اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدChromospheric Variability in SDSS M Dwarfs. II. Short-Timescale H-alpha Variability
187
0
0.0
(
0
)
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
[Abridged] We present the first comprehensive study of short-timescale chromospheric H-alpha variability in M dwarfs using the individual 15 min spectroscopic exposures for 52,392 objects from the Sloan Digital Sky Survey. Our sample contains about 10^3-10^4 objects per spectral type bin in the range M0-M9, with a total of about 206,000 spectra and a typical number of 3 exposures per object (ranging up to a maximum of 30 exposures). Using this extensive data set we find that about 16% of the sources exhibit H-alpha emission in at least one exposure, and of those about 45% exhibit H-alpha emission in all of the available exposures. Within the sample of objects with H-alpha emission, only 26% are consistent with non-variable emission, independent of spectral type. The H-alpha variability, quantified in terms of the ratio of maximum to minimum H-alpha equivalent width (R_EW), and the ratio of the standard deviation to the mean (sigma_EW/<EW>), exhibits a rapid rise from M0 to M5, followed by a plateau and a possible decline in M9 objects. In particular, R_EW increases from a median value of about 1.8 for M0-M3 to about 2.5 for M7-M9, and variability with R_EW>10 is only observed in objects later than M5. For the combined sample we find that the R_EW values follow an exponential distribution with N(R_EW) exp[-(R_EW-1)/2]; for M5-M9 objects the characteristic scale is R_EW-1approx 2.7, indicative of stronger variability. In addition, we find that objects with persistent H-alpha emission exhibit smaller values of R_EW than those with intermittent H-alpha emission. Based on these results we conclude that H-alpha variability in M dwarfs on timescales of 15 min to 1 hr increases with later spectral type, and that the variability is larger for intermittent sources.
قيم البحث
اقرأ أيضاً
We present the V band variability analysis of the point sources in the Faint Sky Variability Survey on time scales from 24 minutes to tens of days. We find that about one percent of the point sources down to V = 24 are variables. We discuss the varia
bility detection probabilities for each field depending on field sampling, amplitude and timescale of the variability. The combination of colour and variability information allows us to explore the fraction of variable sources for different spectral types. We find that about 50 percent of the variables show variability timescales shorter than 6 hours. The total number of variables is dominated by main sequence sources. The distribution of variables with spectral type is fairly constant along the main sequence, with 1 per cent of the sources being variable, except at the blue end of the main sequence, between spectral types F0--F5, where the fraction of variable sources increases to about 2 percent. For bluer sources, above the main sequence, this percentage increases to about 3.5. We find that the combination of the sampling and the number of observations allows us to determine the variability timescales and amplitudes for a maximum of 40 percent of the variables found. About a third of the total number of short timescale variables found in the survey were not detected in either B or/and I. These show a similar variability timescale distribution to that found for the variables detected in all three bands.
We present and analyse new R-band frames of the gravitationally lensed double quasar FBQ 0951+2635. These images were obtained with the 1.5m AZT-22 Telescope at Maidanak (Uzbekistan) in the 2001-2006 period. Previous results in the R band (1999-2001
period) and the new data allow us to discuss the dominant kind of microlensing variability in FBQ 0951+2635. The time evolution of the flux ratio A/B does not favour the continuous production of short-timescale (months) flares in the faintest quasar component B (crossing the central region of the lensing galaxy). Instead of a rapid variability scenario, the observations are consistent with the existence of a long-timescale fluctuation. The flux ratio shows a bump in the 2003-2004 period and a quasi-flat trend in more recent epochs. Apart from the global behaviour of A/B, we study the intra-year variability over the first semester of 2004, which is reasonably well sampled. Short-timescale microlensing is not detected in that period. Additional data in the i band (from new i-band images taken in 2007 with the 2m Liverpool Robotic Telescope at La Palma, Canary Islands) also indicate the absence of short-timescale events in 2007.
M dwarf stars are excellent candidates around which to search for exoplanets, including temperate, Earth-sized planets. To evaluate the photochemistry of the planetary atmosphere, it is essential to characterize the UV spectral energy distribution of
the planets host star. This wavelength regime is important because molecules in the planetary atmosphere such as oxygen and ozone have highly wavelength dependent absorption cross sections that peak in the UV (900-3200 $r{A}$). We seek to provide a broadly applicable method of estimating the UV emission of an M dwarf, without direct UV data, by identifying a relationship between non-contemporaneous optical and UV observations. Our work uses the largest sample of M dwarf star far- and near-UV observations yet assembled. We evaluate three commonly-observed optical chromospheric activity indices -- H$alpha$ equivalent widths and log$_{10}$ L$_{Halpha}$/L$_{bol}$, and the Mount Wilson Ca II H&K S and R$_{HK}$ indices -- using optical spectra from the HARPS, UVES, and HIRES archives and new HIRES spectra. Archival and new Hubble Space Telescope COS and STIS spectra are used to measure line fluxes for the brightest chromospheric and transition region emission lines between 1200-2800 $r{A}$. Our results show a correlation between UV emission line luminosity normalized to the stellar bolometric luminosity and Ca II R$_{HK}$ with standard deviations of 0.31-0.61 dex (factors of $sim$2-4) about the best-fit lines. We also find correlations between normalized UV line luminosity and H$alpha$ log$_{10}$ L$_{Halpha}$/L$_{bol}$ and the S index. These relationships allow one to estimate the average UV emission from M0 to M9 dwarfs when UV data are not available.
The Gaia DR2 sample of short-timescale variable candidates results from the investigation of the first 22 months of Gaia photometry for a subsample of sources at the Gaia faint end. For this exercise, we limited ourselves to the case of suspected rap
id periodic variability. Our study combines fast-variability detection through variogram analysis, high-frequency search by means of least-squares periodograms, and empirical selection based on the investigation of specific sources seen through the Gaia eyes (e.g. known variables or visually identified objects with peculiar features in their light curves). The progressive definition and validation of this selection criterion also benefited from supplementary ground-based photometric monitoring of a few preliminary candidates, performed at the Flemish Mercator telescope (Canary Islands, Spain) between August and November 2017. We publish a list of 3,018 short-timescale variable candidates, spread throughout the sky, with a false-positive rate up to 10-20% in the Magellanic Clouds, and a more significant but justifiable contamination from longer-period variables between 19% and 50%, depending on the area of the sky. Although its completeness is limited to about 0.05%, this first sample of Gaia short-timescale variables recovers some very interesting known short-period variables, such as post-common envelope binaries or cataclysmic variables, and brings to light some fascinating, newly discovered variable sources. In the perspective of future Gaia data releases, several improvements of the short-timescale variability processing are considered, by enhancing the existing variogram and period-search algorithms or by classifying the identified candidates. Nonetheless, the encouraging outcome of our Gaia DR2 analysis demonstrates the power of this mission for such fast-variability studies, and opens great perspectives for this domain of astrophysics.
Aims. We attempt to determine if a dependency on spectral subtype or vsini exists for stars undergoing phase-changes between B and Be states, as well as for those stars exhibiting variability in H{alpha} emission. Methods. We analyse the changes in H
{alpha} line strength for a sample of 55 Be stars of varying spectral types and luminosity classes using five epochs of observations taken over a ten year period between 1998 and 2010. Results. We find i) that the typical timescale between which full phase transitions occur is most likely of the order of centuries, although no dependency on spectral subtype or vsini could be determined due to the low frequency of phase-changing events observed in our sample, ii) that stars with earlier spectral types and larger values of vsini show a greater degree of variability in H{alpha} emission over the timescales probed in this study, and iii) a trend of increasing variability between the shortest and longest baselines for stars of later spectral types and with smaller values of vsini.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
