Study of binary central stars of planetary nebulae with GALEX and corollary optical surveys.
Planetary Nebulae (PNe) are the descendants of low- and intermediate-mass stars (0.8--8.0 M☉). The shape of the PN takes place at some point between the asymptotic giant branch (AGB) and the white dwarf (WD) stellar phases. Among the astrophysical phenomena, the PNe shows an impressive variety of complex morphologies. In fact, most of the PNe known are not spherical (more than 80%) and present shapes from slightly elliptical to bipolar (or multipolar), as well as knots and/or collimated outflows at high velocities, reaching 600 km s-1 in the most extreme cases. The aim of this thesis is to explore how binarity affects the evolution of low- and intermediate-mass stars during the PN phase. We started by looking for photometric data of the known central stars of PNe (CSPNe) in different databases (from the ultraviolet to the near-infrared) in order to search for a double spectral energy distribution (SED) composed by a hot-star (the ionizing star) and a cool companion star. We compiled a catalog of PNe by combining the coordinates PNe catalogs found in the literature, and matched them with the Galaxy Evolution Explorer (GALEX) database (GPNcat). The FUV and NUV photometry, included in GALEX, allow us to extract the hottest stellar objects, which are elusive in optical wavelengths due to its Teff and luminosity. We matched the GPNcat with Sloan Digital Sky Survey (SDSS) and the Panoramic Survey and Rapid Response System (PanSTARRS) databases (GPNcatxSDSSDR14xPS1DR2), both providing five magnitudes in the optical range (SDSS ugriz and PS1 grizy). We separated the resolved PNe, with sizes (diameters) at least twice the resolution of GALEX, from unresolved PNe. For resolved PNe, the flux of the CS were isolated by using aperture photometry techniques. The final catalog, GPNcatxSDSSDR14xPS1DR2, contains 326 PNe with both UV and optical photometry of which 222 are resolved. Most of the PNe are not spherical, and current single-star models cannot explain the morphologies we observe. A binary interaction is the preferred channel to form non-spherical PNe. A fundamental step to corroborate this is to take a sample of PNe without discriminating the morphologies and analyze their CS. We search for spatially unresolved optical excess, by combining GALEX UV with optical SDSS and PS1 photometry, in a sample of 23 resolved CSPNe extracted from GPNcatxSDSSDR14xPS1DR2. We selected the PNe to be analyzed according to their colors in the color-color diagram and according with the accuracy of the match with its UV and optical counterparts. We have constructed a program to identify and characterize binary CSPNe based on the Markov-Chain Monte-Carlo (MCMC) method. We have characterized 11 binary CSPNe of which 7 are new binary CSPNe candidates. We constructed a grid of synthetic SED of PNe using the photoionization code CLOUDY, obtaining synthetic magnitudes in GALEX FUV and NUV, and SDSS and PS1 optical bands. We compared our model colors with other theoretical and observed colors for different astrophysical objects to identify the position of PNe in the color space. After identification of PNe in the color space, we added a cool companion to each model SEDs to identify PNe with binary CS. We found that GALEX FUV-NUV versus SDSS r-i and GALEX NUV-SDSS r versus SDSS r-i color-color diagrams are the best ones for cleanly separating PNe and PNe with binary CS from other astrophysical objects. Finally, as a particular case of study, we present an analysis of the binary central star of NGC 2346 based on archival data from the International Ultraviolet Explorer (IUE), and new low- and high-resolution optical spectroscopic observations. By including the stellar and nebular continuum, we reconciled long-time discrepancy UV and optical diagnostics from the literature and derived E(B-V)=0.18±0.01. We classified the companion star's spectral type as A5IV by analyzing the wings of the Balmer absorption lines in the highresolution (R=67 000) optical spectra (3700-7300Å). Using the distance to the nebula of 1400 pc from Gaia DR2, we constructed a photoionization model based on abundances and line intensities derived from the low-resolution optical spectra to obtain the temperature (Teff=130 000 K) and luminosity (L = 170 L☉) of the ionizing star, which are consistent with the UV continuum. This analysis allows us to better constrain the binary system. We concluded that the progenitor star of NGC 2346 has evolved through a common envelope phase, in which the companion star has evolved off the main-sequence.
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