IRS an outsider in the GC

The nature of the GC source IRS 8 (Becklin & Neugebauer 1975), one of the brightest compact mid-infrared sources in the Central cluster, was unknown until

Figure 13.5. New K-band high-resolution observations and model fits to the AF star (left) and 16NE (right). For the AF star our models indicate a slightly higher but still consistent temperature and luminosity with the values obtained by Najarro et al. (1994, 1997). For 16NE, the presence of metal lines implies a downward correction of up to 6,000 K to the temperature estimate from Najarro et al. (1997).

Figure 13.5. New K-band high-resolution observations and model fits to the AF star (left) and 16NE (right). For the AF star our models indicate a slightly higher but still consistent temperature and luminosity with the values obtained by Najarro et al. (1994, 1997). For 16NE, the presence of metal lines implies a downward correction of up to 6,000 K to the temperature estimate from Najarro et al. (1997).

adaptive-optics H- and K-band imaging revealed that the bulk of its infrared emission originates from a classic bowshock (Rigaut et al. 2003; Geballe et al. 2004). Geballe et al. (2004) showed that the IRS 8 bowshock is a straightforward consequence of the interaction of a dense and high-velocity wind from a hot star (hereafter IRS 8*) that is traversing moderately dense interstellar gas. To investigate the nature of the central source, we obtained mid-resolution (R ~ 900) K-band spectra using the Gemini adaptive-optics module ALTAIR to feed the near-infrared spectrograph NIRI; see also Geballe et al. (2006).

Figure 13.6 shows the resulting normalized K-band spectrum of IRS 8* compared with online-available K-band spectra from the Hanson et al. (1996) catalog for O stars ranging from O4 to O6.5 and various luminosity classes. The resolving powers for all template spectra have been degraded to 800 for direct comparison with the observed spectrum. From Figure 13.6 we judge that IRS 8* falls within the O5-O6.5 and III-If ranges, with likely O5-O6 spectral type and if luminosity class. Given the strong spectral similarities of IRS 8* to the O5-O6 supergiants in Cyg OB2 (see Figure 13.6), we computed model fits covering that parameter domain, drawing on our analysis of the Cyg OB2 stars for which UV, optical, and IR spectra are available (Najarro et al. in preparation). Figure 13.6 displays our best-fitting model (dashed line), which we obtained using the line-blanketing method presented in previous sections. See Geballe et al. (2006) for a thorough discussion of the analysis.

Of concern is the re-identification of the strong emission feature at 2.116 in IRS 8* which has been attributed in the past to C iii and N iii n = 8-7 transitions and is present over a very wide range of O spectral types and luminosities (Hanson

Figure 13.6. Spectral type determination of IRS 8* by comparison of the resulting normalized spectrum with K-band spectra from Hanson et al. (1996) degraded to a resolution of R = 800. Also displayed (dashed) is a model fit with stellar parameters corresponding to an O5.5 If star (see the text).

Figure 13.6. Spectral type determination of IRS 8* by comparison of the resulting normalized spectrum with K-band spectra from Hanson et al. (1996) degraded to a resolution of R = 800. Also displayed (dashed) is a model fit with stellar parameters corresponding to an O5.5 If star (see the text).

et al. 1996). Our investigation (Geballe et al. 2006) indicates that the 2.116^m feature in IRS 8* is dominated by O iii n = 8-7 transitions. Further, the O iii component of the 2.116^m feature depends largely on the oxygen abundance and only slightly on the gravity, effective temperature, wind density, and velocity field. Thus, this feature may be a powerful diagnostic of oxygen abundance, and therefore an important metal-abundance determiner, over a wide range of O spectral types (Najarro et al. in preparation). Using it we obtain an oxygen abundance of 0.8-1.1 times Solar in IRS 8*, which indicates that the cloud in which IRS 8* formed was of Solar metallicity.

Our analysis suggests that IRS 8*, although only 1 pc from the center, does not fit into the current picture of the Central cluster of hot stars. It is of much earlier spectral type than any of the stars classified by Paumard et al. (2006). Currently it is the only known OB star outside the central 0.5 pc region of the cluster. Figure 13.7 shows the position of IRS 8* (solid cross) as estimated from our model fits on the HR diagram compared with various evolutionary scenarios. The age of 2.8 Myr and absence of surface enrichment obtained with tracks of stars without rotation as used

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Figure 13.7. The position of IRS 8* (solid cross) in the HR diagram as estimated from model fits, compared with various evolutionary scenarios. The current location of IRS 8* is reached after 3.6 and 7.1 million years for the evolutionary cases of a single star (dashed) and massive close binary (solid), respectively. See Geballe et al. (2006) for a thorough discussion.

by Paumard et al. (2006) (Maeder & Meynet 2003) is clearly at odds both with the current estimate for the age of the GC cluster and with the abundance pattern derived from our models. The situation improves when evolutionary models accounting for rotation (dashed lines in Figure 13.7) are considered (3.5 Myr and CNO-processed material on the stellar surface). Except for the age, which is still well below the estimate obtained by Paumard et al. (2006), on using rotating models for a single-burst scenario we obtain stellar parameters fully consistent with those derived from our modeling.

The crucial question thus is whether this star is really much younger than the cluster and probes the existence of ongoing (or at least much more recent) star formation, or, on the contrary, the star is either an impostor or a cluster member that underwent a rejuvenation cure. A possible way out is provided if the star was originally a member of a massive close binary system. In such a case, we could be looking now at the secondary star, with the primary either having exploded as a supernova or being in an evolutionary phase during which it is much dimmer in the K band than the secondary. Using the models of Wellstein & Langer (1999) we found that for a massive close binary system with initial masses of 25M0 and

24M© (their model 10a) the current position of IRS 8* may be elegantly explained (see Geballe et al. (2006) for a thorough discussion) without contradiction with the age of the GC cluster (solid lines in Figure 13.7). Similar scenarios are a possible explanation for some of the overluminous He i objects in the central parsec.

6 Conclusions

Our result of Solar metallicity for the Central cluster, the Arches cluster, and the Quintuplet cluster runs counter to the trend in the disk (Rolleston et al. 2000; Smartt et al. 2001) but is consistent with the findings from cool-star studies (Carr et al. 2000; Ramirez et al. 2000). This may imply that the ISM in the disk does not extend inward to the GC, or that the GC stars are forming out of an ISM that has an enrichment history that is distinctly different from that in the disk. Our result is more consistent with the values found for the bulge (Frogel et al. 1999; Felzing & Gilmore 2000).

Acknowledgments

I would like to thank Don Figer, John Hillier, Rolf Kudritzki and, Tom Geballe for invaluable discussions, and acknowledge receipt of grants PNAYA-2003-02785-E and AYA2004-08271-C02-02.

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