TRACING THE ORPHAN STREAM TO 55 kpc WITH RR LYRAE STARS

Abstract

We report positions, velocities and metallicities of 50 ab-type RR Lyrae (RRab) stars observed in the vicinity of the Orphan stellar stream. Using about 30 RRab stars classified as being likely members of the Orphan stream, we study the metallicity and the spatial extent of the stream. We find that RRab stars in the Orphan stream have a wide range of metallicities, from -1.5 dex to -2.7 dex. The average metallicity of the stream is -2.1 dex, identical to the value obtained by Newberg et al. (2010) using blue horizontal branch stars. We find that the most distant parts of the stream (40-50 kpc from the Sun) are about 0.3 dex more metal-poor than the closer parts (within ∼ 30 kpc), suggesting a possible metallicity gradient along the stream's length. We have extended the previous studies and have mapped the stream up to 55 kpc from the Sun. Even after a careful search, we did not identify any more distant RRab stars that could plausibly be members of the Orphan stream. If confirmed with other tracers, this result would indicate a detection of the end of the leading arm of the stream. We have compared the distances of Orphan stream RRab stars with the best-fit orbits obtained by Newberg et al. (2010). We find that model 6 of Newberg et al. (2010) cannot explain the distances of the most remote Orphan stream RRab stars, and conclude that the best fit to distances of Orphan stream RRab stars and to the local circular velocity is provided by potentials where the total mass of the Galaxy within 60 kpc is M 60 ∼ 2.7 × 10 11 M , or about 60% of the mass found by previous studies. More extensive modelling that would consider non-spherical potentials and the possibility of misalignment between the stream and the orbit, is highly encouraged.

Target Selection for the SDSS-IV APOGEE-2 Survey

“…Gray points are other 2MASS sources in the field that were not targeted as LMC stars. Carlberg et al 2012), GD-1 (Koposov et al 2010), Orphan (Newberg et al 2010;Casey et al 2013Casey et al , 2014Sesar et al 2013), and the Sgr tail (Bellazzini et al 2003;Yanny et al 2009;Carrell et al 2012;Pila-Díez et al 2014). Each stream has one to five numbered fields placed along its length (e.g., GD1-1, GD1-2, etc.).…”

Disentangling the Virgo Overdensity With Rr Lyrae Stars

“…Following Sesar et al (2013b) we used fxcor in two different ranges of wavelength, centered on Hβ (4630-5000Å) and Hγ (4160-4630Å). For the WIYN data, better results were obtained by fitting profiles to individual lines and measuring their centers.…”

“…• The errors of systemic velocities were estimated following Sesar (2012) and Sesar et al (2013b), which included the error in the velocity measurement and the error of the template at the phase of observation. Figure 2 illustrates the radial velocity curve fitting for star LSQ 652 (< V >= 17.0) which was observed with the WIYN telescope at two different phases.…”

Motion of halo tracer objects in the gravitational potential of a low-mass model of the Galaxy

“…11 M within 60 kpc (with disk+bulge mass of 1.3 × 10 11 M ; Newberg et al 2010;Sesar et al 2013), significantly less than suggested by the above estimates within 50 kpc. This points to some model-dependent effects.…”

“…In this sense the numerically 3 That is, by minimizing the following mismatch functional δ ≡ [ (Merrifield 1992;Little & Tremaine 1987) or with the value of (2.6−2.7) × 10 11 M recently inferred from the kinematics of the Orphan stream (Newberg et al 2010;Sesar et al 2013) within ∼60 kpc.…”