Volume 547, November 2012
|Number of page(s)||13|
|Section||Galactic structure, stellar clusters and populations|
|Published online||31 October 2012|
In this section we comment on the reported observations for our potential NEMP stars. If these stars are truly polluted with material from hot-bottom burning companions they are expected to be rich in nitrogen, sodium and magnesium (e.g. Stancliffe 2009). Further processing of C and N because of extra mixing may occur in the star if is evolved beyond first dredge up.
Alpha elements such as calcium and titanium are enhanced by about 0.3 − 0.4 dex in normal Galactic halo stars. Some lithium enhancement is also expected because of hot-bottom burning although there are many other processes through which it may be made (Iwamoto 2009; Stancliffe 2010). Heavy s-process elements such as barium and lead are not thought to be produced in intermediate-mass AGB stars so are not expected to be significantly enhanced in NEMP stars (Lugaro et al. 2012).
The typical 1-σ errors on abundance measurements [X/Fe] are 0.2−0.3 dex.
The abundances of carbon and nitrogen in CS22949-037 are reported by Norris et al. (2001), Depagne et al. (2002), Cayrel et al. (2004) and Cohen et al. (2008) who all find [N/Fe] > 1, [C/Fe] > 1 and [N/C] > 0.5. CS22949-037 is thus a CEMP and NEMP star. Spite et al. (2006) find 12C/13C = 4 which implies that the CN cycle is in equilibrium. These abundances are consistent with an intermediate mass AGB star undergoing hot bottom burning. The gravity of this star, log g = 1.5 − 1.7, implies that it has passed through first dredge-up and is at the point where extra mixing begins but has not yet had time to significantly alter surface abundances of carbon and nitrogen (see Figs. 2, 6, and 7 in Stancliffe et al. 2009).
Enhancements of sodium ( [Na/Fe] = + 1.57, Andrievsky et al. 2007) and magnesium ( [Mg/Fe] = + 1.55, Andrievsky et al. 2010) also suggest hot bottom burning is active. Barium is deficient ( [Ba/Fe] = −0.84, Norris et al. 2001; − 0.66, Cohen et al. 2008; − 0.5, Andrievsky et al. 2011) and strontium is mildly enhanced ( [Sr/Fe] = + 0.18, Cohen et al. 2008; +0.17, Andrievsky et al. 2011). These abundances fit the canonical view that most s-process enhancement occurs in low-mass AGB stars.
This star is enriched in carbon and nitrogen with [C/Fe] = + 2.05 and [N/Fe] = + 3.06 according to Aoki et al. (2007) but in contrast Johnson et al. (2007) find [C/Fe] = [N/Fe] = + 1.15. The discrepancy in nitrogen abundances may well be due to the respective use of CN and NH as abundance indicators. We can only conclude that this system may be a NEMP star. The star is also enhanced in magnesium ( [Mg/Fe] = 0.65) and barium ( [Ba/Fe] = 0.86).
If we assume that the molecular abundance measurements of Sivarani et al. (2006), [CH/Fe] = 1.59, [CN/Fe] = 3.07 and [NH/Fe] = 3.00, are indicative of elemental abundances then this star satisfies our (C)NEMP criteria. With log g = 4.0 it is unevolved and there is no mixing on the RGB. Sodium is enhanced, [Na/Fe] = 1.20, but magnesium is not, [Mg/Fe] = 0.40, especially when compared to other alpha elements such as calcium and titanium which show similar enhancements. CS29528-041 is rich in lithium, log ϵLi = 1.71, and barium [Ba/Fe] = 0.97.
CS30314-067 classifies as a NEMP star according to the abundances determined by Aoki et al. (2002), [C/Fe] = + 0.5 and [N/Fe] = + 1.20, but Johnson et al. (2007) instead find [C/Fe] = + 0.25 and [N/Fe] = + 0.50. Some of the discrepancy may result from the different nitrogen abundance indicators used and the different adopted gravities, although the Aoki et al. (2002) study also has greater resolution.
Masseron et al. (2006) have suggested that CS30322-023 is an AGB star because its gravity is low, log g = −0.3, but this is contradicted by Aoki et al. (2007) who claim log g = 1.0. In either case extra mixing on the RGB may have occurred. Both Masseron et al. (2006) and Aoki et al. (2007) claim [C/Fe] = + 0.6 and significant nitrogen enrichment ( [N/Fe] = + 2.81 and + 2.47 respectively) which suggest this is a NEMP star. Its abundances of sodium, magnesium and heavy elements are also enhanced ( [Ba/Fe] ≈ + 0.6 and [Pb/Fe] = + 1.49).
HD 25329 is an unevolved star (log g = 4.6) with [C/Fe] = 0.1, [N/Fe] = + 1.0 and 12C/13C > 40 (Fulbright 2000, Gratton et al. 2000 and Gratton et al. 2003). Its only slight enhancements in sodium and magnesium ( [Na/Fe] = + 0.24 and [Mg/Fe] = + 0.59; Gratton et al. 2003) suggest it has not been polluted by a hot bottom burning AGB star. The origin of nitrogen in this object remains unexplained.
The metallicity of HD 206983 is rather large, [Fe/H] = −0.99 (Masseron et al. 2010), suggesting it is a nitrogen-rich CH-star rather than a NEMP star. This is supported by an enhancement of barium, [Ba/Fe] = + 0.92 (Masseron et al. 2010).
The carbon- and nitrogen-rich star HE0400-2030, with [C/Fe] = + 1.14 and [N/Fe] = + 2.75 according to high-resolution observations (R = 50, 000) of Aoki et al. (2007), qualifies as a CNEMP star. Lower-resolution observations (R = 20, 000) of Lucatello et al. (2006) find less enhancement, [N/Fe] = + 1.0 and [C/Fe] = + 0.8. Sodium ( [Na/Fe] = + 0.71), magnesium ( [Mg/Fe] = + 0.62) and barium ( [Ba/Fe] = + 1.64) are also enhanced.
If this star accreted from a hot bottom burning companion, then the accreted material must have been diluted during the main sequence (e.g. by thermohaline mixing, Stancliffe et al. 2007), as the sodium and magnesium abundances are lower than predicted by theoretical models.
Cohen et al. (2006) find [C/Fe] = + 1.63 and [N/Fe] = + 2.48 for HE1031-002, i.e. it is a CNEMP star. With log g = 2.2 it is a moderately evolved giant and should have finished first dredge up without significant further extra mixing. Magnesium is slightly enhanced [Mg/Fe] = + 0.5 but sodium is not measured. Calcium, heavy s-process elements and lead are enhanced, [Ca/Fe] = + 1.12, [hs/ls] > + 1 and [Pb/Fe] = + 2.66, suggest an odd evolutionary history for this star which does not necessarily involve a hot bottom burning companion.
The most extensive set of abundance measurements for the NEMP star HE1337+0012 (also known as G64–12) is provided by Aoki et al. (2006) who find [C/Fe] = + 0.49 and [N/Fe] = + 1.42. The sodium abundance is low, [Na/Fe] = −1.1, so HE1337+0012 probably did not accrete material from a hot bottom burning companion despite its NEMP status.
This star is in the samples of both Cohen et al. (2006) and Masseron et al. (2010) but the authors disagree on its surface gravity with log g = 3.5 and 2.0 respectively. This gives incompatible abundances of [C/Fe] = + 2.33 and [N/Fe] = + 2.94 according to Masseron et al. (2010) and [C/Fe] = + 1.83 and [N/Fe] = + 1.73 from Cohen et al. (2006). Given the similar resolution of the studies it is unclear which is correct.
Lucatello et al. (2006) find this is an unevolved (log g > 4) CNEMP star with [N/Fe] = + 2.5 and [C/Fe] = + 1.7. Barklem et al. (2005) measure log g = 4.59, [C/Fe] = + 1.45 and [Sr/Fe] = −0.47. The lack of further abundance measurements prevents a reliable determination of the origin of the nitrogen in this star.
HE2150-0825 and HE2253-4217 are NEMP stars according to the carbon and nitrogen abundances measured by Lucatello et al. (2006) who find [C/Fe] = +0.3 and [N/Fe] = +1.3, and [C/Fe] = +1.0 and [N/Fe] = +1.5 respectively. The status of HE2253-4217 as a NEMP is marginal with [N/C] = 0.5, while HE2150-0825 has a low total CN abundance, [(C + N)/Fe] = 0.76.
Neither CS29528-028 nor SDSS1707+58 has measured nitrogen, so they are not classed as NEMP stars, but their sodium and magnesium abundances are unusual. CS29528-028 is an unevolved star with log g = 4.0. Aoki et al. (2007) report that CS29528-028 has [Na/Fe] = + 2.68 and [Mg/Fe] = + 1.69 despite a normal calcium abundance ([Ca/Fe] = + 0.46). Its barium abundance is [Ba/Fe] = + 3.27.
SDSS1707+58 is similarly enhanced with [Na/Fe] = + 2.71 and [Mg/Fe] = + 1.13 as well as [Ba/Fe] = + 3.40 (Aoki et al. 2008). This star was recently found to be an RR Lyr star by Kinman et al. (2012), who also find a lower metallicity [Fe/H] = −2.92 and somewhat lower sodium and barium abundances.
Based on their sodium and magnesium abundances, these stars are quite possibly polluted by a companion undergoing hot bottom burning, although their barium enhancements do not support this view. Unfortunately, until their nitrogen abundance and carbon isotopic ratios are measured the status of these stars remains unknown.
© ESO, 2012
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