Recently, parallax data have become available for the majority of the stars in the SBK2 sample (Gaia Collaboration et al. 2018a,b). We present an update to the kinematics analysis described in Section 3.2 utilizing theGAIA second data release (DR2).
This time we can get precise (triangulated) distances for most of the SBK2 stars from the DR2 parallaxes. We multiply these trigonometric distances by the total proper motion, µ, of each star to obtain a transverse velocity, from vT = 4.74 µ dtrig. We present again a frequency histogram of those velocity space projections, see Figure 3.6. Again, the rapid rotators are shown on the top panel in red while all stars are shown on the bottom panel in black. This confirms the trends already noted in Section 3.2: fast rotators are significantly more frequent at lower velocities and their distribution peaks at a lower velocity than the stars as a whole. Here, the median transverse motion of the fast rotators is 33 km s−1, with a mean absolute deviation (MAD) of 17 km s−1. For all the other stars, the median transverse
Figure 3.6 Frequency histograms of 2D velocity projections for the fast rotators (top) and all stars (bottom) using trigonometric distances. There are more fast rotators at low velocity and their overall distribution peaks at a lower velocity compared to the general population of stars, see Section 3.2. A two-sided KS test of these distributions produces a KS statistic of 0.43 and a p-value of 10−176 indicating that they are drawn from different populations.
motion is 71 km s−1, with a MAD of 41 km s−1. This motion, again, implies that the fast
rotators peak at a transverse velocity of about half that of the slow rotators. A two-sided KS-test returns a KS statistic of 0.43 and a p-value of 10−176. These values confirm that
the motions of the fast rotators are indeed different from the general population of stars as was previously implied using photometric distances. If anything, the difference is even more pronounced.
Figure 3.7 Plot of 2d velocity projections using GAIA DR2 proper motions and parallaxes. The symbols follow the convention established in Figure 3.4
revised kinematics plots, see Figure 3.7. The axes plot the projected velocity vectors (vl,vb) = (4.74 µl dphot, 4.74 µb dtrig), where µl and µb are the components of proper motion in Galactic coordinates, which are multiplied by theGAIAdistances. The velocity components are given in units of km s−1. As before, the boxes represent the general location of known
nearby star clusters and NYMGs utilizing the UVW velocities in Table 3.1.
The updated kinematics plot is similar to the kinematics plot in Section 3.2. The fast rotators (red circles) tend to lie at lower velocities than the general SBK2 stars, implying that they are members of the young disk. We also note the same clumping we saw previously in C04, C05, and C11. Overall, the GAIA DR2 kinematics only confirm the trends already noted above, and this shows that our photometric distances were already reasonably reliable in most cases. However, the increased accuracy of the GAIA parallaxes allows us to better identify the members of clusters and possible NYMGs.
Table 3.5: Fast Rotators Kinematics (Trigonometric) Field < vl >med. M ADvl < vb >med. M ADvb
(km s−1) (km s−1) (km s−1) (km s−1) 0 24 27 -17 14 1 -23 24 -25 24 2 -22 20 -3 17 3 5 33 -23 34 4 29 1 -12 1 5 3 10 -30 14 6 -46 44 -2 27 7 -23 37 -16 10 8 34 43 -20 28 10 -30 53 -14 25 11 -28 13 -8 8 12 18 30 -26 11 14 -13 23 -29 12 15 -27 14 2 22
Tables 3.5 and 3.6 update the values presented in Tables 3 and 4 using GAIA DR2. The average ratio of the MAD for the fast rotators compared to the entire sample is 0.51 in the Galactic latitude direction and 0.53 in the Galactic longitude direction. This fraction
implies that the fast rotator distribution of velocities is 50% smaller than the velocities of the general population of SBK2 targets. This tighter distribution of velocities is consistent with the young, thin disk. This evidence further solidifies our conclusion that these stars are young.
Table 3.6: SBK2 Kinematics (Trigonometric) Field < vl >med. M ADvl < vb >med. M ADvb
(km s−1) (km s−1) (km s−1) (km s−1) 0 36 26 -14 22 1 -25 75 -47 43 2 -43 34 -6 29 3 -10 54 -35 40 4 45 36 -10 37 5 42 60 -31 42 6 -58 68 -17 56 7 -42 35 -19 33 8 52 69 -32 51 10 -41 81 -35 46 11 -59 48 -12 36 12 8 77 -46 47 13 51 43 -3 34 14 1 64 -50 46 15 -65 57 -2 49
Figure 3.8 plots the GAIA distances as a function of G-J color. The left hand side extends to 1 kpc while the right hand side is zoomed in to 250 pc. The figure indicates that our survey does not detect significant numbers of fast rotators at distances >400pc. This is because most of those distant stars are in fact high-velocity stars, i.e. members of the Galactic halo population, so they are not rotating rapidly in general. The majority of the rapid rotators we observe lie within 150pc of the Sun. This is because young stars with low space velocities can only be selected as high proper motion stars if they are relatively close
Figure 3.8 Trigonometric distance as a function of G-J color. Black points are all SBK2 stars and red circles are all fast rotators detected. The left hand side extends to 1000pc while the right hand side is zoomed in to 250pc. Annotations indicate the middle of the mass range as it corresponds to G-J color.
to the Sun. Additionally, as we have seen previously, this subset is comprised mainly of M dwarfs (see Figure 6.1 and Section 3.3 for example). This is simply a bias in our sample towards nearby M dwarfs due to the nature of the SUPERBLINK catalog. The overdensity of fast rotators at a distance of ∼140pc is likely due to members of the Pleiades which is discussed further in Chapter 4.
We can now update Figure 2.15 with the absolute magnitudes calculated from GAIA
DR2, see the left panel of Figure 3.9. While Figure 2.15 informed us about theKepler CCD sensitivity to detecting rotation periods, Figure 3.9 can inform us aboutintrinsic limitations to the detection of rotation periods in GKM dwarfs. We restrict this analysis to disk stars only. The figure indicates that the SBK2 sample of fast rotators is mainly comprised of M dwarfs (MG ∼ 12). We also note a distinct lack of rapid rotators at 5 < MG < 7. These
stars are the more massive K dwarfs. The lack of rapid rotators at these masses indicates that K dwarfs slow down faster than G and M dwarfs are are not detected by our rotation period detection algorithm discussed in Chapter 2. We create a color-absolute magnitude diagram utilizingGAIA DR2, see the right hand side of Figure 3.9. We utilize the GAIA G magnitude and 2MASS J magnitude for the color plotted on the x-axis. The y-axis is the absolute G magnitude calculated from the distance modulus, MG =G−5 log10d+ 5. We
remove all SBK2 stars that lie within the Halo based on their location in the reduced proper motion diagram (see Figure 3.1) and stars with distances greater than 50pc. We make these cuts for the purpose of comparing our sample of rapid rotators only to nearby, disk stars.
The GAIA color magnitude diagram contains many interesting features. The general field stars (black points) trace the standard main-sequence, which is narrow at blue colors, and wider at red colors. The SBK2 sample of fast rotators rotators, on the other hand, is clearly elevated above the ZAMS at very red colors (G-J >2). These colors correspond to late M-type stars which take the longest to contract on to the main sequence, so this suggests that these fast rotators are overluminous, which can happen if a star is extremely young and still contracting. The GAIA color magnitude diagram also features a few members of the red giant branch. However, these objects are not the focus of this work.
The objects of interest here are the very red rapid rotators that are elevated above the MS. As we saw in the RPM diagram in Figure 3.1, these stars were of interest due to their low reduced proper motion value. We speculated that this could be due to either overluminoisty or low transverse motion. We saw in both Figures 3.3 and 3.6 that the subset of 1,113
Figure 3.9 On the left, frequency histogram of absolute GAIA G magnitudes. On the right, Color- absolute magnitude diagram for nearby, disk SBK2 stars (black dots) and the subset of fast rotators (red circles). Absolute magnitudes were calculated using GAIA DR2. SBK2 fast rotators have a statistically significant low velocity when compared to the entire sample fo 58,484 stars. Now we see in Figure 3.9 which utilizes trigonometric distance that the stars appear to be overluminous. These observations (low velocities, UV excess, and overluminosity) taken in aggregate are strong indicators that this subset of fast rotators we identify are a younger population of stars.
What we think, however, is that the fast rotators on this diagram are not actually shifted up (overluminous), but are in fact shifted right, i.e. are redder than expected. This redder color may be an effect due to “magnetic inflation” caused by the rapid rotation and strong magnetic field in a star that is relatively young, but not necessarily so young that it is still contracting. The magnetic inflation results in “puffed up” stars that are larger than a non- inflated star of the same mass. This results in a larger surface area, making the star appear
cooler. Therefore, these stars may not actually be PMS stars, but are bona fide MS stars, albeit young. In any case, the subsample of fast rotators represents a young population of disk stars in the Galaxy.
The technique we describe in this chapter could prove especially useful once data from the upcomingTESS mission becomes available. These data will make it possible to expand the search for young M dwarfs over much of the sky, especially if most of the known nearby M dwarfs are being targeted by TESS. We explore the potential to find NYMGs members via fast rotation in the next chapter.
IDENTIFYING NEW MEMBERS OF NEARBY STAR CLUSTERS AND YOUNG MOVING GROUPS