[Paper Review] RR Lyrae Stars in Dwarf Spheroidal Galaxies

G. Clementini|arXiv (Cornell University)|Feb 8, 2010

Stellar, planetary, and galactic studies3 citations

TL;DR

This paper reviews RR Lyrae stars in Local Group dwarf spheroidal (dSph) galaxies, finding that ultra-faint dSphs predominantly exhibit Oosterhoff type II properties, while bright dSphs are Oosterhoff intermediate, contrasting with the Milky Way's Oosterhoff type I dominance. The results suggest that ultra-faint dSphs—rarely observed in the Milky Way halo—may have contributed to the formation of the Milky Way and Andromeda halos via hierarchical accretion.

ABSTRACT

With ages comparable to the age of the Universe, the variable stars of RR Lyrae type have eyewitnessed the formation of their host galaxies, and thus can provide information on the processes that led to the assembling of large galaxies such as the Milky Way and the Andromeda galaxy. The present knowledge of the RR Lyrae population in Local Group dwarf spheroidal galaxies is reviewed,calling attention to the "ultra-faint" spheroidal systems recently discovered around the Milky Way by the Sloan Digital Sky Survey. The properties of the RR Lyrae stars and the Oosterhoff dichotomy observed for Galactic globular clusters and field RR Lyrae stars are discussed, since they put constrain on the possibility that the Milky Way and Andromeda halos were built up from protogalactic fragments resembling the dwarf spheroidals we observe today.

Motivation & Objective

• To assess the Oosterhoff dichotomy in RR Lyrae stars across dwarf spheroidal galaxies in the Local Group.
• To evaluate whether the properties of RR Lyrae stars in dSphs support the hypothesis that the Milky Way and Andromeda halos formed via hierarchical merging of protogalactic fragments.
• To compare the pulsation periods and metallicities of RR Lyrae stars in dSphs with those in Galactic globular clusters and field stars.
• To examine the implications of Oosterhoff type distributions for the formation history of the Milky Way and M31 halos.
• To summarize the current state of RR Lyrae studies in M31 dSphs, where data remain limited but suggest Oosterhoff type I/intermediate characteristics.

Proposed method

• Compilation and analysis of RR Lyrae data from 15 dSph galaxies, including both 'bright' and 'ultra-faint' systems, using color-magnitude diagrams (CMDs) and period-luminosity relations.
• Classification of RR Lyrae stars into RRab (fundamental mode), RRc (first overtone), and RRd (double-mode) types based on light curve morphology.
• Calculation of mean pulsation periods ⟨Pab⟩ and ⟨Pc⟩ for RRab and RRc stars to determine Oosterhoff type (I, II, or intermediate).
• Comparison of metallicities [Fe/H] and distances to assess correlations between stellar population properties and Oosterhoff classification.
• Use of Hubble Space Telescope (HST) and Sloan Digital Sky Survey (SDSS) data to derive periods and magnitudes for RR Lyrae stars in dSphs.
• Cross-referencing results with existing data on Galactic globular clusters and field RR Lyrae stars to contextualize the Oosterhoff dichotomy.

Experimental results

Research questions

• RQ1 Do the RR Lyrae stars in ultra-faint dSphs exhibit Oosterhoff type I, II, or intermediate properties?
• RQ2 How do the pulsation periods and metallicities of RR Lyrae stars in dSphs compare to those in Galactic globular clusters and field stars?
• RQ3 Can the Oosterhoff type distribution in dSphs support the hypothesis that the Milky Way halo formed via accretion of protogalactic fragments resembling today’s dSphs?
• RQ4 What is the current state of knowledge on RR Lyrae stars in M31 dSphs, and do they show Oosterhoff type I or II characteristics?
• RQ5 Are the 'bright' dSphs more similar to Galactic globular clusters in their RR Lyrae properties, or do they differ significantly in Oosterhoff classification?

Key findings

• Ultra-faint dSphs such as Ursa Major I, Ursa Major II, and Canes Venatici I predominantly exhibit Oosterhoff type II characteristics, with mean periods ⟨Pab⟩ ≈ 0.64–0.74 days and metallicities [Fe/H] ≈ -2.2 to -2.5 dex.
• The 'bright' dSphs, including Fornax and Draco, show Oosterhoff intermediate (Oo-Int) properties, with ⟨Pab⟩ ≈ 0.60–0.69 days and metallicities [Fe/H] ≈ -1.5 to -2.3 dex.
• Sagittarius dSph is the only 'bright' dSph with Oosterhoff type I properties, suggesting a distinct formation history or metallicity evolution.
• RR Lyrae stars in M31 dSphs show mixed Oosterhoff types: And I and And II are Oosterhoff type I, while And III and And V are Oosterhoff type II, with ⟨Pab⟩ values ranging from 0.571 to 0.685 days.
• The M31 field and cluster B514 show Oosterhoff type I or borderline I/intermediate, indicating a possible difference in pulsation properties between M31 and the Milky Way.
• The Milky Way halo is dominated by Oosterhoff type I RR Lyrae stars, but contains a significant Oosterhoff type II component in its inner regions, suggesting complex accretion histories.