اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدMultiwavelenght analysis of Gl 355 (LQ Hya)
386
0
0.0
(
0
)
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
We discuss ROSAT, ASCA, {it Beppo}SAX and optical observations of the young active star Gl 355. During the ROSAT observation a strong flare was detected with a peak flux more than an order of magnitude larger than the quiescent level. Spectral analysis of the data allows us to study the temperature and emission measure distribution, and the coronal metal abundance, for the quiescent phase and, in the case of ROSAT, also during the evolution of the flare. We have modeled the flare and derived a loop semi--length of the order of $sim 1.5$ stellar radii. ROSAT, ASCA and {it Beppo}SAX data suggest that the coronal abundance of Gl 355 is subsolar, in the range $0.1 div 0.3 Z/Z_odot$. A preliminary analysis of optical spectra allows us to compare the photospheric and coronal metal abundances.
قيم البحث
اقرأ أيضاً
We investigate the spot activity of the young chromospherically active main sequence star LQ Hya. Our aims are to identify possible active longitudes, estimate the differential rotation and study long and short term changes in the activity. Our analy
sis is based on 24 years of Johnson V-band photometry. We use the previously published Continuous Period Search (CPS) method to model the evolution of the light curve of LQ Hya. The CPS fits a Fourier series model to short overlapping subsets of data. This enables us to monitor the spot configuration of the star with a higher time resolution. We find seasonal variability in the mean level and amplitude of the light curve of LQ Hya. The variability of the light curve amplitude seems not to be cyclic, but the long-term variations in the mean magnitude could be explained by an approximately 13 year cycle. Because of the limited length of the observed time series, it is not yet possible to determine whether this structure really is periodic and represents an activity cycle. We estimate the differential rotation of the star to be small, and the star is potentially very close to a rigid rotator. We search for active longitudes and find that on time scales up to six months there are typically one or two relatively stable active areas on the star with limited phase migration. On time scales longer than one year, no stable active longitudes have been present except for the period between 2003 and 2009 and possibly also some time before 1995. We find any signs of flip-flops with a regular period. The mean time scale of change of the light curve during the observation period is determined to be of the same order of magnitude as the predicted convective turnover time for the star.
Previous studies have related surface temperature maps, obtained with the Doppler imaging (DI) technique, of LQ Hya with long-term photometry. We compare surface magnetic field maps, obtained with the Zeeman Doppler imaging (ZDI) technique, with cont
emporaneous photometry, with the aim of quantifying the stars magnetic cycle characteristics. We inverted Stokes IV spectropolarimetry into magnetic field and surface brightness maps using a tomographic inversion code that models high signal-to-noise ratio mean line profiles produced by the least squares deconvolution (LSD) technique. The magnetic field and surface brightness maps reveal similar patterns to previous DI and ZDI studies: non-axisymmetric polar magnetic field structure, void of fields at mid-latitudes, and a complex structure in the equatorial regions. There is a weak but clear tendency of the polar structures to be linked with strong radial field and the equatorial ones with the azimuthal. We find a polarity reversal in the radial field between 2016 and 2017 coincident with an activity minimum seen in the long-term photometry. The inverted field strengths cannot easily be related with the observed spottedness, but we find that they are partially connected with the retrieved field complexity. Comparing to global magnetoconvection models for rapidly rotating young Suns, this field topology and dominance of the poloidal field component could be explained by a turbulent dynamo, where differential rotation does not play a major role (so called alpha^2 Omega or alpha^2 dynamos), and axi- and non-axisymmetric modes are excited simultaneously. The complex equatorial magnetic field structure could arise from the twisted (helical) wreaths often seen in these simulations, while the polar feature would be connected to the mostly poloidal non-axisymmetric component having a smooth spatial structure.
We study LQ Hya photometry for 1982-2014 with the carrier fit (CF) -method and compare our results to earlier photometric analysis and recent Doppler imaging maps. We first utilize different types of statistical methods to estimate various candidates
for the carrier period for the CF method. Secondly, a global fit to the whole data set and local fits to shorter segments are computed with the period that is found to be the optimal one. The harmonic least-squares analysis of all the available data reveals a short period close to 1.6 days as a limiting value for a set of significant frequencies. We interpret this as the rotation period of the spots near the equatorial region. In addition, the distribution of the significant periods is found to be bimodal, hinting of a longer-term modulating period, which we set out to study with a two-harmonic CF model. Weak modulation signal is, indeed retrieved, with a period of roughly 6.9 years. The phase dispersion analysis gives a clear symmetric minimum for coherence times lower than and around 100 days. We interpret this as the mean rotation period of the spots (1.60514 days), and this value is chosen to be used as the carrier period for the CF analysis. With the CF method we seek for any systematic trends in the spot distribution in the global time frame, and locally look for abrupt phase changes earlier reported in rapidly rotating objects. During 2005-2008 the global CF reveals a coherent structure rotating with a period of 1.6037 days, while during most other times the spot distribution appears rather random in phase. The evolution of the spot distribution of the object is found to be very chaotic, with no clear signs of an azimuthal dynamo wave that would persist over longer time scales, although the short-lived coherent structures observed occasionally do not rotate with the same speed as the mean spot distribution.
There are many formulas that express interesting properties of a finite group G in terms of sums over its characters. For estimating these sums, one of the most salient quantities to understand is the character ratio trace(pi(g)) / dim(pi), for an
irreducible representation pi of G and an element g of G. It turns out [Gurevich-Howe15, Gurevich-Howe17] that for classical groups G over finite fields there are several (compatible) invariants of representations that provide strong information on the character ratios. We call these invariants collectively rank. Rank suggests a new way to organize the representations of classical groups over finite and local fields - a way in which the building blocks are the smallest representations. This is in contrast to Harish-Chandras philosophy of cusp forms that is the main organizational principle since the 60s, and in it the building blocks are the cuspidal representations which are, in some sense, the LARGEST. The philosophy of cusp forms is well adapted to establishing the Plancherel formula for reductive groups over local fields, and led to Lusztigs classification of the irreducible representations of such groups over finite fields. However, analysis of character ratios might benefit from a different approach. In this note we discuss further the notion of tensor rank for GL_n over a finite field F_q and demonstrate how to get information on representations of a given tensor rank using tools coming from the recently studied eta correspondence, as well as the well known philosophy of cusp forms, mentioned just above. A significant discovery so far is that although the dimensions of the irreducible representations of a given tensor rank vary by quite a lot (they can differ by large powers of q), for certain group elements of interest the character ratios of these irreps are nearly equal to each other.
We prove an analogue of Yaus Caccioppoli-type inequality for nonnegative subharmonic functions on graphs. We then obtain a Liouville theorem for harmonic or non-negative subharmonic functions of class Lq, 1<=q<infty, on any graph, and a quantitative
version for q > 1. Also, we provide counterexamples for Liouville theorems for 0 < q < 1.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
