ترغب بنشر مسار تعليمي؟ اضغط هنا

Spectropolarimetric follow-up of 8 rapidly rotating, X-ray bright FK Comae candidates

176   0   0.0 ( 0 )
 نشر من قبل James Sikora
 تاريخ النشر 2020
  مجال البحث فيزياء
والبحث باللغة English




اسأل ChatGPT حول البحث

Our understanding of the evolved, rapidly rotating, magnetically active, and apparently single FK Comae stars is significantly hindered by their extreme rarity: only two stars in addition to FK Com itself are currently considered to be members of this class. Recently, a sample of more than 20 candidate FK Comae type stars was identified within the context of the emph{Kepler-Swift} Active Galaxies and Stars (KSwAGS) survey. We present an analysis of high-resolution Stokes $V$ observations obtained using ESPaDOnS@CFHT for 8 of these candidates. We found that none of these targets can be considered members of the FK Comae class based primarily on their inferred rotational velocities and on the detection of spectroscopic binary companions. However, 2 targets show evidence of magnetic activity and have anomalously high projected rotational velocities ($vsin{i}$) relative to typical values associated with stars of similar evolutionary states. EPIC 210426551 has a $vsin{i}=209,{rm km,s}^{-1}$, an estimated mass of $1.07,M_odot$, and, based in part on its derived metallicity of ${rm [M/H]}=-0.4$, it is either an evolved main sequence (MS) star or a pre-MS star. KIC 7732964 has a mass of $0.84,M_odot$, lies near the base of the red giant branch, and exhibits a $vsin{i}=23,{rm km,s}^{-1}$. We find that these two objects have similar characteristics to FK Com (albeit less extreme) and that their rapid rotation may be inconsistent with that predicted for a single star evolutionary history. Additional observations are necessary in order to better constrain their evolutionary states and whether they have short-period binary companions.



قيم البحث

اقرأ أيضاً

We present a three-dimensional simulation of the corona of an FK Com-type rapidly rotating G giant using a magnetohydrodynamic model that was originally developed for the solar corona in order to capture the more realistic, non-potential coronal stru cture. We drive the simulation with surface maps for the radial magnetic field obtained from a stellar dynamo model of the FK Com system. This enables us to obtain the coronal structure for different field topologies representing different periods of time. We find that the corona of such an FK Com-like star, including the large scale coronal loops, is dominated by a strong toroidal component of the magnetic field. This is a result of part of the field being dragged by the radial outflow, while the other part remains attached to the rapidly rotating stellar surface. This tangling of the magnetic field,in addition to a reduction in the radial flow component, leads to a flattening of the gas density profile with distance in the inner part of the corona. The three-dimensional simulation provides a global view of the coronal structure. Some aspects of the results, such as the toroidal wrapping of the magnetic field, should also be applicable to coronae on fast rotators in general, which our study shows can be considerably different from the well-studied and well-observed solar corona. Studying the global structure of such coronae should also lead to a better understanding of their related stellar processes, such as flares and coronal mass ejections, and in particular, should lead to an improved understanding of mass and angular momentum loss from such systems.
FK Comae is a rapidly rotating magnetically active star, the light curve of which is modulated by cool spots on its surface. It was the first star where the flip-flop phenomenon was discovered. Since then, flip-flops in the spot activity have been re ported in many other stars. Therefore, it is of interest to perform a more thorough study of the evolution of the spot activity in FK Com. In this study, we analyse 15 years of photometric observations with two different time series analysis methods, with a special emphasis on detecting flip-flop type events from the data. We apply the continuous period search and carrier fit methods on long-term standard Johnson-Cousins V-observations from the years 1995--2010. The observations were carried out with two automated photometric telescopes, Phoenix-10 and Amadeus T7 located in Arizona. We identify complex phase behaviour in 6 of the 15 analysed data segments. We identify five flip-flop events and two cases of phase jumps, where the phase shift is Delta phi < 0.4. In addition we see two mergers of spot regions and two cases where the apparent phase shifts are caused by spot regions drifting with respect to each other. Furthermore we detect variations in the rotation period corresponding to a differential rotation coefficient of |k|>0.031. The flip-flop cannot be interpreted as a single phenomenon, where the main activity jumps from one active longitude to another. In some of our cases the phase shifts can be explained by differential rotation: Two spot regions move with different angular velocity and even pass each other. Comparison between the methods show that the carrier fit utility is better in retrieving slow evolution especially from a low amplitude light curve, while the continuous period search is more sensitive in case of rapid changes.
87 - L. Jetsu 2016
Recently, we presented a general model for the light curves of chromospherically active stars, where the observed light curve is interference of two real constant period light curves of long-lived starspots. In this first paper, we make six specific questions which undermine this argument, because it contradicts the current widely held views about the stellar surface differential rotation and the starspots. Our aim is to answer these six questions. We present evidence that the long-lived starspots of our general model have already been detected in the earlier surface imaging studies. The Lomb-Scargle power spectrum method analysis of the real and the simulated data of FK Com reveals that this method fails to detect the two real constant period light curves of our general model. If our model is valid, this method gives incompatible period, amplitude and minimum epoch estimates telling nothing about the real periods, the real amplitudes and the real minimum epochs of the two real light curves. This would mean that all earlier one-dimensional period analyses of the light curves of chromospherically active stars have given spurious results which have been widely and uncritically accepted since the discovery of the starspots in the year 1947. However, we arrive at a dead end, because we can not solve the real light curves of FK Com. In our second paper, we solve these real light curves with a new two-dimensional period finding method, prove the validity of our general model, and answer all six questions made in this first paper.
In this paper, we present results from the spectropolarimetric follow-up of photometrically selected candidate magnetic B stars from the MOBSTER project. Out of four observed targets, one (HD 38170) is found to host a detectable surface magnetic fiel d, with a maximum longitudinal field measurement of 105$pm$14 G. This star is chemically peculiar and classified as an $alpha^2$ CVn variable. Its detection validates the use of TESS to perform a photometric selection of magnetic candidates. Furthermore, upper limits on the strength of a putative dipolar magnetic field are derived for the remaining three stars, and we report the discovery of a previously unknown spectroscopic binary system, HD 25709. Finally, we use our non-detections as case studies to further inform the criteria to be used for the selection of a larger sample of stars to be followed up using high-resolution spectropolarimetry.
COCOA-PUFS is an energy-diverse, time-domain study of the ultra-fast spinning, heavily spotted, yellow giant FK Com (HD117555; G4 III). This single star is thought to be a recent binary merger, and is exceptionally active by measure of its intense ul traviolet and X-ray emissions, and proclivity to flare. COCOA-PUFS was carried out with Hubble Space Telescope in the UV (120-300 nm), using mainly its high-performance Cosmic Origins Spectrograph, but also high-precision Space Telescope Imaging Spectrograph; Chandra X-ray Observatory in the soft X-rays (0.5-10 keV), utilizing its High-Energy Transmission Grating Spectrometer; together with supporting photometry and spectropolarimetry in the visible from the ground. This is an introductory report on the project. FK Com displayed variability on a wide range of time scales, over all wavelengths, during the week-long main campaign, including a large X-ray flare; super-rotational broadening of the far-ultraviolet hot-lines (e.g., Si IV 139 nm (T~80,000 K) together with chromospheric Mg II 280 nm and C II 133 nm (10,000-30,000 K); large Doppler swings suggestive of bright regions alternately on advancing and retreating limbs of the star; and substantial redshifts of the epoch-average emission profiles. These behaviors paint a picture of a highly extended, dynamic, hot (10 MK) coronal magnetosphere around the star, threaded by cooler structures perhaps analogous to solar prominences, and replenished continually by surface activity and flares. Suppression of angular momentum loss by the confining magnetosphere could temporarily postpone the inevitable stellar spindown, thereby lengthening this highly volatile stage of coronal evolution.
التعليقات
جاري جلب التعليقات جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
mircosoft-partner

هل ترغب بارسال اشعارات عن اخر التحديثات في شمرا-اكاديميا