Búsqueda fotométrica de variables cataclísmicas adecuadas para estudios dinámicos

Abstract

En este trabajo presentamos un método rápido diseñado para detectar la estrella secundaria en el visible/infrarrojo en variables cataclísmicas a partir de sus distribuciones de energía espectral (SEDs) calculadas a partir de las magnitudes y distancias publicadas por diferentes mapeos (surveys) de todo el cielo. Nuestra muestra contiene novas de nuestra Galaxia cuyas erupciones ocurrieron hace más de 50 años y que a día de hoy se encuentran en estado de quietud. La detección de la estrella secundaria permitirá realizar estudios dinámicos que permitan respaldar o refutar los modelos teóricos que deducen la masa de la enana blanca –––en muchas ocasiones superior a 1 M⊙— a partir de la curva de decaimiento de brillo de la nova. Hemos implementado una serie de códigos de PYTHON que permiten automatizar el proceso de generación de las SEDs. De los 44 sistemas estudiados, hemos observado indicios de la presencia de la estrella secundaria en 12 novas no simbióticas. Los resultados obtenidos han sido comparados con estudios espectroscópicos de la literatura, que han corroborado la detección certera de la secundaria en tres casos (GK Per, V841 Oph y BD Pav). Además, hemos identificado en estos trabajos previos otros candidatos que necesitan espectros de mejor calidad para dar una respuesta definitiva.

Cataclysmic variables are semi-detached binary systems composed of a white dwarf primary star and a low mass, typically main sequence, secondary star. The latter overfills its Roche lobe and transfers matter into the Roche lobe of the white dwarf. The way this mass is driven on to the primary star will depend on its magnetic properties. For weakly-magnetic white dwarfs, the stream of transferred material, which has a non-zero angular momentum relative to the white dwarf, goes into orbit around it eventually forming an accretion disc. If, on the contrary, the white dwarf is strongly magnetic, the formation of an accretion disc is not possible and the material is channelled along the magnetic field lines of the primary onto its surface to end up colliding near its magnetic poles. Cataclysmic variables are given their exotic name as a consequence of the characteristic outbursts they undergo. According to their recurrence time and brightness amplitude, these are classified into three groups: classical novae, recurrent novae and dwarf novae. Novae draw their energy from thermonuclear reactions on the white dwarf surface, while dwarf novae rely on accretion disc instabilities to power their milder eruptions. The main distinction between classical and recurrent novae is that the former ones have not been observed to erupt more than once, while recurrent novae seem to recur on timescales of decades or centuries. Theoretical models based on the novae brightness decay curves predict that white dwarfs in novae would have masses larger than 1 M⊙, even close to the Chandrasekhar mass limit, which makes these systems type Ia supernova candidate progenitors. To test these predictions, it is necessary to carry out dynamical studies that can provide accurate masses for both the white dwarf and its companion. This can only be done if the absorption lines of the secondary star are detected in the spectrum. However, the accretion disc generally happens to be the main contributor to the optical light, often overshining the secondary star. For this reason, it is important to search for many systems where signatures of the secondary stars can be seen. In this work, we have compiled a sample of Galactic novae whose outbursts were registered at least 50 years ago, so that they have had enough time to return to their quiescence state. We have analyzed the morphology of their spectral energy distributions (SEDs), and determined whether the secondary star contributes significantly to the brightness of the system. The SED of an object accounts for the variation with wavelength of its emitted energy, and can be considered as a very low resolution spectrum that gives us an idea of the shape of the continuum. The spectral continua of the white dwarf and the secondary star can be approximated as black bodies with their same temperatures. However, the signature of the accretion discs is different: while the outer parts are relatively cold (≃ 5000 K), the inner parts get hotter the closer they are to the white dwarf, reaching temperatures of ≃ 30000 K in the innermost regions. Therefore, the spectral continuum of such a disc can be regarded as the sum of the black-body emissions of a series of rings whose temperatures decrease smoothly outwards. The result of this superposition is an emission curve that follows a power law as a function of the wavelength. In those cases where the emission of the accretion disc is the main contributor to the optical light, the shape of the resulting SED will be that of a power law. On the contrary, if the secondary star contributes significantly to the total flux, the shape of the SED will be the result of the sum of a power law and a typical blackbody curve. It is precisely this that we will take advantage of to make a selection of novae in which the secondary star may be detected. To calculate the SEDs of our sample we have compiled photometric measurements from the optical to the mid-infrared from the following sky surveys: PanSTARRS Data Release 1 (PS1), Sloan Digital Sky Survey (SDSS), The AAVSO Photometric All-Sky Survey (APASS), Two Micron All Sky Survey (2MASS) and Wide-field Infrared Survey Explorer (WISE). The magnitudes have been corrected for interstellar extinction using the distances inferred from Gaia data given by Bailer-Jones et al. (2021) and the three-dimensional dust reddening map by Green et al. (2019). We retrieved all this information using the VizieR Catalogue Service, an astronomical catalogue search application provided by the Centre de Données astronomiques de Strasbourg (CDS). To make the data treatment easier and produce the SEDs of the objects in our sample, we have developed PYTHON codes that automate the calculation process. We classified the systems based on the morphology of their SEDs and searched for spectroscopic evidence of their secondary stars in the literature. From our results we can conclude that it is possible to infer whether the secondary star appreciably contributes to the optical and infrared light of a nova remnant from its photometric SED. Of the 44 novae studied, besides six symbiotic stars, we found indication of the presence of the secondary star in 12 of them. In three of these 12 cases previous spectroscopic studies had confirmed that the spectrum of the secondary star is appreciable: GK Per, V841 Oph and BD Pav. This nicely shows the utility of our photometric method. Signatures of the secondary star are not unambiguously detected in the case of V368 Aql, in which a brighter spectrum towards the red is observed. This, together with its long orbital period, is compatible with the secondary contributing significantly to the system brightness. Finally, NSV 11561 has been proposed as a K4 V star, but it might be the bright secondary star dominating its spectrum. It would be interesting to carry out radial velocity studies of this system to clarify its nature.