X-ray emission regimes and rotation sequences in M 35

Abstract

Context. Late-type stars in young open clusters show two kinds of dependence of their X-ray emission on rotation. They also tend to group into two main sub-populations that lie on narrow sequences in diagrams where their rotation periods are plotted against their (B − V) colour indices. A correlation between these two regimes of X-ray emission and the rotation sequences has been recently observed in the M 34 open cluster. Sun-like M 34 stars also show a drop of their X-ray to bolometric luminosity ratio by about one order of magnitude at a Rossby number of about 0.3. Aims. The present study looks for similar connections between X-ray activity and rotation in an other open cluster. The aim is to consolidate a model of X-ray activity evolution on the main sequence and to provide observational constraints on dynamo processes in the interiors of late-type stars. Methods. The paper compares XMM-Newton measurements of X-ray stellar emission in M 35 with X-ray luminosity distributions derived from rotation period measurements assuming either an X-ray regime transition at a critical Rossby number or a correlation between X-ray emission regimes and rotation sequences. Results. This second hypothesis could account for the low number of M 35 stars detected in X-rays. A model of X-ray activity evolution is proposed based on the correlation. One major output is that the transition from saturated to non-saturated X-ray emission occurs at Rossby numbers between about 0.13 and 0.4 for each star depending on its mass and initial period of rotation on the ZAMS. This prediction agrees with observations of stellar X-ray emission in M 34. It explains the large range of X-ray luminosities observed among Sun-like stars in young open clusters. Conclusions. I conclude that the correlation between X-ray emission regimes and rotation sequences could be a fundamental property of the early evolution of stellar magnetic activity on the main sequence. I argue that the angular momentum redistribution mechanism(s) responsible for the transition between rotation sequences could result in a changing mixture of dynamo processes occurring side by side, possibly including a turbulent dynamo at high rotation rate and an interface-type dynamo whose efficiency decreases progressively at later stage of stellar evolution when rotation dies away.

Stellar activity with LAMOST – II. Chromospheric activity in open clusters

“…Barnes 2003b;Gondoin 2012Gondoin , 2013. In our work, we detected a similar correspondence in Hα emission among cool stars in Pleiades, Praesepe and Hyades, as discussed in previous sections.…”

“…Such a turbulent dynamo can naturally explain the weak dependency of activity on rotation among fast rotators in the saturation regime. Moreover, the generation of small-scale magnetic field supports some observational behaviour shown by very active stars, e.g., large spot coverage of stars in young open clusters may be due to many small and randomly located spots on the stellar photosphere , 2013Fang et al 2016), and the possibility of multiple small flares such as nanoflares that might be responsible for the departures of relation between various chromospheric emissions among very active late-K and M dwarfs reported by previous studies and discussed above in current work. We thus conclude that a turbulent-like dynamo dominates in fast rotating cool stars in the saturated regime.…”

“…In fact, Gondoin (2012Gondoin ( , 2013 showed the evidence for transition from saturated to non-saturated X-ray emission that occurs at Ro between ∼0.13 and 0.4 in open clusters M34 and M35. Indeed, this transition scenario could describe the scatter in Ro region of 0.1-0.4.…”

Wiyn Open Cluster Study. Lxvi. Spectroscopic Binary Orbits in the Young Open Cluster M35 (Ngc 2168)

“…0 ″ . Modeling by Gondoin (2013) shows that beyond 4. 0 ″ , the number of spurious optical counterpart matches continues to increase while the number of reliable counterpart matches has reached its maximum.…”

“…M35 has been the subject of several photometric studies (Sung & Bessell 1999;von Hippel et al 2002;Kalirai et al 2003), proper-motion studies (Cudworth 1971;McNamara & Sekiguchi 1986;McNamara et al 2011), studies of stellar rotation and tidal evolution in binary stars (Meibom & Mathieu 2005;Meibom et al 2006Meibom et al , 2007, a study of X-ray sources (Gondoin 2013), abundance studies (Barrado y Navascués et al 2001a ;Steinhauer & Deliyannis 2004), an investigation of the mass function (Barrado y Navascués et al 2001b), a search for variable stars in the field (Hu et al 2005), and several studies of its white dwarf population (Williams et al 2004(Williams et al , 2006(Williams et al , 2009. The first paper in this series, Geller et al (2010), presented our initial radial-velocity membership study of M35, upon which we expand here.…”