Do you want to publish a course? Click here

$K2$ Ultracool Dwarfs Survey. VI. White light superflares observed on an L5 dwarf and flare rates of L dwarfs

79   0   0.0 ( 0 )
 Added by Rishi Paudel
 Publication date 2020
  fields Physics
and research's language is English




Ask ChatGPT about the research

Kepler K2 long cadence data are used to study white light flares in a sample of 45 L dwarfs. We identified 11 flares on 9 L dwarfs with equivalent durations of (1.3 - 198) hr and total (UV/optical/IR) energies of $geq$0.9 $times$ 10$^{32}$ erg. Two superflares with energies of $>$10$^{33}$ erg were detected on an L5 dwarf: this is the coolest object so far on which flares have been identified. The larger superflare on this L5 dwarf has an energy of 4.6$times$ 10$^{34}$ ergs and an amplitude of $>$300 times the photospheric level: so far, this is the largest amplitude flare detected by the $Kepler/K2$ mission. The next coolest star on which we identified a flare was an L2 dwarf: 2MASS J08585891+1804463. Combining the energies of all the flares which we have identified on 9 L dwarfs with the total observation time which was dedicated by $Kepler$ to all 45 L dwarfs, we construct a composite flare frequency distribution (FFD). The FFD slope is quite shallow (-0.51$pm$0.17), consistent with earlier results reported by Paudel et al. (2018) for one particular L0 dwarf, for which the FFD slope was found to be -0.34. Using the composite FFD, we predict that, in early and mid-L dwarfs, a superflare of energy 10$^{33}$ erg occurs every 2.4 years and a superflare of energy 10$^{34}$ erg occurs every 7.9 years. Analysis of our L dwarf flares suggests that magnetic fields of $geq$0.13-1.3 kG are present on the stellar surface: such fields could suppress Type II radio bursts.



rate research

Read More

We observed strong superflares (defined as flares with energy in excess of 10^33 erg) on three late-M dwarfs: 2MASS J08315742+2042213 (hereafter 2M0831+2042; M7 V), 2MASS J08371832+2050349 (hereafter 2M0837+2050; M8 V) and 2MASS J08312608+2244586 (hereafter 2M0831+2244; M9 V). 2M0831+2042 and 2M0837+2050 are members of the young (~700 Myr) open cluster Praesepe. The strong superflare on 2M0831+2042 has an equivalent duration (ED) of 13.7 hr and an estimated energy of 1.3 X 10^35 erg. We observed five superflares on 2M0837+2050, on which the strongest superflare has an ED of 46.4 hr and an estimated energy of 3.5 X 10^35 erg. This energy is larger by 2.7 orders of magnitude than the largest flare observed on the older (7.6 Gyr) planet-hosting M8 dwarf TRAPPIST-1. Furthermore, we also observed five superflares on 2M0831+2244 which is probably a field star. The estimated energy of the strongest superflare on 2M0831+2244 is 6.1 X 10^34 erg. 2M0831+2042, 2M0837+2050 and 2MASS J0831+2244 have rotation periods of 0.556pm0.002, 0.193pm0.000 and 0.292pm0.001 d respectively, which are measured by using K2 light curves. We compare the flares of younger targets with those of TRAPPIST-1 and discuss the possible impacts of such flares on planets in the habitable zone of late-M dwarfs.
We present photometric measurements of two superflares observed on a very young brown dwarf CFHT-BD-Tau 4, observed during Campaign 13 of the textit{Kepler K2} mission. The stronger of the two superflares brightened by a factor of $sim$48 relative to the quiescent photospheric level, with an increase in textit{Kepler} magnitude $Delta tilde{K_{p}}$ = -4.20. It has an equivalent duration of $sim$107 hour, a flare duration of 1.7 day, and an estimated total bolometric (ultraviolet/optical/infrared) energy up to 2.1 $times$ 10$^{38}$ erg. The weaker of the two superflares is a complex (multipeaked) flare with an estimated total bolometric (UV/optical/IR) energy up to 4.7 $times$ 10$^{36}$ erg. They are the strongest flares observed on any brown dwarf so far. The flare energies are strongly dependent on the value of visual extinction parameter $A_{V}$ used for extinction correction. If we apply a solar flare-model to interpret the two superflares, we find that the magnetic fields are required to be stronger by as much as an order of magnitude than previous reports of field measurements in CFHT-BD-Tau 4 by Reiners et al. (2009b). On the other hand, if we interpret our data in terms of accretion, we find that the requisite rate of accretion for the stronger superflare exceeds the rates which have been reported for other young brown dwarfs.
We present a volume-limited, spectroscopically-verified sample of M7$-$L5 ultracool dwarfs within 25,pc. The sample contains 410 sources, of which $93%$ have trigonometric distance measurements ($80%$ from textit{Gaia} DR2), and $81%$ have low-resolution ($Rsim120$), near-infrared (NIR) spectroscopy. We also present an additional list of 60 sources which may be M7$-$L5 dwarfs within 25,pc when distance or spectral type uncertainties are taken into account. The spectra provide NIR spectral and gravity classifications, and we use these to identify young sources, red and blue $J-K_S$ color outliers, and spectral binaries. We measure very low gravity and intermediate gravity fractions of $2.1^{+0.9}_{-0.8}%$ and $7.8^{+1.7}_{-1.5}%$, respectively; fractions of red and blue color outliers of $1.4^{+0.6}_{-0.5}$% and $3.6^{+1.0}_{-0.9}$%, respectively; and a spectral binary fraction of $1.6^{+0.5}_{-0.5}%$. We present an updated luminosity function for M7$-$L5 dwarfs continuous across the hydrogen burning limit that agrees with previous studies. We estimate our completeness to range between $69-80%$ when compared to an isotropic model. However, we find that the literature late-M sample is severely incomplete compared to L dwarfs, with completeness of $62^{+8}_{-7}%$ and $83^{+10}_{-9}%$, respectively. This incompleteness can be addressed with astrometric-based searches of ultracool dwarfs with textit{Gaia} to identify objects previously missed by color- and magnitude-limited surveys.
We report the discovery and classification of SDSS~J053341.43+001434.1 (SDSS0533), an early-L dwarf first discovered during a powerful $Delta V < -11$ magnitude flare observed as part of the ASAS-SN survey. Optical and infrared spectroscopy indicate a spectral type of L0 with strong H$alpha$ emission and a blue NIR spectral slope. Combining the photometric distance, proper motion, and radial velocity of SDSS0533 yields three-dimensional velocities of $(U,V,W)=(14pm13,-35pm14,-94pm22)$~km~s$^{-1}$, indicating that it is most likely part of the thick disk population and probably old. The three detections of SDSS0533 obtained during the flare are consistent with a total $V$-band flare energy of at least $4.9times10^{33}$~ergs (corresponding to a total thermal energy of at least $E_{rm tot}>3.7times10^{34}$~erg), placing it among the strongest detected M dwarf flares. The presence of this powerful flare on an old L0 dwarf may indicate that stellar-type magnetic activity persists down to the end of the main sequence and on older ML transition dwarfs.
We conducted a volume-limited survey at 4.9 GHz of 32 nearby ultracool dwarfs with spectral types covering the range M7 -- T8. A statistical analysis was performed on the combined data from the present survey and previous radio observations of ultracool dwarfs. Whilst no radio emission was detected from any of the targets, significant upper limits were placed on the radio luminosities that are below the luminosities of previously detected ultracool dwarfs. Combining our results with those from the literature gives a detection rate for dwarfs in the spectral range M7 -- L3.5 of ~ 9%. In comparison, only one dwarf later than L3.5 is detected in 53 observations. We report the observed detection rate as a function of spectral type, and the number distribution of the dwarfs as a function of spectral type and rotation velocity. The radio observations to date point to a drop in the detection rate toward the ultracool dwarfs. However, the emission levels of detected ultracool dwarfs are comparable to those of earlier type active M dwarfs, which may imply that a mildly relativistic electron beam or a strong magnetic field can exist in ultracool dwarfs. Fast rotation may be a sufficient condition to produce magnetic fields strengths of several hundreds Gauss to several kilo Gauss, as suggested by the data for the active ultracool dwarfs with known rotation rates. A possible reason for the non-detection of radio emission from some dwarfs is that maybe the centrifugal acceleration mechanism in these dwarfs is weak (due to a low rotation rate) and thus cannot provide the necessary density and/or energy of accelerated electrons. An alternative explanation could be long-term variability, as is the case for several ultracool dwarfs whose radio emission varies considerably over long periods with emission levels dropping below the detection limit in some instances.
comments
Fetching comments Fetching comments
Sign in to be able to follow your search criteria
mircosoft-partner

هل ترغب بارسال اشعارات عن اخر التحديثات في شمرا-اكاديميا