1. XMM-Newton observations of open clusters and star forming regions R. Pallavicini and E. Franciosini INAF- Osservatorio Astronomico di Palermo, Italy S. Randich Osservatorio Astrofisico di Arcetri, Florence, Italy XMM-Newton EPIC Observations of Stellar Clusters and Star Forming Regions R. Pallavicini and E. Franciosini Osservatorio Astronomico di Palermo, Italy S. Randich Osservatorio Astrofisico di Arcetri, Florence, Italy EPIC Consortium Meeting, Palermo, 14-16 October 2003

2. Why observing stellar clusters in X-rays Open clusters constitute homogeneous samples of stars with approximately the same age, distance and chemical composition They are fundamental tools to study stellar structure and evolution as well as the dependence of chromospheric and coronal activity on stellar rotation and dynamo-generated magnetic fields The X-ray emission of late-type stars in clusters is in fact a strong function of magnetic activity as measured e.g. by the Rossby number (a combination of rotation and convection zone properties) Since late-type stars suffer magnetic braking during their evolutionary history, coronal activity is a decreasing function of age through the influence of stellar rotation on the dynamo mechanism X-ray sources in star forming regions and in young clusters provide therefore a powerful tool to identify PMS stars and cluster members

4. The cluster around sigma Orionis Equivalent MOS exposure time for the combined image: 217 ksec, sensitivity at center > 2.5 x 10 28 erg/sec 241 X-ray sources detected with ML>10, 86 identified with cluster members, including the hot stars  Ori AB and E, and 7 very-low mass stars (later than M6) including the brown-dwarf candidate S Ori 25 and the planetary mass object S Ori 68 Cluster discovered by ROSAT around the O9.5V star  Ori. age 2-5 Myr d = 350 pc

5. An X-ray flare from the hot star  Ori E A similar flare seen with ROSAT by Groote & Schmitt (2003) who attributed it to the hot star. We believe it more likely that the flare originated from an unseen late-type companion of the hot star because of the unusually high temperature of the quiescent preflare corona Flare spectrum: T1= 0.75 keV, T2=3.4 keV, EM2/EM1 = 5 Quiescent spectrum : T1= 0.3 keV, T2=1.3 keV, EM2/EM1= 1

6. EPIC spectra of sigma Ori stars  Ori E B2Vp Source 4 K3  Ori AB O9.5 V Source  

7. Rotationally modulated source in the  Ori cluster The MOS light curve of source 4 (Sp. K3) appears to be rotationally modulated with a period of about 9 hours due to surface activity.

8. EPIC MOS observation of the T Tauri star SU Aur The MOS field containing about 130 X-ray sources mostly PMS stars The light curve of the central source SU Aur (a classical T Tauri star) showing frequent flaring

9. The EPIC MOS spectrum of SU Aur Exp. Time 130 ks T1= 0.67 keV EM1= 1.02 E53 T2 = 1.67 keV EM2 = 2.24 E53 T3 = 4.75 keV EM3 =1.64 E53 Fe = 0.6 solar

10. Is Praesepe really different from the Hyades ? Praesepe is a cluster with about the same age as the Hyades and slightly lower metallicity. On the basis of ROSAT observations a much lower detection rate of cluster sources was reported for Praesepe than for the Hyades (Randich & Schmitt 1995) suggesting that a cluster of a given age may not be representative of all clusters with the same age. This discrepancy could not be explained on the basis of different distributions of rotation rates or as due to contamination by non-members. Age 600 Myr, metallicity about solar Combined exp. time : 279 ks Sensitivity at center: 5 x 10 27 erg/s (a factor 4 better than ROSAT)

11. The XLDF of Praesepe and Hyades seen by ROSAT and XMM EPIC From XMM-Newton dataFrom ROSAT data Praesepe might result from the merging of two clusters of different ages

12. Spectra and variability of Praesepe stars XMM-Newton vs. ROSAT EPIC PN spectra of the 3 brightest sources in the Praesepe field

13. XMM EPIC observations of the  Persei cluster Cluster age about 50 Myr, d=190 pc 275 ks combined exp time, 154 X-ray sources detected with ML > 10: only 13 cluster members in the XMM field, all but one detected (sensitivity at field center : 7x10 27 erg/s, an order of magnitude higher than the previous ROSAT observations). XMM-Newton vs. ROSAT

14. A Hyades field around VB71 (Theta 1 Tau) 135 X-ray sources detected in the field, of which only 4 belonging to the cluster. The flaring source VA 479 is a field M dwarf. Time variability of Hyades and field stars

15. Conclusions and future work XMM-Newton EPIC observations of open clusters and star forming regions are providing new insights into the physics of magnetic activity in stars. With its combination of high sensitivity and good spectral resolution EPIC observations (in combination with RGS observations for the brightest sources) allow investigating the coronal properties of stars over a wide range of ages (from a few million years to about 1 Gyr). The GT observations discussed here, together with those of other GT and GO programmes, are starting to cover the full age-metallicity plane of nearby open clusters. They allow addressing the question whether a cluster of a given age is representative of all clusters with the same age. XMM-Newton is sensitive enough to allow also the investigation of coronal emission in clusters older than 1 Gyr (e.g. solar-type stars in the 2 Gyr cluster NGC 752 or active binaries in the 5 Gyr cluster M67).