The most massive stars in the Arches cluster

Abstract

Aims. We study a sample composed of 28 of the brightest stars in the Arches cluster. Our aim is to constrain their stellar and wind properties and to establish their nature and evolutionary status. Methods. We analyze K-band spectra obtained with the integral field spectrograph SINFONI on the VLT. Atmosphere models computed with the code CMFGEN are used to derive the effective temperatures, luminosities, stellar abundances, mass loss rates and wind terminal velocities. Results. We find that the stars in our sample are either H-rich WN7-9 stars (WN7-9h) or supergiants, two being classified as OIf + . All stars are 2-4 Myr old. There is marginal evidence for a younger age among the most massive stars. The WN7-9h stars reach luminosities as large as 2 × 10 6 L ⊙ , consistent with initial masses of ∼ 120 M ⊙ . They are still quite H-rich, but show both N enhancement and C depletion. They are thus identified as core H-burning objects showing products of the CNO equilibrium at their surface. Their progenitors are most likely supergiants of spectral types earlier than O4-6 and initial masses > 60 M ⊙ . Their winds follow a well defined modified wind momentum -luminosity relation (WLR): this is a strong indication that they are radiatively driven. Stellar abundances tend to favor a slightly super solar metallicity, at least for the lightest metals. We note however that the evolutionary models seem to under-predict the degree of N enrichment.

Spatial distribution of Galactic Wolf–Rayet stars and implications for the global population

“…In Table 7 we show the range and standard deviation in MK S within a WR spectral type. Ranges are typically < 1, although WN9 stars in the Arches cluster show a larger range due to the anomalously faint WR 102aj; we classify this star based on a K-band spectrum published by Martins et al (2008) and thus can only claim a ±1 accuracy on the WN9 spectral type. Typical standard deviations of 0.3-0.6 mag in MK S suggest that WR absolute magnitudes intrinsically vary by at least ± 0.3 within a spectral type.…”

KMOS view of the Galactic centre

“…Ramírez et al (2000) found that the iron abundance Fe/H of the Galactic centre stars is roughly solar. However, the α-element abundance is super-solar (Cunha et al 2007;Martins et al 2008). Paumard et al (2006) and Lu et al (2013) showed that the young population in the Galactic centre is 3−8 Myr old.…”

The binary nature of the Galactic centre X-ray source CXOGC J174536.1-285638

“…β = 0.8 was adopted by Martins et al (2008), for which similar synthetic spectra are predicted, except for somewhat weaker ; with the exception of oxygen for which the value given by Asplund et al 2004, was used). Spectroscopic fits to IRTF/SpeX observations are presented in Fig.…”

“…Arches WN9h members F2, F6 and F7 are of particular interest in view of their spectroscopic similarities to CXO J1745-28 (recall Fig. 2), for which Martins et al (2008) estimates, Brγ-derived surface hydrogen contents in the range 40% ≤ X H ≤ 55% are obtained for F2, F6 and F7, versus 25% for CXO J1745-28, while the nitrogen mass fraction obtained for CXO J1745-28 is also at the lower end of the range observed for the Arches members; 1.2% versus 1.1-2.8% respectively.…”