LMC Novae Research Articles

A Light Curve Analysis of Recurrent and Very Fast Novae in Our Galaxy, Magellanic Clouds, and M31

Abstract We analyzed optical, UV, and X-ray light curves of 14 recurrent and very fast novae in our Galaxy, Magellanic Clouds, and M31, and obtained their distances and white dwarf (WD) masses. Among the 14 novae, we found that eight novae host very massive (≳1.35 M ☉) WDs and are candidates for Type Ia supernova (SN Ia) progenitors. We confirmed that the same timescaling law and time-stretching method as in galactic novae can be applied to extragalactic fast novae. We classify the four novae V745 Sco, T CrB, V838 Her, and V1534 Sco as V745 Sco type (rapid decline); the two novae RS Oph and V407 Cyg as RS Oph type (circumstellar matter (CSM) shock); and the two novae U Sco and CI Aql as U Sco type (normal decline). The V light curves of these novae almost overlap with each other in the same group, if we properly stretch in the time direction (timescaling law). We apply our classification method to the Large Magellanic Cloud (LMC), Small Magellanic Cloud (SMC), and M31 novae. YY Dor, LMC N 2009a, and SMC N 2016 belong to the normal-decline type, LMC N 2013 to the CSM-shock type, and LMC N 2012a and M31 N 2008-12a to the rapid-decline type. We obtained the distance to SMC N 2016 to be d = 20 ± 2 kpc, suggesting that SMC N 2016 is a member of our Galaxy. Rapid-decline type novae have very massive WDs of M WD = 1.37–1.385 M ☉ and are promising candidates for SN Ia progenitors. Novae of this type are much fainter than the maximum magnitude versus rate of decline relations.

On the narrow emission line components of the LMC novae 2004 (YY Doradus) and 2009a

We present early decline spectra of the two Large Magellanic Cloud novae: LMC 2004 (YY Dor) and LMC 2009a and discuss their spectral an line profile evolution with special emphasis on the existence and appearance of a sharp component. We show that the narrow component that characterizes the emission lines in the maximum spectra of nova LMC 2004 originates in the ejecta. The HeII 4686 A, narrow emission which appears at the onset of the nebular phase in both novae is somewhat controversial. Our observations suggest that the corresponding line forming region is physically separated from the rest of the ejecta (the broad line region) and environmentally different. However, the lack of late time observations covering the super-soft source (SSS) phase, the post-SSS phase and the quiescence state does not allow to securely establish any non-ejecta origin/contribution as, instead, in the case of U Sco and KT Eri.

PHOTOMETRIC AND SPECTROSCOPIC PROPERTIES OF NOVAE IN THE LARGE MAGELLANIC CLOUD

The photometric and spectroscopic properties of the 43 known LMC nova candidates are summarized and reviewed. Of these, photometric data sufficient to establish decline rates are available for 29 novae, while spectroscopic data sufficient to establish the spectroscopic classes are available for 18 systems. Half of the 18 novae belong to the Fe II class, with the remaining nine belonging to either the He/N or the Fe IIb classes. As seen in previous nova studies of M31 and M33, the He/N and Fe IIb novae have on average faster photometric developments than do their Fe II counterparts. Overall, the available photometry confirms earlier studies, and shows conclusively that LMC novae have faster rates of decline than do novae in the Galaxy and M31. It appears that the increased fraction of faster, He/N and Fe IIb novae observed in the LMC compared with M31 is almost certainly the result of differences in the underlying stellar population between the two galaxies. We propose that the younger population seen in the LMC compared with M31's bulge (where most of the novae are found) produces progenitor binaries with higher average white dwarf masses. The higher mean white dwarf mass not only produces a larger fraction of fast, He/N novae compared with M31, but also results in a relatively large recurrent nova population.

Light curves and absolute magnitudes of four recent fast LMC novae

AbstractThe light curves of four recent fast LMC novae (Nova LMC 1988a, 1992, 1995, 2000) have been analysed to obtain the parameter t2 the time for a two magnitude decline below maximum light. Using the calibration of Della Valle & Livio (1995), values of MV at maximum are obtained. The weighted mean distance modulus to the LMC based on these novae is 18.89 ± 0.16. This differs significantly from the distance modulus adopted by Della Valle & Livio of 18.50, but only differs at the 1σ-level from Feast’s (1999) value of 18.70 ± 0.10. The evidence based on these novae suggests that either: (i) DMLMC = 18.50 is too close for the LMC; or (ii) some novae in the LMC, including these four, are significantly underluminous at maximum light compared with those in M31, by about 0.4 mag. This could be a metallicity effect, given that more metal-rich M31 novae were predominantly used by Della Valle & Livio to obtain their calibration.

Recent advances in studies of the nova outburst

The nova outburst is a consequence of the accretion of hydrogen-rich material onto a white dwarf in a close binary system and observations show that the accreting material must have mixed with core material at some phase of the outburst. Abundance determinations of novae ejecta show that the core material is either carbon and oxygen or oxygen, neon, and magnesium. I report on the methods and results of a project to determine the ejecta abundances for a number of recent Galactic and LMC novae using combined optical and ultraviolet data. The abundance determinations also provide a value for amount of mass ejected in the outburst. There are large discrepancies between the observed and predicted values of the ejecta mass in that the observed ejecta masses are much larger than predicted for both carbon–oxygen and oxygen–neon–magnesium novae. I also review the X-ray observations and present new evolutionary calculations done to try to understand the missing ejecta mass problem and suggest a possible solution to this problem.

Analysis of Three Fast Oxygen‐Neon‐Magnesium Novae

Since astronomers have only known about the existence of oxygen-neon-magnesium (ONeMg) novae for the last decade, little work has been done on them as a group. The purpose of this work was to study three fast ONeMg novae: V838 Nova Herculis 1991 (Her 91), V693 Nova Coronae Australis 1981 (CrA 81), and Nova LMC 1990 No. 1 (LMC 90a). Each of these novae has been studied in detail, and a comparison of the results is provided in order to better understand ONeMg novae as a class. My analyses have provided values of the elemental abundances, ejection velocities, ejected masses, white dwarf (WD) masses, maximum outburst luminosities, and white dwarf turn-off times for these novae. All three novae have abundances enhanced relative to solar values, with the oxygen abundance of Her 91 being the exception (see Table 1). They show high N/O ratios, indicating high-mass WD progenitors. We find CrA 81 and LMC 90a to have almost identical abundances, even though the initial metal abundances for the LMC novae are one-third of solar values. Each of the novae exhibited a super-Eddington luminosity phase at maximum light. Our analysis of the Her 91 spectrum taken 2 months after outburst, however, required a luminosity almost 2 orders of magnitude lower than the Eddington limit for a 1.4 M, WD. The best fits for CrA 81 and LMC 90a, on the other hand, were consistent with a constant luminosity of ∼1038 ergs s for at least 2 months after outburst. While we were unable to accurately determine ejected masses for either CrA 81 or LMC 90a owing to the lack of optical data, we found an ejected mass of ∼1024 M, for Her 91. This is in agreement with the values found by other groups using different data and different techniques. The results of this study can be used in many ways to help further the understanding of novae and stellar evolution. The elemental abundances that have been determined can be used as input to theoretical hydrodynamic models. This will provide better constraints on the models in order to determine the energetics of the outburst and the mass of the underlying white dwarf, properties that are difficult to obtain observationally. Knowledge of the abundances will also help determine the contribution of fast ONeMg novae to the heavy-element content of the interstellar medium. In particular, with the abundance results, the mass fraction of Al in the nova ejecta can be estimated. These estimates will help to determine if novae are important for Galactic nucleosynthesis. Analysis of LMC 90a provides the means to compare Galactic with extragalactic novae. For example, the LMC novae can be used to calibrate the luminosities of (and therefore distances to) Galactic novae. This calibration will allow a better determination of the fast end of the absolute-magnitude decay-rate relationship or perhaps develop a separate relationship for this class of novae.

Are Novae Standard Candles?

Novae in outburst can be used as distance indicators. The distance measurement process is considerably simplified if novae could be regarded as standard candles. The term standard candles in the context of this paper means that the absolute magnitude at maximum for different novae is, to a good approximation, constant. The t3 – Mb diagram (Schmidet 1957) implies that this is the case for M31 and LMC novae (Della Valle & Livio 1995). However, this conclusion does not necessarily apply to non Local Group novae (Livio 1992; Della Valle & Livio 1995); for example the Virgo novae are not similar to Local Group novae (Della Valle & Livio 1995). Here we focus on a question of principle, namely: Does the fact that novae have similar spectra imply that they have similar luminosities?

The ROSAT observations of classical novae

We observed a number of classical and recurrent novae in the Galaxy and the LMC with the ROSAT X-ray telescope and searched the archival data for other serendipitous observations. Preliminary results show that only 9 out of 37 observed objects were bright enough in X-rays to be detected with ROSAT, either in outburst or at quiescence.Three basic mechanisms can cause X-ray emission from classical or re-current novae. The first is hot hydrogen burning in a thin shell of the remnant envelope left on the white dwarf after the nova explosion. Hydrogen burning post-novae should be blackbody like emitters at nearly Eddington luminosity (as per the ‘supersoft’ X-ray sources). In our sample, only GQ Mus (Nova Mus 1983, see Ögelman et al. 1993; Shanley et al. 1995) and V1974 Cyg 1992 (Krautter et al. 1996) had these characteristics. Remarkably, among 10 LMC novae that had an outburst in the last 47 yr none was detected as a ‘supersoft’ X-ray source. The 3 σ upper limits for the black-body temperatures of the post-nova white dwarfs are mainly in the range 20… 30 eV. A post-nova can also emit X-rays because of shocks occurring in the ejected shell (e.g. O’Brien et al. 1994). Three out of four classical novae that were observed in outburst displayed a hard X-ray component in the ROSAT energy band, which might be due to a shocked shell. Finally, X-ray emission is expected from quiescent nearby novae because of accretion. Only four nearby accreting sources were detected; the ROSAT upper limits for the non-detected quiescent novae are Lx < 1031… 1032 ergs−1, assuming a thermal plasma at kT = a few keV.

Simultaneous Optical and UV Spectra of the Two LMC Novae 1988

AbstractThe two classical novae of 1988 in the LMC are the first extragalactic novae, which have been studied in the UV range. IUE spectra were taken under the Nova Target of Opportunity program, and quasi-simultaneous optical spectra were obtained by the authors. The spectral development in the UV and visual ranges is discussed for both objects.