X-SHOOTER OBSERVATIONS OF MAIN-SEQUENCE STARS IN THE GLOBULAR CLUSTER NGC 2808: FIRST CHEMICAL TAGGING OF A He-NORMAL AND A He-RICH DWARF

Journal Article · Wed Sep 01 00:00:00 EDT 2010 · Astrophysical Journal Letters

DOI:https://doi.org/10.1088/2041-8205/720/1/L41· OSTI ID:21452842

Bragaglia, A; Carretta, E ^[1]; Gratton, R G; Lucatello, S; D'Orazi, V ^[2]; Milone, A; Piotto, G ^[3]; Cassisi, S ^[4]; Sneden, C ^[5]

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1. INAF, Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna (Italy)
2. INAF, Osservatorio Astronomico di Padova, vicolo dell'Osservatorio 5, 35122 Padova (Italy)
3. Dipartimento di Astronomia, Universita di Padova, Vicolo dell'Osservatorio 2, I-35122 Padova (Italy)
4. INAF, Osservatorio Astronomico di Collurania, via M. Maggini, I-64100 Teramo (Italy)
5. Department of Astronomy and McDonald Observatory, University of Texas, Austin, TX 78712 (United States)

We present the first chemical composition study of two unevolved stars in the globular cluster NGC 2808, obtained with the X-shooter spectrograph at VLT. NGC 2808 shows three discrete, well-separated main sequences. The most accepted explanation for this phenomenon is that their stars have different helium contents. We observed one star on the bluest main sequence (bMS, claimed to have a high helium content, Y {approx}0.4) and the other on the reddest main sequence (rMS, consistent with a canonical helium content, Y = 0.245). We analyzed features of NH, CH, Na, Mg, Al, and Fe. While Fe, Ca, and other elements have the same abundances in the two stars, the bMS star shows a huge enhancement of N, a depletion of C, an enhancement of Na and Al, and a small depletion of Mg with respect to the rMS star. This is exactly what is expected if stars on the bMS formed from the ejecta produced by an earlier stellar generation in the complete CNO and MgAl cycles whose main product is helium. The elemental abundance pattern differences in these two stars are consistent with the differences in the helium content suggested by the color-magnitude diagram positions of the stars.

• OSTI ID: 21452842
• Journal Information: Astrophysical Journal Letters, Journal Name: Astrophysical Journal Letters Journal Issue: 1 Vol. 720; ISSN 2041-8205
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ABUNDANCE
CHEMICAL COMPOSITION
COLOR
ELEMENTS
EVOLUTION
FLUIDS
GASES
HELIUM
MAIN SEQUENCE STARS
NONMETALS
OPTICAL PROPERTIES
ORGANOLEPTIC PROPERTIES
PHYSICAL PROPERTIES
RARE GASES
STAR EVOLUTION
STARS