ABSTRACT Empirical calibrations of the stellar age-rotation-activity relation (ARAR) rely on observations of the co-eval populations of stars in open clusters. We used the Chandra X-ray Observatory ...to study M37, a 500-Myr-old open cluster that has been extensively surveyed for rotation periods ( ). M37 was observed almost continuously for five days, for a total of 440.5 ks, to measure stellar X-ray luminosities ( ), a proxy for coronal activity, across a wide range of masses. The cluster's membership catalog was revisited to calculate updated membership probabilities from photometric data and each star's distance to the cluster center. The result is a comprehensive sample of 1699 M37 members: 426 with , 278 with X-ray detections, and 76 with both. We calculate Rossby numbers, , where τ is the convective turnover time, and ratios of the X-ray-to-bolometric luminosity, , to minimize mass dependencies in our characterization of the rotation-coronal activity relation at 500 Myr. We find that fast rotators, for which , show saturated levels of activity, with log( ) . For , activity is unsaturated and follows a power law of the form , where β = . This is the largest sample available for analyzing the dependence of coronal emission on rotation for a single-aged population, covering stellar masses in the range 0.4-1.3 , in the range 0.4-12.8 days, and in the range . Our results make M37 a new benchmark open cluster for calibrating the ARAR at ages of Myr.
We present observations of the Rossiter-McLaughlin effect for two exoplanetary systems, revealing the orientations of their orbits relative to the rotation axes of their parent stars. HAT-P-4b is ...prograde, with a sky-projected spin-orbit angle of Delta *l = --4.9 ? 11.9 deg. In contrast, HAT-P-14b is retrograde, with Delta *l = 189.1 ? 5.1 deg. These results conform with a previously noted pattern among the stellar hosts of close-in giant planets: hotter stars have a wide range of obliquities and cooler stars have low obliquities. This, in turn, suggests that three-body dynamics and tidal dissipation are responsible for the short-period orbits of many exoplanets. In addition, our data revealed a third body in the HAT-P-4 system, which could be a second planet or a companion star.
We report the discovery of HAT-P-8b, a transiting planet with mass M p = 1.52+0.18 -0.16 M J, radius R p = 1.50+0.08 -0.06 R J, and photometric period P = 3.076 days. HAT-P-8b has a somewhat inflated ...radius for its mass, and a somewhat large mass for its period. The host star is a solar-metallicity F dwarf, with mass M = 1.28 +/- 0.04 M and R = 1.58+0.08 -0.06 R. HAT-P-8b was initially identified as one of the 32 transiting-planet candidates in HATNet field G205. We describe the procedures that we have used to follow up these candidates with spectroscopic and photometric observations, and we present a status report on our interpretation for 28 of the candidates. Eight are eclipsing binaries with orbital solutions whose periods are consistent with their photometric ephemerides; two of these spectroscopic orbits are single-lined and six are double-lined.
We present the measured projected obliquity-the sky-projected angle between the stellar spin axis and orbital angular momentum-of the inner planet of the HAT-P-17 multi-planet system. We measure the ...sky-projected obliquity of the star to be lambda = 19+ super(14) sub(-16) deg by modeling the Rossiter-McLaughlin effect in Keck/HIRES radial velocities (RVs). The anomalous RV time series shows an asymmetry relative to the midtransit time, ordinarily suggesting a nonzero obliquity-but in this case at least part of the asymmetry may be due to the convective blueshift, increasing the uncertainty in the determination of lambda. We employ the semi-analytical approach of Hirano et al. that includes the effects of macroturbulence, instrumental broadening, and convective blueshift to accurately model the anomaly in the net RV caused by the planet eclipsing part of the rotating star. Obliquity measurements are an important tool for testing theories of planet formation and migration. To date, the measured obliquities of ~50 Jovian planets span the full range, from prograde to retrograde, with planets orbiting cool stars preferentially showing alignment of stellar spins and planetary orbits. Our results are consistent with this pattern emerging from tidal interactions in the convective envelopes of cool stars and close-in planets. In addition, our 1.8 yr of new RVs for this system show that the orbit of the outer planet is more poorly constrained than previously thought, with an orbital period now in the range of 10-36 yr.
We report on the discovery of HAT-P-38b, a Saturn-mass exoplanet, transiting the
$ V$
$ =$
12.56 dwarf star GSC 2314-00559 on a
$ P$
$ =$
4.6404 d circular orbit. The host star is a 0.89
$ M_{\odot}$
...late G dwarf, with solar metallicity and a radius of 0.92
$ R_{\odot}$
. The planetary companion has a mass of 0.27
$ M_{\rm J}$
and a radius of 0.82
$ R_{\rm J}$
. HAT-P-38b is one of the planets the mass and radius of which have ever been discovered to be the closest to those of Saturn.
We perform a search for transiting planets in the NASA K2 observations of the globular cluster (GC) M4. This search is sensitive to larger orbital periods (P 35 days, compared to the previous best of ...P 16 days), and at the shortest periods, smaller planet radii (Rp 0.3 RJ, compared to the previous best of Rp 0.8 RJ) than any previous search for GC planets. Seven planet candidates are presented. An analysis of the systematic noise in our data shows that most, if not all, of these candidates are likely false alarms. We calculate planet occurrence rates assuming our highest significance candidate is a planet and occurrence rate upper limits assuming no detections. We calculate 3 occurrence rate upper limits of 6.1% for 0.71-2 RJ planets with 1-36 days periods and 16% for 0.36-0.71 RJ planets with 1-10 days periods. The occurrence rates from Kepler, TESS, and radial velocity studies of field stars are consistent with both a nondetection of a planet and detection of a single hot Jupiter in our data. Comparing to previous studies of GCs, we are unable to place a more stringent constraint than Gilliland et al. for the radius-period range they were sensitive to, but do place tighter constraints than both Weldrake et al. and Nascimbeni et al. for the large-radius regimes to which they were sensitive.
We present the first secondary eclipse and phase curve observations for the highly eccentric hot Jupiter HAT-P-2b in the 3.6, 4.5, 5.8, and 8.0 mu m bands of the Spitzer Space Telescope. We present ...an improved non-parametric method for removing the intra-pixel sensitivity variations in Spitzer data at 3.6 and 4.5 mu m that robustly maps position-dependent flux variations. We compare our measured secondary eclipse depths to the predictions from a one-dimensional radiative transfer model, which suggests the possible presence of a transient day side inversion in HAT-P-2b's atmosphere near periapse. We also derive improved estimates for the system parameters, including its mass, radius, and orbital ephemeris. We also find evidence for a long-term linear trend in the radial velocity data. The trend suggests the presence of another sub-stellar companion in the HAT-P-2 system, which could have caused HAT-P-2b to migrate inward to its present-day orbit via the Kozai mechanism.
By using subsets of the HATNet and K2 (Kepler two-wheel) Campaign 1 databases, we examine the effectiveness of filtering out systematics from photometric time series while simultaneously searching ...for periodic signals. We carry out tests to recover simulated sinusoidal and transit signals added to time series with both real and artificial noise. We find that the simple (and more traditional) method that performs correction for systematics first and signal search thereafter, produces higher signal recovery rates on the average, while also being substantially faster than the simultaneous method. Independently of the method of search, once the signal is found, a far less time consuming full-fledged model, incorporating both the signal and systematics, must be employed to recover the correct signal shape. As a by-product of the tests on the K2 data, we find that for longer period sinusoidal signals the detection rate decreases (after an optimum value is reached) as the number of light curves used for systematics filtering increases. The decline of the detection rate is observable in both methods of filtering, albeit the simultaneous method performs better in the regime of relative high template number. We suspect that the observed phenomenon is linked to the increased role of low amplitude intrinsic stellar variability in the space-based data. This assumption is also supported by the substantially higher stability of the detection rates for transit signals against the increase of the template number.
The Kepler spacecraft is monitoring more than 150,000 stars for evidence of planets transiting those stars. We report the detection of two Saturn-size planets that transit the same Sun-like star, ...based on 7 months of Kepler observations. Their 19.2- and 38.9-day periods are presently increasing and decreasing at respective average rates of 4 and 39 minutes per orbit; in addition, the transit times of the inner body display an alternating variation of smaller amplitude. These signatures are characteristic of gravitational interaction of two planets near a 2:1 orbital resonance. Six radial-velocity observations show that these two planets are the most massive objects orbiting close to the star and substantially improve the estimates of their masses. After removing the signal of the two confirmed giant planets, we identified an additional transiting super-Earth-size planet candidate with a period of 1.6 days.
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