Magnetic Activity of Pre-main Sequence Stars near the Stellar-Substellar Boundary
aa r X i v : . [ a s t r o - ph . S R ] D ec Young Stars & Planets Near the SunProceedings IAU Symposium No. 314, 2015J. H. Kastner, B. Stelzer, & S. A. Metchev, eds. c (cid:13) Magnetic Activity of Pre-main SequenceStars near the Stellar-Substellar Boundary
David A. Principe , , Joel. H. Kastner and David Rodriguez N´ucleo de Astronom´ıa, Facultad de Ingenier´ıa, Universidad Diego Portales, Santiago, Chileemail: [email protected] Millennium Nucleus Protoplanetary Disks, Chile Center for Imaging Science, School of Physics & Astronomy, and Laboratory forMultiwavelength Astrophysics, Rochester Institute of Technology, Rochester, NY 14623, USA Departamento de Astronom´ıa, Universidad de Chile, Casilla 36-D, Santiago, Chile
Abstract.
X-ray observations of pre-main sequence (pre-MS) stars of M-type probe coronalemission and offer a means to investigate magnetic activity at the stellar-substellar boundary.Recent observations of main sequence (MS) stars at this boundary display a decrease in fractionalX-ray luminosity ( L X / L bol ) by almost two orders of magnitude for spectral types M7 and later.We investigate magnetic activity and search for a decrease in X-ray emission in the pre-MSprogenitors of these MS stars. We present XMM-Newton X-ray observations and preliminaryresults for ∼
10 nearby (30-70 pc), very low mass pre-MS stars in the relatively unexplored agerange of 10-30 Myr. We compare the fractional X-ray luminosities of these 10-30 Myr old starsto younger (1-3 Myr) pre-MS brown dwarfs and find no dependence on spectral type or agesuggesting that X-ray activity declines at an age later than ∼
30 Myr in these very low-massstars.
Keywords.
Stars: formation, Stars: magnetic field, Stars: late-type, X-rays: stars.
1. Introduction
The early evolution of magnetic activity in very low mass pre-MS stars –stars of mid-M-type, which lie near the H-burning limit of 0.08 M ⊙ – is very poorly understood. Yetunderstanding their pre-MS evolution is crucial for determining the emerging differencesbetween very low-mass MS stars and brown dwarfs. X-ray emission offers a means toindirectly probe the effects of internal and surface magnetic activity in both pre-MS andMS stars alike (Vidotto et al. 2014). Pre-MS and MS M-type stars are magnetically activeand thus can be bright X-ray sources, as indicated by their high values of ( L X / L bol ∼ − ). However, observations of nearby late M-type MS stars suggest that stars of ∼ M7and later appear to be under luminous in X-rays (e.g., L X / L bol ∼ − ; Berger et al.2010). The narrow range of spectral types where these M stars become X-ray underluminous is roughly the same spectral type where a transition to predominantly neutralatmospheres occurs (Mohanty et al. 2002). Berger (2006) concluded that the decrease inX-ray activity (as well as H α ) toward late M-types is related to changes in magnetic fieldconfiguration or the decreasing ionization fractions in the atmospheres of these stars.By determining the age at which this dramatic decrease in X-ray activity occurs for M-type stars, we can gain insight into the early pre-MS stellar evolution of such stars whichlie at the low-mass-star/brown dwarf (H-burning) boundary. A recent survey combiningGALEX, 2MASS, WISE and catalog proper motions have revealed a population of nearbylate-M-type stars in the 10-30 Myr age range (Rodriguez et al. 2013) where X-ray activityof such stars has remained, until now, essentially unexplored.1 D. A. Principe, J. H. Kastner, & D. Rodriguez
2. Data and Preliminary Results
We performed XMM-Newton EPIC X-ray observations of 8 nearby ( <
70 pc) ∼ M5members of the Tuc-Hor and β Pic moving groups (ages ∼
30 Myr and 12 Myr, respec-tively; Rodriguez et al. 2013 and ref. therein). Stellar evolution models (D’Antona &Mazzitelli 1997) suggest pre-MS stars of this age and spectral type will evolve to becomeMS ∼ M7 (i.e., they may be progenitors of the under luminous MS M7 stars). Standardone and two temperature thermal plasma models were fit to the data to determine spec-tral parameters such as plasma temperature and L X . Bolometric luminosities for eachof our sources was estimated using their J band flux and the intrinsic colors of 5-30 Myrstars from Pecaut & Mamajek (2013). The fractional X-ray luminosity for each source isshown in Fig. 1 and compared to pre-MS stars of similar spectral type in younger (e.g1-3 Myr) star-forming regions.
3. Conclusions
We find no trend of decreasing fractional X-ray luminosity with age in these 10-30Myr ∼ M5 stars (Fig. 1). If MS stars of ∼ M7 and later are under luminous in X-rays,these preliminary results suggest that either X-ray activity decreases at ages later than ∼
30 Myr or that our sample of pre-MS stars is unusually magnetically active. The latterscenario may be more likely due to the fact that the sample from which we chose oursources required a GALEX UV detection (i.e., only M stars that were UV bright). UVemission is also an indicator of magnetic activity and thus our sample might be biasedtowards magnetically active (i.e., X-ray bright) pre-MS stars. More X-ray observationsof both UV bright and UV faint mid-to-late M-type pre-MS stars are required to explorethe age at which X-ray activity may diminish. A more detailed analysis of these datawill be presented in Principe et al. (2015), in prep.
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Figure 1.
Fractional X-ray luminosity as a function of M spectral subclass for ∼ ∼ ββ