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

Title: Atypical Mg-poor Milky Way Field Stars with Globular Cluster Second-generation-like Chemical Patterns

Abstract

We report the peculiar chemical abundance patterns of 11 atypical Milky Way (MW) field red giant stars observed by the Apache Point Observatory Galactic Evolution Experiment (APOGEE). These atypical giants exhibit strong Al and N enhancements accompanied by C and Mg depletions, strikingly similar to those observed in the so-called second-generation (SG) stars of globular clusters (GCs). Remarkably, we find low Mg abundances ([Mg/Fe] < 0.0) together with strong Al and N overabundances in the majority (5/7) of the metal-rich ([Fe/H] ≳ −1.0) sample stars, which is at odds with actual observations of SG stars in Galactic GCs of similar metallicities. This chemical pattern is unique and unprecedented among MW stars, posing urgent questions about its origin. These atypical stars could be former SG stars of dissolved GCs formed with intrinsically lower abundances of Mg and enriched Al (subsequently self-polluted by massive AGB stars) or the result of exotic binary systems. We speculate that the stars Mg-deficiency as well as the orbital properties suggest that they could have an extragalactic origin. This discovery should guide future dedicated spectroscopic searches of atypical stellar chemical patterns in our Galaxy, a fundamental step forward to understanding the Galactic formation and evolution.

Authors:
; ; ; ;  [1]; ; ; ;  [2]; ;  [3];  [4];  [5];  [6];  [7];  [8]; ; ;  [9];  [10] more »; « less
  1. Departamento de Astronomía, Universidad de Concepción, Casilla 160-C, Concepción (Chile)
  2. Instituto de Astrofísica de Canarias, E-38205 La Laguna, Tenerife (Spain)
  3. Observatório Nacional, Rua Gal. José Cristino 77, Rio de Janeiro, RJ—20921-400 (Brazil)
  4. Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF (United Kingdom)
  5. New Mexico State University, Las Cruces, NM 88003 (United States)
  6. University of Texas at Austin, McDonald Observatory, Fort Davis, TX 79734 (United States)
  7. Centro de Investigaciones de Astronomía, AP 264, Mérida 5101-A (Venezuela, Bolivarian Republic of)
  8. Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States)
  9. Department of Astronomy, University of Virginia, Charlottesville, VA 22903 (United States)
  10. Department of Physics and JINA Center for the Evolution of the Elements, University of Notre Dame, Notre Dame, IN 46556 (United States)
Publication Date:
OSTI Identifier:
22654403
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal Letters; Journal Volume: 846; 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; ABUNDANCE; ALUMINIUM; CARBON; GALACTIC EVOLUTION; ISOCHRONOUS CYCLOTRONS; MAGNESIUM; METALLICITY; METALS; MILKY WAY; NITROGEN; RED GIANT STARS; STAR CLUSTERS

Citation Formats

Fernández-Trincado, J. G., Geisler, D., Tang, B., Villanova, S., Mennickent, R. E., Zamora, O., García-Hernández, D. A., Dell’Agli, F., Prieto, Carlos Allende, Souto, Diogo, Cunha, Katia, Schiavon, R. P., Hasselquist, Sten, Shetrone, M., Vieira, K., Zasowski, G., Sobeck, J., Hayes, C. R., Majewski, S. R., Placco, V. M., E-mail: jfernandezt@astro-udec.cl, E-mail: jfernandezt87@gmail.com, and and others. Atypical Mg-poor Milky Way Field Stars with Globular Cluster Second-generation-like Chemical Patterns. United States: N. p., 2017. Web. doi:10.3847/2041-8213/AA8032.
Fernández-Trincado, J. G., Geisler, D., Tang, B., Villanova, S., Mennickent, R. E., Zamora, O., García-Hernández, D. A., Dell’Agli, F., Prieto, Carlos Allende, Souto, Diogo, Cunha, Katia, Schiavon, R. P., Hasselquist, Sten, Shetrone, M., Vieira, K., Zasowski, G., Sobeck, J., Hayes, C. R., Majewski, S. R., Placco, V. M., E-mail: jfernandezt@astro-udec.cl, E-mail: jfernandezt87@gmail.com, & and others. Atypical Mg-poor Milky Way Field Stars with Globular Cluster Second-generation-like Chemical Patterns. United States. doi:10.3847/2041-8213/AA8032.
Fernández-Trincado, J. G., Geisler, D., Tang, B., Villanova, S., Mennickent, R. E., Zamora, O., García-Hernández, D. A., Dell’Agli, F., Prieto, Carlos Allende, Souto, Diogo, Cunha, Katia, Schiavon, R. P., Hasselquist, Sten, Shetrone, M., Vieira, K., Zasowski, G., Sobeck, J., Hayes, C. R., Majewski, S. R., Placco, V. M., E-mail: jfernandezt@astro-udec.cl, E-mail: jfernandezt87@gmail.com, and and others. 2017. "Atypical Mg-poor Milky Way Field Stars with Globular Cluster Second-generation-like Chemical Patterns". United States. doi:10.3847/2041-8213/AA8032.
@article{osti_22654403,
title = {Atypical Mg-poor Milky Way Field Stars with Globular Cluster Second-generation-like Chemical Patterns},
author = {Fernández-Trincado, J. G. and Geisler, D. and Tang, B. and Villanova, S. and Mennickent, R. E. and Zamora, O. and García-Hernández, D. A. and Dell’Agli, F. and Prieto, Carlos Allende and Souto, Diogo and Cunha, Katia and Schiavon, R. P. and Hasselquist, Sten and Shetrone, M. and Vieira, K. and Zasowski, G. and Sobeck, J. and Hayes, C. R. and Majewski, S. R. and Placco, V. M., E-mail: jfernandezt@astro-udec.cl, E-mail: jfernandezt87@gmail.com and and others},
abstractNote = {We report the peculiar chemical abundance patterns of 11 atypical Milky Way (MW) field red giant stars observed by the Apache Point Observatory Galactic Evolution Experiment (APOGEE). These atypical giants exhibit strong Al and N enhancements accompanied by C and Mg depletions, strikingly similar to those observed in the so-called second-generation (SG) stars of globular clusters (GCs). Remarkably, we find low Mg abundances ([Mg/Fe] < 0.0) together with strong Al and N overabundances in the majority (5/7) of the metal-rich ([Fe/H] ≳ −1.0) sample stars, which is at odds with actual observations of SG stars in Galactic GCs of similar metallicities. This chemical pattern is unique and unprecedented among MW stars, posing urgent questions about its origin. These atypical stars could be former SG stars of dissolved GCs formed with intrinsically lower abundances of Mg and enriched Al (subsequently self-polluted by massive AGB stars) or the result of exotic binary systems. We speculate that the stars Mg-deficiency as well as the orbital properties suggest that they could have an extragalactic origin. This discovery should guide future dedicated spectroscopic searches of atypical stellar chemical patterns in our Galaxy, a fundamental step forward to understanding the Galactic formation and evolution.},
doi = {10.3847/2041-8213/AA8032},
journal = {Astrophysical Journal Letters},
number = 1,
volume = 846,
place = {United States},
year = 2017,
month = 9
}
  • Cosmological simulations of Population III star formation suggest an initial mass function (IMF) biased toward very massive stars (M {approx}> 100 M{sub sun}) formed in minihalos at redshift z {approx}> 20, when the cooling is driven by molecular hydrogen. However, this result conflicts with observations of extremely metal-poor (EMP) stars in the Milky Way (MW) halo, whose r-process elemental abundances appear to be incompatible with those expected from very massive Population III progenitors. We propose a new solution to the problem in which the IMF of second-generation stars formed at z {approx}> 10, before reionization, is deficient in sub-solar massmore » stars, owing to the high cosmic microwave background temperature floor. The observed EMP stars are formed preferentially at z {approx}< 10 in pockets of gas enriched to metallicity Z {approx}> 10{sup -3.5} Z{sub sun} by winds from Population II stars. Our cosmological simulations of dark matter halos like the MW show that current samples of EMP stars can only constrain the IMF of late-time Population III stars, formed at z {approx}< 13 in halos with virial temperature T{sub vir} {approx} 10{sup 4} K. This suggests that pair instability supernovae were not produced primarily by this population. To begin probing the IMF of Population III stars formed at higher redshift will require a large survey, with at least 500 and probably several thousand EMP stars of metallicities Z {approx} 10{sup -3.5} Z{sub sun}.« less
  • Many observational studies have revealed the presence of multiple stellar generations in Galactic globular clusters. These studies suggest that second-generation stars make up a significant fraction of the current mass of globular clusters, with the second-generation mass fraction ranging from {approx}50% to 80% in individual clusters. In this Letter, we carry out hydrodynamical simulations to explore the dependence of the mass of second-generation stars on the initial mass and structural parameters and stellar initial mass function (IMF) of the parent cluster. We then use the results of these simulations to estimate the fraction f{sub SG,H} of the mass of themore » Galactic stellar halo composed of second-generation stars that originated in globular clusters. We study the dependence of f{sub SG,H} on the parameters of the IMF of the Galactic globular cluster system. For a broad range of initial conditions, we find that the fraction of mass of the Galactic stellar halo in second-generation stars is always small, f{sub SG,H} < 4%-6% for a Kroupa-1993 IMF and f{sub SG,H} < 7%-9% for a Kroupa-2001 IMF.« less
  • We present analyses of variability in the red giant stars in the metal-poor globular cluster NGC 6397, based on data obtained with the Hubble Space Telescope. We use a nonstandard data reduction approach to turn a 23 day observing run originally aimed at imaging the white dwarf population, into time-series photometry of the cluster's highly saturated red giant stars. With this technique we obtain noise levels in the final power spectra down to 50 parts per million, which allows us to search for low-amplitude solar-like oscillations. We compare the observed excess power seen in the power spectra with estimates ofmore » the typical frequency range, frequency spacing, and amplitude from scaling the solar oscillations. We see evidence that the detected variability is consistent with solar-like oscillations in at least one and perhaps up to four stars. With metallicities 2 orders of magnitude lower than those of the Sun, these stars present so far the best evidence of solar-like oscillations in such a low-metallicity environment.« less
  • Zinn in 1973 and 1977 and Norris and Zinn in 1977 showed that in M92 and several other metal-poor globular clusters the G bands (mostly due to CH) in the spectra of asymptotic giant branch (AGB) stars are systematically weaker than those found in the less highly evolved subgiant branch (SGB) stars. If carbon is depleted in the atmospheres of evolved stars because material at the base of the envelope, processed through the CN cycle, has been mixed with the material above, then the atmospheric nitrogen abundance should be correspondingly increased. In this paper we test the hypothesis that Cmore » and N abundances in M92 giants are negatively correlated as the evolutionary state becomes more advanced. We find that this simple hypothesis is not adequate to describe the complex behavior of C and N in the cluster giants.« less
  • We derive carbon and nitrogen abundances for 33 giant-star members of the globular cluster M15, thus extending the earlier work of Carbon et al. in M92 to another cluster of comparably low metallicity ((M/H)< or approx. =-2). We describe a technique for obtaining C and N abundances which is based on spectrophotometric indices that measure the strengths of NH, CN, and CH bands in low-resolution scanner spectra. The spectrophotometric indices are calibrated using scanner spectra of M92 giants stars in which estimates of C and N abundances had previously been obtained by the method of spectrum synthesis. The C abundancesmore » derived here for M15 giants are comparable in quality to those given for M92 giants previously; observational errors associated with the N abundances for M15 stars are typically about twice those derived for M92 stars.« less