Proposal #76

LS 4948: A Merger Progenitor

Abstract: LS 4948 is a massive contact binary consisting of two B1.5nnn stars in an orbit of ~1.03 d. While contact binaries are not particularly rare, there are few well studied massive contacts. Additionally this star show a clear period change between two different epochs of photometry. However, this period change is limited to two distinct points and is not sufficient two determine what this period is. By adding adding contemporary data 5 years after the last known observation we hope to solidify the period change in this object and confirm what will be the first ever massive binary merger progenitor with a confirmed spin down rate.

Scientific Justification:

Massive star evolution is dominated by binary interactions (Sana et al. 2012), with possible av-
enues of evolution including mergers (24%), mass transfer ( 33%) and spin-up/accretion (14%).
Only ≈29% of main sequence massive stars should follow the predictions of traditional single star
evolutionary models (Sana et al. 2012, Ekstrom et al. 2012). Given the predicted importance of
stellar mergers it is critical that we characterize some of the pre- and post-merger systems in the
Galaxy. This was accomplished for the presumed merger V838 Mon by Chesneau et al. (2014),
and the confirmed merger V1309 Scorpii (Tylenda & Kaminski 2016). However, a rigorous system
analysis of a massive pre-merger system has yet to be accomplished, an issue we will be able to
rectify with the system LS 4948.

LS 4948 is contact binary consisting of two B1 main sequence stars discovered using MOST data
. With an orbital period just a few minutes longer than 1 day and two nearly identical
stars, it is an excellent candidate for merger. However, while contact binaries with such massive
stars aren’t common, there are others that are known, at least one of which being far more massive
(Ibanoglu et al. 2013). However, LS 4948 is unique for several reasons. First, this binary is located
in M17, which is an extremely well studied cluster. The age of the system is known precisely from
an analysis of cluster isochrones (Hanson et al. 1997) and at ≈ 1 Myr it is among the youngest
clusters known. Additionally, unlike all other massive contact binaries, photometric analysis of
LS 4948 reveals a clear change of its orbital period, signifying that the two stars are spiraling in
towards an eventual merger.

This period change has been derived using data from ASAS as well as MOST, however, due to baseline and observations limits we have only been able to derive two measurements. This fact makes it impossible to adequately determine the spin down rate which is crucial for the understanding of this system. As the last observations were in 2012, sufficient time has passed for there to be a measureable change in the binary period. Using observations from just a handful of observing nights, in combination with our nearly two decade long baseline we should be able to determine the spin down rate of this system. Additionally, with spectra that have already been obtained by the GRACES spectrograph on Gemini this past June, we will be able to use the combination of photometric and radial velocity data to determine fundamental parameters for both components of this system.

Technical Justification:

LS 4948 is a bright target (10.8 in V, 11.5 in B) with large (~1 Magnitude deep) eclipses so it should be an easy target to observe. However, the known period change makes it difficult to predict times of minima, so we will use a Phase dispersion Minimization approach to obtaining a period. However, this requires that the phase be well covered. To cover the phase sufficiently we need images every 5-10 minutes over the entire orbit. Since this system has a period of ~1.03 days it takes roughly 21 days for all phases to be visible. At this time of year this is an early evening target and can generally only be observed for 2-4 hours depending on the observatory location, and the altitude restrictions of the individual telescopes. We therefore request high cadence observations of this object, when it is visible, in V and ( if possible) B bands for a 30 day span, starting as soon as possible, as the object will become steadily less and less viewable over the next month. From the information available it appears that the BSM in Australia would be the most appropriate for this proposal, though it should be viewable from virtually every scope.

References:

Chesneau et al. 2014, 569, L3

Ekstrom et al. 2012, 537, A146

Hanson, Howarth & Conti. 1997, 489, 698
Ibanoglu, & Sipahi 2013, 436, 750

Sana et al. 2012, Science, 337, 444
Tylenda & Kaminski, 2016, A134