skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: THE MOST DISTANT STARS IN THE MILKY WAY

Abstract

We report on the discovery of the most distant Milky Way (MW) stars known to date: ULAS J001535.72+015549.6 and ULAS J074417.48+253233.0. These stars were selected as M giant candidates based on their infrared and optical colors and lack of proper motions. We spectroscopically confirmed them as outer halo giants using the MMT/Red Channel spectrograph. Both stars have large estimated distances, with ULAS J001535.72+015549.6 at 274 ± 74 kpc and ULAS J074417.48+253233.0 at 238 ± 64 kpc, making them the first MW stars discovered beyond 200 kpc. ULAS J001535.72+015549.6 and ULAS J074417.48+253233.0 are both moving away from the Galactic center at 52 ± 10 km s{sup –1} and 24 ± 10 km s{sup –1}, respectively. Using their distances and kinematics, we considered possible origins such as: tidal stripping from a dwarf galaxy, ejection from the MW's disk, or membership in an undetected dwarf galaxy. These M giants, along with two inner halo giants that were also confirmed during this campaign, are the first to map largely unexplored regions of our Galaxy's outer halo.

Authors:
;  [1]; ;  [2];  [3];  [4];  [5]
  1. Haverford College, 370 Lancaster Ave, Haverford, PA 19041 (United States)
  2. Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138 (United States)
  3. Kapteyn Astronomical Institute, P.O. Box 800, 9700 AV Groningen (Netherlands)
  4. Department of Astronomy, Boston University, 725 Commonwealth Avenue, Boston, MA 02215 (United States)
  5. Michigan State Astronomy Group, Michigan State University, Biomedical Physical Sciences Building, 567 Wilson Road, Room 3261 East Lansing, MI 48824-2320 (United States)
Publication Date:
OSTI Identifier:
22365575
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal Letters; Journal Volume: 790; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ASTROPHYSICS; COLOR; DISTANCE; INFRARED RADIATION; MILKY WAY; STARS

Citation Formats

Bochanski, John J., Willman, Beth, Caldwell, Nelson, Brown, Warren, Sanderson, Robyn, West, Andrew A., and Strader, Jay, E-mail: jbochans@haverford.edu. THE MOST DISTANT STARS IN THE MILKY WAY. United States: N. p., 2014. Web. doi:10.1088/2041-8205/790/1/L5.
Bochanski, John J., Willman, Beth, Caldwell, Nelson, Brown, Warren, Sanderson, Robyn, West, Andrew A., & Strader, Jay, E-mail: jbochans@haverford.edu. THE MOST DISTANT STARS IN THE MILKY WAY. United States. doi:10.1088/2041-8205/790/1/L5.
Bochanski, John J., Willman, Beth, Caldwell, Nelson, Brown, Warren, Sanderson, Robyn, West, Andrew A., and Strader, Jay, E-mail: jbochans@haverford.edu. Sun . "THE MOST DISTANT STARS IN THE MILKY WAY". United States. doi:10.1088/2041-8205/790/1/L5.
@article{osti_22365575,
title = {THE MOST DISTANT STARS IN THE MILKY WAY},
author = {Bochanski, John J. and Willman, Beth and Caldwell, Nelson and Brown, Warren and Sanderson, Robyn and West, Andrew A. and Strader, Jay, E-mail: jbochans@haverford.edu},
abstractNote = {We report on the discovery of the most distant Milky Way (MW) stars known to date: ULAS J001535.72+015549.6 and ULAS J074417.48+253233.0. These stars were selected as M giant candidates based on their infrared and optical colors and lack of proper motions. We spectroscopically confirmed them as outer halo giants using the MMT/Red Channel spectrograph. Both stars have large estimated distances, with ULAS J001535.72+015549.6 at 274 ± 74 kpc and ULAS J074417.48+253233.0 at 238 ± 64 kpc, making them the first MW stars discovered beyond 200 kpc. ULAS J001535.72+015549.6 and ULAS J074417.48+253233.0 are both moving away from the Galactic center at 52 ± 10 km s{sup –1} and 24 ± 10 km s{sup –1}, respectively. Using their distances and kinematics, we considered possible origins such as: tidal stripping from a dwarf galaxy, ejection from the MW's disk, or membership in an undetected dwarf galaxy. These M giants, along with two inner halo giants that were also confirmed during this campaign, are the first to map largely unexplored regions of our Galaxy's outer halo.},
doi = {10.1088/2041-8205/790/1/L5},
journal = {Astrophysical Journal Letters},
number = 1,
volume = 790,
place = {United States},
year = {Sun Jul 20 00:00:00 EDT 2014},
month = {Sun Jul 20 00:00:00 EDT 2014}
}
  • We present a new catalog of 404 M giant candidates found in the UKIRT Infrared Deep Sky Survey (UKIDSS). The 2400 deg{sup 2} available in the UKIDSS Large Area Survey Data Release 8 resolve M giants through a volume four times larger than that of the entire Two Micron All Sky Survey. Combining near-infrared photometry with optical photometry and proper motions from the Sloan Digital Sky Survey yields an M giant candidate catalog with less M dwarf and quasar contamination than previous searches for similarly distant M giants. Extensive follow-up spectroscopy of this sample will yield the first map ofmore » our Galaxy's outermost reaches over a large area of sky. Our initial spectroscopic follow-up of ∼30 bright candidates yielded the positive identification of five M giants at distances ∼20-90 kpc. Each of these confirmed M giants have positions and velocities consistent with the Sagittarius stream. The fainter M giant candidates in our sample have estimated photometric distances ∼200 kpc (assuming [Fe/H] = 0.0), but require further spectroscopic verification. The photometric distance estimates extend beyond the Milky Way's virial radius, and increase by ∼50% for each 0.5 dex decrease in assumed [Fe/H]. Given the number of M giant candidates, initial selection efficiency, and volume surveyed, we loosely estimate that at least one additional Sagittarius-like accretion event could have contributed to the hierarchical build-up of the Milky Way's outer halo.« less
  • An empirical comparison of the observed H-R diagrams for the supergiants in our region of the Galaxy and the Large Magellanic Cloud reveals comparable distributions of spectral types and luminosities in the two galaxies. Supergiants of similar spectral types have the same luminosities, except for the A-type stars, where selection effects may be important. These results suggest that the same basic physical processes govern the evolution of the most massive stars in the two galaxies.Variations in the blue-to-red supergiant ratio with galactocentric distance and with luminosity involve chemical composition gradients and varying rates of mass loss. Since the relative numbersmore » of the most luminous stars are more sensitive to mass loss, the B/R ratio from the less luminous supergiants may be a better indicator of galactic abundance gradients.The upper luminosity boundary for both the galactic and the LMC supergiants is characterized by (1) decreasing luminosity with decreasing temperature for the hottest stars and (2) an upper limit to the luminosity near M/sub bol/approx. =-9.5 to -10 mag for stars cooler than 15,000 K. We suggest that the observed luminosity limits are due primarily to the effects of large mass loss on the evolution of the most massive stars. The examples of eta Car and P Cyg suggest that mass-loss rates can be very rapid and unsteady--higher on the average than presently observed for most of the hot supergiants. The evolution of stars greater than 60 M/sub sun/ to cooler temperatures is consequently limited by instabilities and the accompanying high mass loss. An initial mass near 50--60 M/sub sun/ may be an empirical upper limit to the mass at which a star can evolve to the region of the M supergiants and probably accounts for the observed upper bound to the luminosities of the cooler supergiants.« less
  • In situ observations of 247 faint, distant stars with B-V less than or equal to 1.00 are reported near the north galactic pole in the galactic halo or thick disk. Omega velocities for all of the stars and luminosities, distances, and metallicities for half of the stars are given. Thick-disk and halo stars comprise the majority of the sample which is divided into three populations. The specific metallicities and velocity dispersions of the thin disk, thick disk, and the halo are presented, and speculations are made for the stars that comprise a more distant, faint halo. The stars in themore » thick disk are described as red and old, and two faint stars are employed to estimate the mass of the galaxy. About 10 percent of the observed stars are in fact galaxies, although none of the motions of the halo, O, A, B, or zero-metal stars show any evidence of imbalance. The possibility that a halo moving group exists within the sample is shown to be improbable. 80 refs.« less
  • Based on long baseline (5-7 years) multi-epoch HST/ACS photometry, used previously to measure the proper motion of M31, we present the proper motions (PMs) of 13 main-sequence Milky Way halo stars. The sample lies at an average distance of r {approx_equal} 24 kpc from the Galactic center, with a root-mean-square spread of 6 kpc. At this distance, the median PM accuracy is 5 km s{sup -1}. We devise a maximum likelihood routine to determine the tangential velocity ellipsoid of the stellar halo. The velocity second moments in the directions of the Galactic (l, b) system are {sup 1/2}= 123{sup +29}{submore » -23} km s{sup -1}, and {sup 1/2}= 83{sup +24}{sub -16} km s{sup -1}. We combine these results with the known line-of-sight second moment, {sup 1/2}= 105{+-}5 km s{sup -1}, at this (r) to study the velocity anisotropy of the halo. We find approximate isotropy between the radial and tangential velocity distributions, with anisotropy parameter {beta}= 0.0{sup +0.2}{sub -0.4}. Our results suggest that the stellar halo velocity anisotropy out to r {approx} 30 kpc is less radially biased than solar neighborhood measurements. This is opposite to what is expected from violent relaxation, and may indicate the presence of a shell-type structure at r {approx} 24 kpc. With additional multi-epoch HST data, the method presented here has the ability to measure the transverse kinematics of the halo for more stars, and to larger distances. This can yield new improved constraints on the stellar halo formation mechanism, and the mass of the Milky Way.« less
  • Building on the legacy of the Sloan Digital Sky Survey (SDSS-I and II), SDSS-III is a program of four spectroscopic surveys on three scientific themes: dark energy and cosmological parameters, the history and structure of the Milky Way, and the population of giant planets around other stars. The Baryon Oscillation Spectroscopic Survey (BOSS) will measure redshifts of 1.5 million massive galaxies and Ly{alpha} forest spectra of 150,000 quasars, using the baryon acoustic oscillation (BAO) feature of large scale structure to obtain percent-level determinations of the distance scale and Hubble expansion rate at z < 0.7 and at z {approx} 2.5.more » SEGUE-2, a now-completed continuation of the Sloan Extension for Galactic Understanding and Exploration, measured medium-resolution (R = {lambda}/{Delta}{lambda} 1800) optical spectra of 118,000 stars in a variety of target categories, probing chemical evolution, stellar kinematics and substructure, and the mass profile of the dark matter halo from the solar neighborhood to distances of 100 kpc. APOGEE, the Apache Point Observatory Galactic Evolution Experiment, will obtain high-resolution (R {approx} 30,000), high signal-to-noise ratio (S/N {ge} 100 per resolution element), H-band (1.51 {micro}m < {lambda} < 1.70 {micro}m) spectra of 10{sup 5} evolved, late-type stars, measuring separate abundances for {approx} 15 elements per star and creating the first high-precision spectroscopic survey of all Galactic stellar populations (bulge, bar, disks, halo) with a uniform set of stellar tracers and spectral diagnostics. The Multi-object APO Radial Velocity Large-area Survey (MARVELS) will monitor radial velocities of more than 8000 FGK stars with the sensitivity and cadence (10-40 m s{sup -1}, {approx} 24 visits per star) needed to detect giant planets with periods up to two years, providing an unprecedented data set for understanding the formation and dynamical evolution of giant planet systems. As of January 2011, SDSS-III has obtained spectra of more than 240,000 galaxies, 29,000 z {ge} 2.2 quasars, and 140,000 stars, including 74,000 velocity measurements of 2580 stars for MARVELS. In keeping with SDSS tradition, SDSS-III will provide regular public releases of all its data, beginning with SDSS Data Release 8 (DR8) in January 2011.« less