New results on the Galactic Center Helium stars

Abstract

The cluster of helium stars around Sgr A* has been re‐observed with the BEAR spectro‐imager on CFHT, in the 2.06 μm helium line, at a spectral resolution of 52 km s^(–1) and on a field of ≃40″. This new analysis confirms and completes a previous study at a spectral resolution of 74 km s^(–1) and on a smaller field of 24″ , corresponding to the central parsec (Paumard et al. 2001). Nineteen stars are confirmed as helium stars. These observations led to a clear differentiation between two groups of hot stars based on their emission linewidth, their magnitude and their positions relative to Sgr A*. The first class of 6 members is characterized by narrow‐line profiles (FWHM ≃200 km s^(–1)) and by their brightness. The other, fainter in K by an average of 2 mag, has a much broader emission component of width ≃1,000 km s^(–1). Several of the emission lines show a P Cygni profile. From these results, we propose that the narrow‐line group is formed of stars in the LBV phase, while the broad‐line group is formed of stars in or near the WR phase. The division into two groups is also shown by their spatial distribution, with the narrow‐line stars in a compact central cluster (IRS 16) and the other group distributed at the periphery of the central cluster of hot stars. HST‐NICMOS data in Paα (1.87 μm) of the same field reveal a similar association. The identification of the Paα counterpart to the He I stars provides an additional element to characterize the two groups. Bright Paα emitters are found generally associated with the narrow‐line class stars while the weak Paαemitters are generally associated with the broad‐line stars. A few particular cases are discussed. This confirms the different status of evolution of the two groups of massive, hot stars in the central cluster. As a by‐product, about 20 additional candidate emission stars are detected in the central, high‐resolution 19″ field from the NICMOS data.