Detailed Chemical Analysis of Two Giants in the SGR DSPH

2 Pages·0.05 MB·English

by  P. Bonifacio

DETAILED CHEMICAL ANALYSIS OF TWO GIANTS IN THE
SGR DSPH
1
P. BONIFACIO,P. MOLARO
0
Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11,
0
2 34131 - Trieste, Italy
E-mail: [email protected], [email protected]
n
a
J The8mclasstelescopes allowforthefirsttimetostudystarsofexternalgalaxies
with the same resolution and S/N ratio which has been so far used for Galactic
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stars. It is quite likely that this study will shake some of our current beliefs.
2 In this poster we highlight some of the results which have been obtained for two
giantsintheSgrdSphthankstotheUVESspectrographontheESO8.2mKueyen
1 telescope. Further details on the observations and data analysis maybe foundin
v Bonifacioetal1.
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4 1 Results
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0 Thetwostarsturnedouttoberathermetal-richwith[Fe/H]≈−0.3. Thiswas
1 somewhatasurprisesincethestarshaveroughlythesameV−I colourbutdif-
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ferinV by0.18magandwerethoughttoberepresentativeofthespreadinSgr
/
h RGBwhichcommon–wisdomhasattributedtoaspreadinmetallicity2,3,4,5,6,7.
p
GiventhesteepnessoftheRGBinthiscolourrangeitmaywellbethatthetwo
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o starslie infactonthe sameisochroneandtheirV −I coloursaredifferentbut
r photometric errors of ≈0.04 mag make them equal. However there is another
t
s possible explanation whose consequences are rather intriguing and cannot be
a
easily discarded: the two stars havea different distance. A difference of about
:
v 2 kpc would be enough to explain the difference in V mag. This value is not
Xi unreasonablesinceIbataetal5estimatealine–of–sightdepthofabout1.2kpc.
Fromourspectrawemayfindevidencewhichfurthersupportsthispossibility:
r
a the Na I D lines of the brighter star, # 143, appear perfectly symmetric with
no hintofthe presenceofinterstellarcomponentsatthe radialvelocityofSgr,
whilethefainterstar,#139,showsaweakbutdefinite asymmetryinbothNa
I D lines, which may be interpreted as due to interstellar material associated
with Sgr.
ThehighmetallicityfoundforbothstarsimpliesthatSgrhasundergonea
high levelofchemicalprocessing;it alsosuggeststhat a sizeable populationof
Sgris this metal–rich. The abundance patternis interesting,althoughmostof
theelementswithA≤39areconsistentwithinerrorwiththesolarratio,taken
at face value they are all slightly sub-solar. On the other hand Ba and more
massive elements show over-solar ratios. The pattern may be summarized as
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follows: Na and Sc are over-deficient with respect to iron by about 0.4 dex;
heavy neutron–capture elements Ba, La, Ce, Nd, Eu are over–abundant with
respect to iron by 0.3 to 0.5 dex; Y is over-deficient with respect to iron by
about 0.4 dex.
It is interesting to compare the abundances of these two Sgr giants with
thoseobtainedbyShetroneetal8forthreeotherLocalGroupGalaxies(Draco,
Ursa Minor and Sextans). One should also keep in mind that Sagittarius is
considerably more massive than the other three galaxies and this is, perhaps,
thekeytounderstanditshigh–metallicitypopulation. The[Mg/Fe]ratioseems
to drop, with increasing [Fe/H], faster in Local Group Galaxies than it does
in the Galactic Halo or disk and this could be the signature of a very low
or episodic star formation. It shall be very interesting to see what is the
[Mg/Fe] ratio of the metal–poor population of Sagittarius, when its existence
isconfirmed. DampedLymanαsystems,observedinabsorptionintheline–of–
sightofQSO’shavemetallicitiesintherange-2,-1andshow[α/Fe]ratioswhich
are solar9,10. Could Damped Lyman α systems be linked to dwarf galaxies ?
This possibility is appealing, although these galaxies are now devoid of gas,
they musthavebeenrelativelyrichingaswhenthe starsweobservenowwere
formed.
The over-deficiency of Na seems a common occurrence in Local Group
galaxies, although there is no definite trend and a large scatter. On the
other hand Y is always over-deficient with very few exceptions and this over-
deficiency is mirrored by an overabundance of heavy elements.
References
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