4.5. Binary star contamination
5.3.3. Rotation-Activity relation
Our catalog of Hyades K and M stars allows to probe stellar rotation and chromospheric and coronal activity from a large sample of common objects. This enables direct correlations between the observed consequences of the magnetic field generation and its most prominent driver. We show in Fig. 5.13 and 5.14 the correspondence of vsini to Hα emission and X-ray emission, respectively. As rotation is generally accepted to be a prerequisite of stellar magnetic fields (in a rotationally driven dynamo), and hence magnetic fields emanate in activity proxies, more rotation equates to stronger activity. The data clearly resembles this mechanism in both tracers, with the fastest objects being among the most active. Thus, the rotation-activity relation known from field stars (spanning the spectral type range F- down to M) is still valid in the Hyades low-mass dwarfs, i.e. both in the slow/inactive K-dwarf and in the fast/active M-dwarf regime. There seems to be a limiting maximum of activity strength that these stars can produce, which is the same for all rotational velocities. This means that even among those stars, whosevsiniis too small to be detected, we find the same upper bounds of log(LHα/Lbol)sat ≈ −3.5 and log(LX/Lbol)sat≈ −2.5 as for the most rapidly rotating stars in the sample. This saturation limit is seen in the same way than in old field stars (Schrijver & Zwaan 2008;Pizzolato et al. 2003), albeit in the latter only at later spectral types. However, a few stars in the sample exhibit considerably stronger activity, argueably caused by powerful magnetic outbursts, and probably randomly caught during such events.
One would expect that the most rapid individuals are, at the same time, also the most active
Figure 5.12.: Comparison of measured vsini for objects in our sample with previously de-termined rotational velocities. Left: vsinimeasured in this work versus values obtained by other studies. Stars included in S97 (Stauffer et al. 1997a) and RM2000 (Reid & Mahoney 2000) are plotted as triangles and diamonds, respec-tively. Circles denote other works (for references, see Table 5.4). Filled symbols are confirmed binary systems. Right: comparison of vsini for objects in com-mon derived in S97 and RM2000. Note that stars on the identity line were only measured by S97, others were also remeasured by RM2000. See Fig. 5.8 for further explanations of plot symbols.
ones. While this is true for the general distribution in the sample, the three objects with vsini >30 km/s attract attention. Their Hα strength is slightly below log(LHα/Lbol)sat. It must be noted though, that the corresponding coronal emission remains at very high levels.
There is a puzzling subset of targets in Fig. 5.13, however, that seems to contradict the rotation-activity connection. Eleven objects in the sample populate a range of rotational velocities vsini ≥ 6 km/s (excluding upper limits), but surprisingly at very low or non-detected Hα emission strengths. One third of these objects are K stars, the others very early M-type. Even if we exclude the known binaries among them (33%) for their potentially different evolution of rotation, the behaviour is still striking. The HAIRs are summarized in Table 5.3 with their properties. Interestingly, the same objects with detected rotation but with marginal or no Hα emission are not all inactive when we look at X-ray activity.
There is only one star among the HAIRs, 2M 04181763+1500339, with a non-detection in LX, the remaining showing logLX/Lbol between −4.6 (rather inactive) and as high as−2.8 (close to log(LX/Lbol)sat). Again, even if we exclude the binaries (which show LX above average among the HAIRs), strong X-ray emitters remain that are non-detections in Hα.
The strength of X-ray activity for the objects in question grows with decreasing effective temperature. One possible explanation is that all the HAIRs are hidden binaries which unidentifiably broadened the XCFs and so mimicked enhanced rotation. This would first require a small difference in RV and/or a much lower-mass companion, because otherwise the impact should be visible in the deformation of the XCFs (we have identified one object among the HAIRs, 2M 04412966+1313164, as a probable SB2 before; cf. Sec. 5.1). We estimate the
5.3. Rotational velocities 55
Figure 5.13.: Chromospheric activity versus rotational velocity vrotsinifor the Hyades K–
M stars. Objects of spectral type K are denoted in red, M-type stars in blue.
Upper (detection) limits are indicated by their respective arrows. For coding of symbols, refer to Fig. 5.3.
effect of such binary stars on the resulting Hα line profile, and find that for combinations of an earlier type star with Hα in absorption and a later type one with Hα in strong emission, together with a small enough ∆RV, the line EW effectively vanishes or cancels out. We see indications of this mechanism in two of the binary HAIRs, namely 2M 04341113+1133285 and 2M 04285080+16172043, which may explain their positions in Fig.5.13. Thus, one component in these objects should be active, but we are unable to disentangle them from the current epoch of data. The same may be true for 2M 04510241+1458167, the fastest presumably single HAIR, which also shows a high level of X-ray activity (logLX/Lbol =−3.5). Its Hα profile resembles both parts of an absorption and emission line, which leads us to the conclusion that this may indeed be a so far unnoticed binary star, with an unresolved but chromospherically active late-type companion. As a test, we try to reconstruct the Hα emission line of the active component in this case (assuming a superposed gaussian absorption profile). This yields a lower limit of logLHα/Lbol & −4.8 for 2M 04510241+1458167 (as we do not know the bolometric luminosity of the active star, which should be lower), placing the object well towards the rotation-activity regime in Fig.5.13, yet at considerably lower activity level than other stars with similar rotation4.
The remaining apparently single HAIRs do not bear indications of such hidden, active com-panions. Their Hα lines are moderate to strong absorption profiles, on display in Fig. 5.15.
Any spectroscopic binary companion to these objects would have to show a significant flux contribution (ie. be within a few spectral types) in order to bias the measurement of vsini (and not be at orbital phase close to zero), but this would mean a significant and measurable
3Another binary star, 2M 04480086+1703216, withvsini= 5.7±1.1 not listed as a HAIR, reveals the same properties.
4This estimate assumes that both components rotate with the same rotational velocityvsini, which is not necessarily the case.
Figure 5.14.: Coronal activity versus rotational velocity vrotsinifor the Hyades K–M stars.
Colour coding and plot symbols as in Fig. 5.13. See Fig. 5.3 for further expla-nations of symbol shapes.
distortion of the resulting Hα profile (either emission or absorption). Or a companion would have an insignificant flux contribution, in which case vsini would not be compromised and Hα might be disturbed (in case of emission) or not. Either scenario does not explain the ob-servations. If we do not detect secondary Hαemission, then such a low-luminosity companion cannot be a contributor to the detected rotation either. This renders the binary hypothesis for the remaining HAIRs unlikely or irrelevant.
Furthermore, this would not explain the fact that the presumably single HAIRs do show X-ray activity, but at levels around the lowest that we detect with =−4.6≤logLX/Lbol ≤ −4.3 (there are three stars with higher values, −3.7 ≤ logLX/Lbol ≤ −3.6, but they have lower vsini ≤ 6 km/s). This level is substantially below the average of confirmed binaries with detected rotation (around logLX/Lbol =−3.0).
If the HAIRs do not form Hαemission lines in their chromospheres, these stars cannot have a high level of activity. With increasing activity level, Hαis known to increase in absorption before the line core fills in, and eventually turns into emission, while at the same time other activity tracers show an emission line from the beginning (Walkowicz & Hawley 2009). We suspect that the HAIR stars might be in a phase where absorption is still present, but filling of the line with emission has not yet started (in terms of activity level). We therefore search these objects for emission in the CaIIH & K lines. We find that all the stars show H and K with emission cores, while Hα is in absorption. Hence, indications are that the HAIR stars do still exhibit chromospheric activity, although another proxy for it, Hα, does not show this. The level of activity in these stars is therefore comparably low, in concordance with a reduced level of coronal activity. In effect, the Hα-inactive but rotating objects thus fall into two categories: the faster rotating among them (vsini≤14 km/s) are likely binaries with at least one component showing activity, while the slowest detected rotators (vsini≤8 km/s) without measurable Hα emission still show chromospheric activity as Calcium emission and coronal X-ray activity. In both categories, rotation is associated with signs of magnetic
5.3. Rotational velocities 57 activity, so that the rotation-activity relation does not seem to be violated. Rotation thus remains a governing principle for magnetic activity throughout all spectral types K to M, and moreover at younger ages.
Figure 5.15.: Hα line profiles for nine of the eleven chromospherically inactive yet rotating stars from Table5.3(2M 04285080+1617204 and 2M 04181763+1500339 are not shown).
5.3.Rotationalvelocities59
Table 5.3.: Activity and rotation for Hα inactive yet rotating stars. For notes and references, please refer to TableA.1.
Objecta Other Nameb Teffc SpTd loglogLLbolX loglogLLH αbol vsini±σe SB?f Refg Notesh
2MASS J [◦K] [km s−1]
04341113+1133285 vB 294 3475 M3 -2.78 -5.38 7.2 ±1.2 X (9) a) d) 04285080+1617204 vB 190 3690 M1 -3.48 < -5.75 14.3 . . . X (1) a) d) 04222568+1118205 vB 259 3850 M0 -3.89 < -5.66 8.3 ±1.1 X (9) a) d) 05013603+1355586 vB 348 3860 M0 -4.42 < -5.61 7.9 ±1.4 (9) a) d) 04254922+1531165 vA 366 3880 M0 -4.35 < -5.51 6.4 ±1.1 (9) c) f) 04510241+1458167 . . . 3945 M0 -3.48 < -5.70 10.6 ±1.4 ? (9) c) e) 04340530+1413029 vA 731 4025 K7 -3.71 < -5.57 6.0 ±0.8 (9) a) d) 04412966+1313164 vB 316 4070 K7 -4.26 < -5.56 9.8 ±1.5 ? (9) c) d) 04151038+1423544 vA 72 4085 K7 -4.29 < -5.43 6.0 ±1.0 (9) a) d) 04033902+1927180 RHy 44 4625 K4 -4.47 < -5.67 6.3 ±0.3 (9) a) d) 04181763+1500339 RHy 200 4865 K2 < -4.63 -5.35 7.5 . . . (7) c) h)
this work other Ref
04325009+1600210 K1 5.7 6.7 (2)
04181926+1605181 K2 < 2.5 2.5 (6) 04265434+1308175 K2 < 2.5 5.4 (6) 04395095+1243426 K2 < 2.5 3.9 (6) 04033902+1927180 K4 6.3 < 10 (3)
04322565+1306476 K4 23.4 24 (4)
04235440+1403075 K4 < 2.5 < 3 (6)
04070122+1520062 K5 3.0 6.5 (6)
04333716+2109030 K5 < 2.5 2.7 (6)
04334192+1900504 K6 3.8 2.8 (6)
04412966+1313164 K7 9.8 < 10 (3) 04063463+1332566 K7 < 2.5 < 6 (4) 04340530+1413029 K7 6.0 < 10 (3)
04161310+1853042 M0 3.5 2 (6)
04222568+1118205 M0 8.3 11.3 (6)
04244401+1046192 M0 < 2.5 6 (4)
04285243+1558539 M0 5.7 < 10 (3)
04480086+1703216 M2 5.7 8 (1)
04291234+1516259 M2 3.5 < 10 (3)
04060221+1815033 M2 4.9 < 6 (4)
04360416+1853189 M2 18.4 15 (3)
04350255+0839304 M3 11.4 25 (4)
04341113+1133285 M3 7.2 < 10 (3) 04271663+1714305 M3 < 2.5 < 10 (3)
04303385+1444532 M3 3.0 6 (4)
04332699+1302438 M3 3.0 < 6 (4)
04295572+1654506 M3 2.8 < 10 (3)
04322373+1745026 M3 23.6 33.4 (5)
04251456+1858250 M4 5.3 < 10 (3)
04290099+1840254 M4 18.7 15.5 (3)
References.(1)Griffin et al.(1985); (2)Radick et al.(1987); (3)Stauffer et al.(1987); (4)Stauffer et al.(1997a); (5)Bleach et al.(2002b); (6)Mermilliod et al.(2009).