---
abstract: 'The development of surveys which will be able to cover a large region of the sky several times per year will allow the massive detection of transient events taking place in timescales of years. In addition, the projected full digitalization of the Harvard plate collection will open a new window to identify slow transients taking place in timescales of centuries. In particular, these projects will allow the detection of stars undergoing slow eruptions as those expected during late helium flashes in the post-AGB evolution. In order to identify those transients which correspond with late helium flashes the development of synthetic light curves of those events is mandatory. In this connection we present preliminary results of a project aimed at computing grids of theoretical light curves of born again stars.'
author:
- 'Marcelo M. Miller Bertolami$^{1,2,3}$& René D. Rohrmann,$^{2,4}$'
bibliography:
- 'M3B\_RR.bib'
title: 'Synthetic Light Curves for Born Again Events: Preliminary Results'
---

Introduction
============

About 20% of the stars leaving the Asymptotic Giant Branch (AGB) are supposed to undergo a final thermal pulse during their post-AGB evolution [@1984ApJ...277..333I]. In those cases, the star is predicted to be temporarily reborn as a yellow giant in the so-called “Born Again AGB” scenario [@1984ApJ...277..333I]. The transition from the prewhite dwarf to the giant configuration is expected to be very rapid; being of a few years in the very late thermal pulse (VLTP) case [@1995LNP...443...48I] and of the order of a century in the case of a late thermal pulse (LTP) . Due to the short duration of these events, although $\sim 10$% of all post-AGB stars are expected to undergo a VLTP, the observation of those events is very rare. Indeed, only two objects (V605 Aql and V4334 Sgr)[^1] have been identified as stars undergoing a VLTP , while a third has been identified with the LTP flavor of the scenario (FG Sge, see and references therein). Individual born again stars are very valuable to test our understanding of the s-processes during the thermal pulses on the AGB and also to test and validate the simulations of convective reactive burning in the interior of the stars [@2011ApJ...727...89H]. In addition, these events are key to understand the formation of carbon-rich H-deficient stars and H-deficient white dwarfs [@2006PASP..118..183W]. Consequently, the identification of more stars undergoing late helium shell flashes will boost our understanding of the late phases of low and intermediate mass stars and their nucleosynthesis. In this connection, a rough estimate suggests that the birth rate of planetary nebulae in our galaxy is of about one every year . If 10% of their central stars become VLTP giants, then we should expect such events in our galaxy to take place at a rate of about one per decade. Due to their high intrinsic brightnesses ($M_{\rm V}$=-2...-4) these objects can be easily detected at large distances within our galaxy. In this context, the development of surveys which will be able to cover a large region of the sky several times per year, like the Panoramic Survey Telescope & Rapid Response System (PanSTARRS, @2012ApJ...750...99T) and the Large Synoptic Survey Telescope (LSST, @2012IAUS..285..158W), will allow the systematic detection of born again events. In addition, the projected full digitalization of the Harvard plate collection (Digital Access to a Sky Century at Harvard, DASCH, @2009ASPC..410..101G) will open a new window to identify slower born again events taking place in timescales of centuries like FG Sge. Due to the lack of enough observational counterparts of bonafide born again stars the identifications of such events has to rely partially in our present theoretical knowledge of their evolution. We have recently started a project to compute, and provide, theoretical light curves for born again events that will help in the identification of those transients. In what follows we present some preliminary results and discuss future developments.

Numerical modeling
==================

In order to compute synthetic light curves appropriate for Born Again AGB stars proper stellar evolution models and atmospheres are needed. As a first step we computed the evolution of stellar models during fast born again events (low mass VLTP events). During VLTP events the hydrogen envelope remaining on the hot white dwarf is convectively mixed into the interior of the star and violently burned —H-flash, see . Stellar evolution sequences were computed with [LPCODE]{} stellar evolution code which is perfectly suited to compute the mixing & burning during the violent H-flash . Initial models for a wide range of hot-white dwarf masses were taken from [@2007MNRAS.380..763M], which is the only available grid of detailed pre-born again models to date. The masses of the models studied in the present work are 0.530M$_\odot$, 0.542M$_\odot$, 0.561M$_\odot$, 0.564M$_\odot$, 0.584M$_\odot$ and 0.609M$_\odot$. As mentioned above, in order to compute appropriate light curves for different filters, a proper assesment of the emergent spectral energy distribution of the models is necessary. Synthetic magnitudes for the stellar evolution models are computed with a modified version of the stellar atmosphere code described in Rohrmann et al. (2011) which includes a simplified treatment of the effects of C, N and O in the structure of the atmosphere and radiative transfer equations. In brief, the modelling of the stellar atmosphere includes several ions and molecules of H and He (see @2011MNRAS.411..781R for a detailed description) and the neutral and first three ionization stages of C, N and O. The chemical balance of these species is determined by the occupation probability formalism for H and He [@1988ApJ...331..794H] and by the ideal partition functions for the C, N and O ions. Opacity sources for H and He ions and molecules are included in a very detailed and exhaustive way [@2011MNRAS.411..781R] while C, N and O ion opacity sources are included within the hydrogenic approximation and no molecules have been included so far.

Preliminary results and future work
===================================

As expected, eruption light curves are dependent not only on the timescales set by the evolution of the stellar interior, but also on the specific element abundances of the photosphere of the models. In Fig. \[Figure\] (top right panel) we show the predicted light curves obtained from the same stellar evolution sequence when different surface abundances are adopted in the computation of the synthetic magnitudes. Preliminary synthetic light curves were computed for the UBVRJ [@1990PASP..102.1181B] and the Sloan Digital Sky Survey (ugriz; @1996AJ....111.1748F) photometric systems. In Fig. \[Figure\] we show the computed light curves for all our stellar evolution sequences in the SDSS photometric system, when abundances similar to those observed in V4334 Sgr are assumed. Note that the SDSS photometric system is similar to the one adopted by the PanSTARRS1 survey [@2012ApJ...750...99T]. In order to assess the importance of the detailed treatment of the atmosphere and also the importance of the different atmospheric abundances, two test cases have been computed by assuming different atmospheric compositions for the 0.561M$_\odot$ sequence: a solar-like atmosphere ($X_{\rm H}/X_{\rm
  He}/X_{\rm C}/X_{\rm N}/X_{\rm O}/= 0.74/0.25/2.4\times 10^{-3}/
7\times 10^{-4}/ 5.8\times 10^{-3}$, brown lines) and a V4334 Sgr-like atmosphere ($X_{\rm H}/X_{\rm He}/X_{\rm C}/X_{\rm N}/X_{\rm O}/=
7.4\times 10^{-4}/ 0.74/0.22/8.2\times 10^{-3}/ 2.9\times 10^{-2}$, black lines, ). Clearly, a correct assessment of the radiative transfer in the atmosphere is necessary to compute born again light curves.

![Synthetic light curves computed for the SDSS photometric system under the assumption of V4334 Sgr-like compositions for all our computed stellar evolution sequences. The effect of different compositions is shown in the top right panel for the 0.561M$_\odot$ where the light curves for a solar-like atmosphere are also shown (grey curves). Note that the much slower eruption light curve of the 0.609M$_\odot$ sequence is due to the fact that massive remnants do not harbour enough hydrogen for the H-flash to drive a fast expansion as already explained by [@2007MNRAS.380..763M].[]{data-label="Figure"}](lightcurves_ugriz.eps){width="10cm"}

Several improvements in theoretical lightcurves, as well as a proper assessment of the involved uncertainties, are still necessary. In particular we plan to improve the stellar atmospheric modelling by including a better treatment of the C, N and O ions and the inclusion of related molecules. Also a calibration of the mixing efficiency in stellar evolution sequences during the violent H-flash is necessary and will be performed. In addition we plan to expand the grid of masses to include higher masses, with slower VLTP events, and to compute lightcurves for LTP episodes.

M3B thanks the organizers of the EUROWD12 for the finantial assistance that helped him to attend the conference. This research was supported by PIP 112-200801-00940 from CONICET and PICT-2010-0861 from ANCyT.

[^1]: In addition two other eruptions have been suggested to be VLTP events; CK Vul [@2002MNRAS.332L..35E] and more recently NSV 11749 [@2011ApJ...743L..33M], but neither of these have been confirmed.
