Title: An Ancient Metal-Poor Population in M32, and Halo Satellite Accretion in M31, Identified by RR Lyrae Stars
Authors: Ata Sarajedini, Soung-Chul Yang, Antonela Monachesi, Tod R. Lauer, Scott C. Trager
First Author’s Institution: University of Florida – Gainesville, Florida
The variability of the star RR Lyrae was reported at the turn of the 20th century by Edward Pickering, after its discovery by Wilhelmina Fleming. RR Lyrae is the prototype for the class of variable stars that are given its name; these “RR Lyrae stars” show short period, high amplitude pulsations. RR Lyrae stars are evolved stars – that is, they have stopped hydrogen fusion in their cores and moved off the main sequence. Their progenitors were poor in heavy elements, but otherwise normal stars with masses about 70% the Sun’s mass. The American Association of Variable Star Observers (AAVSO) has a fantastic page on these stars, describing their history, evolution and application.
Variable stars have a history of being exploited to help us study the properties of distant objects. Perhaps most famously, the relationship between the period of pulsation and the absolute brightness of Cepheid variable stars (through Henrietta Leavitt’s period-luminosity relation) allowed Edwin Hubble to determine the distance to the “nebula” M31, now known as the Andromeda Galaxy. The distance he found (900,000 light years) settled the debate over whether nebulae such as M31 were part of the Milky Way, or distant galaxies. Cepheid variables still play an important role in helping astronomers measure distances to far-off galaxies in the Universe.
One thing that drew me to this paper is the small connection to RR Lyrae history: the subject is RR Lyrae stars in a galaxy none other than Andromeda (and also in M32, one of Andromeda‘s small companion galaxies). Another fact I found interesting was that the study of RR Lyrae stars in Andromeda, which I’ll discuss, was not the original motivation for the research. Instead, the observations of Andromeda were intended as a control for their study of M32 – monitoring the contamination of their M32 sample by stars actually in Andromeda – and were re-purposed for research of their own.
A patch of sky on the outskirts of Andromeda was observed for 7.5 hours with the Hubble space telescope, in which they found 630 RR Lyraes. 446 of the RR Lyrae stars the authors found are “ab-type,” which exhibit saw tooth-like pulsations. They measure the period and amplitude of these pulsations for each star using an automated code. Testing their variable star finding code against a simulation shows that they are able to measure the periods and amplitudes of these stars without bias.
A key point in this paper is that the period and amplitude of an ab-type RR Lyrae star is related to its metallicity (heavy element abundance). By applying a previously published relation, determined from observations of RR Lyrae stars in the Milky Way, Sarajedini et al. use the periods and amplitudes to determine the metallicities of their stars.
They start by considering the global consequences of the metallicity distributions. Recall that RR Lyrae stars contain very few heavy elements. In addition, if a star forms in a galaxy where many stars have already formed (and subsequently evolved and formed heavier elements), then the new stars will have more heavy elements. This means that RR Lyrae stars formed early on in the history of a galaxy, before many other stars had formed. Both Andromeda and its companion galaxy M32 contain RR Lyrae stars of similar metallicities, which is telling us that the old stellar populations in both these systems formed at roughly the same time. In fact, similar RR Lyrae stars are found in the Milky Way, so all three experienced an early era of star formation together.
Now, how about the details of the metallicity distribution? When Sarajedini et al. calculate the metallicities of their ab-type RR Lyrae stars, they find something strange. All previous studies of RR Lyrae stars in Andromeda show a single-peaked distribution of metallicities, but theirs is double-peaked (the second peak is smaller, to the left of the major one). It looks as if there are two star formation epochs represented in this field! The authors suggest that the primary peak originates from star formation in Andromeda, but that the secondary peak is associated with star formation in a satellite galaxy that has been disrupted and consumed by Andromeda.
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