Evolución IVA y Renta (%PIB)

Finally, we cross-matched all sources with theGALEX catalog of UV sources to investigate whether fast rotators would also normally be detected as chromospherically active stars from excess UV emission. The NASA GALEX mission is led by the California Institute of Technology and has been in operation since 2003. The primary purpose of the mission is to study the evolution of galaxies using ultra violet light. TheGALEX space telescope has

imaged a significant fraction (> 70%) of the sky in two ultraviolet bands: far-ultraviolet (FUV) which corresponds to wavelengths of 1344 to 1786 ˚A, and near ultraviolet (NUV) which corresponds to wavelengths of 1771 to 2831 ˚A. While the primary purpose was to obtain images of the young stars in nearby galaxies,GALEX has also detected and measured the UV brightness of many nearby stars.

Table 3.3: SBK2 Kinematics (Photometric) Field < vl >med. M ADvl < vb >med. M ADvb

(km s−1) (km s−1) (km s−1) (km s−1) 0 60 41 -22 36 1 -46 130 -77 73 2 -49 41 -8 36 3 -18 72 -51 60 4 66 53 -13 48 5 56 81 -50 63 6 -86 98 -23 83 7 -67 59 -29 54 8 72 100 -44 69 10 -61 109 -49 62 11 -73 66 -12 44 12 10 103 -62 62 13 65 55 -5 45 14 2 106 -83 78 15 -105 91 -4 77

Figure 3.5 is a color-color diagram for all SBK2 stars with a counterpart in the GALEX

source catalog. We utilize GALEX NUV magnitudes (when available) as well as optical G magnitudes from GAIA, and infrared J magnitudes from the two-micron all-sky survey (2MASS), to build this diagram. The black points are all SBK2 targets with GALEX

counterparts; the red circles are the fast rotators identified in this work. In this color-color diagram, most stars are expected to fall along a straight line, because the NUV-G and G-J

colors normally follow a linear correlation; this is noticeable in the 0.5<G-J<2.0 color range.2

We don’t see the standard relationship extending beyond G-J>2.0, because these stars are generally too red to show substantial NUV emission, and are thus too faint in the NUV to be detected by GALEX. However, the relationship at this point appears to split between two groups: those stars that continue to follow the normal trend (undetected) and a group of stars that show an excess of NUV emission and creates a “plateau” in the color-color diagram. This split occurs because an active star has more UV radiation than an inactive star of a similar mass. This higher flux (resulting in a lower number magnitude) produces a smaller, i.e. bluer, NUV-G color term, placing the active star lower on the color-color diagram. Indeed, the stars that we see follow this plateau trend are members of our fast rotator sample. We expect the fast rotators to fall in this region because their fast rotation creates a stronger magnetic dynamo producing more chromospheric activity, which creates excess NUV flux, and thus results in a bluer-than-normal NUV-G color for the specific G-J color of the star.

We assume a star to be an M dwarf if it has G−J > 1.8, following L´epine et al. (2013). We further define an M dwarf to be “active” if it has N U V −G < 9.0, indicative of a significant UV excess. Using these criteria, we identify a total of 2,455 “active M dwarfs” in the sample. Of these, we find that 187 are also identified as fast rotators, which represents

∼ 8% of the “active M dwarf” subset. By comparison, there are 36,559 stars in the full SBK2 subset that have have M dwarf colors, and of these only 770 are fast rotators, or a 2_{Eric Mamajek determined FGKM spectral types corresponding to G-J colors to be: F 1-1.2, G}

1.2 -1.3, K 1.3-1.8, and M > 1.8. Full convection begins at M4.5 or G-J = 2.8. See his table at

Figure 3.5 NUV-V vs G-J for SBK2 targets. Black points are all SBK2 stars while red circles are the rapid rotators with GALEX magnitudes. We find that the M dwarfs identified as fast rotators have a 20% chance of showing UV excess, see Table 3.4.

fast rotator rate of just 2%. Another way to look at it is this: of the 36,559 stars in the SBK2 sample with M Dwarf colors, 2,455 are flagged as active by GALEX (6%). For the 770 M dwarf fast rotators, we identify 187 active stars (24%). These statistics both indicate that the M dwarf fast rotators selected from the K2 light curves are more than three times more likely to have UV excess, which supports the connection between fast rotation and chromospheric activity. We additionally note that for the stars which are not M dwarfs, i.e. G−J < 1.8, there is no evidence that the fast rotators have significant NUV excess over the other stars. These observations indicate that, while NUV excess can be a reliable age

diagnostic, it is not as reliable as an indicator of youth than the detection of fast rotation from the K2 light curves, especially for nearby low-mass stars.

Indeed approximately 75% of the M dwarf rapid rotators we identify do not fit the criterion for UV excess. Part of this may be due to the patchy sky-coverage of GALEX. In particular, GALEX did not observe many regions close to the Galactic plane due to high star density. Of the fields we analyzed, C00 and C07 fall directly on the Galactic plane. Campaigns C02 and C04 lie near the Galactic plane. Only C01, C03, C05, C06, and C08 are well within the coverage of the GALEX survey. Table 5 presents a breakdown of the number of SBK2 M dwarfs with UV excess per field and the number of those stars which are identified as fast rotators.

Table 3.4 indicates that most of the fast rotator M dwarfs we identify are not in the low Galactic latitude fields. Of the fields that are well covered by GALEX, we still find that only

∼25% of the SBK2 M dwarfs show evidence of UV excess from GALEX data. Therefore, it would appear likely that many of the rapid rotators in our sample do have some level of UV excess from chromospheric activity, butGALEX was unable to detect these targets, possibly due to the intrinsic faintness of those M dwarfs. From Figure 3.5, we expect that an active M dwarf should have 6<NUV-G<8 approximately. That means if an active M star has a magnitude G=15, which is typical of M dwarfs in the SBK2 subset, then its NUV magnitude will be in the range 21<NUV<23. This magnitude range is quite close to the detection limit of GALEX (NUV=22). This observation indicates that GALEX cannot always detect an active star if it is fainter than G=15. Indeed, there are 32,644 SBK2 M dwarfs fainter than

15th _{magnitude in G which is 90% of the M dwarfs in our sample. We believe that this is} strong evidence that the search for rapid rotators in fields observed with Kepler and K2 can identify young stars that lack a UV detection inGALEX, which can significantly expand the search for nearby young M dwarfs.

Table 3.4: M Dwarf Fast Rotators and NUV Emission Field Fast Rotation Fast Rotation

+NUV 0 22 0 1 53 21 2 33 1 3 21 1 4 200 15 5 57 21 6 55 9 7 35 4 8 50 26 10 43 18 11 24 0 12 35 22 13 82 15 14 63 33 15 47 1

An interesting pattern that emerges in Figure 3.5 is the fact that most of the M dwarf fast rotators have N U V −G > 6. That is, the majority of M dwarfs with extremely large UV excess (N U V −V <6) are not identified as fast rotators in our study. This is consistent with the hypothesis that most M dwarfs with very large NUV excess are actually low-mass stars with a white dwarf companion (Skinner et al. 2017). The white dwarf in this case is the one responsible for the large NUV excess, and not any sort of chromospheric activity in the M dwarf. In fact, systems with a white dwarf companion are relatively older (since all

white dwarfs are the remnants of evolved stars), which means one would not expect the M dwarf component to be a fast rotator, unless the rotation of the M dwarf is driven by tidal forces from the white dwarf in a tight binary system. We do indeed find a small number of fast rotators that have NUV-G>6.0, these could be one of two things: either they are M dwarfs with unusually high levels of NUV emission, perhaps M dwarfs caught during an intense flare (see Chapter 4), or they are tidally interacting systems of an M dwarf with a white dwarf companion.