon a Bayesian statistical approach. From those objects, we selected a sample of PNe with good quality parallax measurements and distance derivations, we which refer to as our Golden Astrometry PNe sample (GAPN), and obtained literature values of their apparent sizes, radial and expansion velocities, visual magnitudes, interstellar reddening, and effective temperatures. Results. We found that the distances derived from DR2 parallaxes compare well with previous astrometric derivations of the United States Naval Observatory and Hubble Space Telescope, but that distances inferred from non-LTE model fitting are overestimated and need to be carefully reviewed. From literature apparent sizes, we calculated the physical radii for a subsample of nebulae that we used to derive the so-called kinematical ages, taking into account literature expansion velocities. Luminosities calculated with DR2 distances were combined with literature central stars T e f f values in a Hertzsprung-Russell (HR) diagram to infer information on the evolutionary status of the nebulae. We compared their positions with updated evolutionary tracks finding a rather consistent picture. Stars with the smallest associated nebular radii are located in the flat luminosity region of the HR diagram, while those with the largest radii correspond to objects in a later stage, getting dimmer on their way to become a white dwarf. Finally, we commented on the completeness of our catalogue and calculated an approximate value for the total number of PNe in the Galaxy.

Gaia DR2 Distances to Planetary Nebulae

“…If we compare our distance estimations results with those derived by other authors (see [1] for details), we appreciate a significant agreement with those obtained from astrometric methods (USNO and HST). On the other hand, we found that distances obtained from non-LTE model fitting are in most cases overestimated and would need to be carefully reviewed.…”

“…Considering physical radii and observational expansion velocities taken from the literature, we derived the so-called kinematical ages of the nebulae. Although most of the PNe are rather young, with ages under 15,000 yr, we also found nebulae spanning ages well beyond those values (see [1] for details). From the average kinematical age value and the mean physical radius of the sample, we obtained a value for the visibility time (i.e., the amount of time elapsed since the PN was created) of the PNe population, < T VT >, similar to that derived by Jacob et al [5].…”

“…The procedure followed for identifying central stars with astrometry in DR2 is carefully explained in [1].…”

“…We excluded from this analysis those objects displaying a clear non-spherical morphology, those known to have a binary central star (CS) and those with H-deficient spectra. Then, based mostly on the literature values of expansion velocities from Frew [4] we obtained self-consistent values for a subsample of 45 PNe (see section 5 in [1]). Furthermore, following the prescription by Jacob et al [5], we applied an overall correction factor for the expansion velocities of 1.5 to take into account expected observational limitations.…”

“…As explained in [1], we retrieved visible magnitudes and extinction and temperature values from the literature, and together with distance estimations we calculated their absolute visible magnitudes. Then, using the calibration by Vacca et al [13], we were able to obtain the absolute bolometric magnitude:…”