Young stars are highly variable in the X-ray regime. In particular, bright X-ray flares can substantially enhance ionization in the surrounding protoplanetary disk. Since disk chemical evolution is impacted by ionization, X-ray flares have the potential to fundamentally alter the chemistry of planet forming regions. We present 2D disk chemical models that incorporate a stochastic X-ray flaring module, named \xgen, and examine the flares' overall chemical impact compared to models that assume a constant X-ray flux. We examine the impact of 500 years of flaring events and find global chemical changes on both short time scales (days) in response to discrete flaring events and long time-scales (centuries) in response to the cumulative impact of many flares. Individual X-ray flares most strongly affect small gas-phase cations, where a single flare can temporarily enhance the abundance of species such as H$_3^+$, HCO$^+$, CH$_3^+$, and C$^+$. We find that flares can also drive chemistry out of "steady state" over longer time periods, where the disk-integrated abundance of some species, such as O and O$_2$, changes by a few percent over the 500 year model. We also explore whether the specific history of X-ray flaring events (randomly drawn but from the same energy distribution) impacts the chemical evolution and find that it does not. Finally, we examine the impact of X-ray flares on the electron fraction. While most molecules modeled are not highly sensitive to flares, certain species, including observable molecules, are very reactive to the dynamic environment of a young star.
We present continuum observations at 1.3 and 2.7 mm using the Combined Array for Research in Millimeter-wave Astronomy (CARMA) toward six protoplanetary disks in the Taurus molecular cloud: CI Tau, DL Tau, DO Tau, FT Tau, Haro 6-13, and HL Tau. We constrain physical properties of the disks with Bayesian inference using two disk models; flared power-law disk model and flared accretion disk model. Comparing the physical properties, we find that the more extended disks are less flared and that the dust opacity spectral index (beta) is smaller in the less massive disks. In addition, disks with a s…
We present multi-epoch Spitzer Space Telescope observations of the transitional disk LRLL 31 in the 2-3 Myr-old star forming region IC 348. Our measurements show remarkable mid-infrared variability on timescales as short as one week. The infrared continuum emission exhibits systematic wavelength-dependent changes that suggest corresponding dynamical changes in the inner disk structure and variable shadowing of outer disk material. We propose several possible sources for the structural changes, including a variable accretion rate or a stellar or planetary companion embedded in the disk. Our res…
The energy in the solar acoustic spectrum is known to be correlated with flares, but it is not known if the same is true for stellar flares? In order to answer this question, we have analyzed 73 flares in 39 solar-like stars. These flares were identified in the 854 solar-like stars observed by the Kepler spacecraft that have stellar parameters measured with asteroseismology. Though we were not able to identify a statistically significant enhancement of the energy in the high-frequency part of the post-flare acoustic spectra compared to the pre-flare spectra of these stars, we did identify a la…
Based on the long-term monitoring data of the water maser since 2019.0 to 2021.0 allowed us detecting in IRAS 16293-2422 two powerful phenomena lasted about year at radial velocities near 6 and 8 km s$^{-1}$. In both cases, powerful short flares were located on the tops of less powerful, but more prolonged ones (2.5 and 0.5 kJy), radiation of which initiated the emission of more powerful flares. For the first time, configurations of several emitting maser spots located at the line of sight to the observer were discovered experimentally. This made it possible to confirm the hypothesis of activa…
One of the defining characteristics of a solar flare is the impulsive formation of very high temperature plasma. The properties of the thermal emission are not well understood, however, and the analysis of solar flare observations is often predicated on the assumption that the flare plasma is isothermal. The EUV Variability Experiment (EVE) on the Solar Dynamics Observatory (SDO) provides spectrally resolved observations of emission lines that span a wide range of temperatures (e.g., Fe XV-Fe XXIV) and allow for thermal flare plasma to be studied in detail. In this paper we describe a method f…
We present new photometric data and analysis of the long-duration UBV photoelectric observations for three candidates to protoplanetary objects - F-supergiants with IR-excesses located at large galactic latitudes, IRAS 18095+2704, IRAS 19386+0155, and IRAS 19500-1709. All three stars have revealed quasiperiodic low-amplitude variabilities caused by pulsations observed against the long-term trends of brightnesses. For IRAS 18095+2704=V887 Her we have found a pulsation period of 109 days and a linear trend of brightness under the constant colours if being averaged over the year timescale. The li…
It is generally accepted that densities of quiet sun and active region plasma are sufficiently low to justify the optically thin approximation, and it is commonly used in the analysis of line emissions from plasma in the solar corona. However, densities of solar flare loops are substantially higher, compromising the optically thin approximation. This study begins with a radiative transfer model that uses typical solar flare densities and geometries to show that hot coronal emission lines are not generally optically thin. Further, the model demonstrates that the observed line intensity should e…
KIC 7341653 is one of several late-type M dwarfs observed in the main mission of Kepler with peculiar infrared colors placing them in the domain of suspected young stellar objects (YSO). It is likely associated with a powerful X-ray emitter with X-ray flares. Kepler light curves reveal two distinct types of activity: frequent flares lasting from less than 30 min to a few hours and a periodic variability with a period of 0.5463441(7) d. The largest flare detected increased the flux in the Kepler passband by a factor of 2.8 and released an estimated 4$\times10^{34}$ erg of energy in the Kepler b…
Superflares on solar-type stars has been a rapidly developing field ever since the launch of $\it Kepler$. Over the years, there have been several studies investigating the statistics of these explosive events. In this study, we present a statistical analysis of stellar flares on solar-type stars made using photometric data in 2-min cadence from $\it TESS$ of the whole southern hemisphere (sectors 1 - 13). We derive rotational periods for all stars in our sample from rotational modulations present in the lightcurve as a result of large starspot(s) on the surface. We identify 1980 stellar flare…
Based on one-month long MMT time-series observations of the open cluster M37, we monitored light variations of nearly 2500 red dwarfs and successfully identified 420 flare events from 312 cluster M dwarf stars. For each flare light curve, we derived observational and physical parameters, such as flare shape, peak amplitude, duration, energy, and peak luminosity. We show that cool stars produce serendipitous flares energetic enough to be observed in the $r$-band, and their temporal and peak characteristics are almost the same as those in traditional $U$-band observations. We also found many lar…
We present here optical I-band photometric variability study down to $\simeq$ 19 mag of a young ($\sim$2-3 Myr) star-forming region IC 348 in the Perseus molecular cloud. We aim to explore the fast rotation (in the time-scales of hours) in Very Low Mass stars (VLMs) including Brown Dwarfs (BDs). From a sample of 177 light-curves using our new I-band observations, we detect new photometric variability in 22 young M-dwarfs including 6 BDs, which are bonafide members in IC 348 and well-characterized in the spectral type of M-dwarfs. Out of 22 variables, 11 M dwarfs including one BD show hour-scal…
As the Lyman-alpha (Lya) line of neutral hydrogen is the brightest emission line in the solar spectrum, detecting increases in irradiance due to solar flares at this wavelength can be challenging due to the very high background. Previous studies that have focused on the largest flares have shown that even these extreme cases generate enhancements in Lya of only a few percent above the background. In this study, a superposed-epoch analysis was performed on ~8500 flares greater than B1 class to determine the contribution that flares make to changes in the solar EUV irradiance. Using the peak of …
The chromospheric Lyman-alpha line of neutral hydrogen (\lya; 1216Å) is the strongest emission line in the solar spectrum. Fluctuations in \lya\ are known to drive changes in planetary atmospheres, although few instruments have had the ability to capture rapid \lya\ enhancements during solar flares. In this paper we describe flare-associated emissions via a statistical study of 477 M- and X-class flares as observed by the EUV Sensor on board the 15th Geostationary Operational Environmental Satellite, which has been monitoring the full-disk solar \lya\ irradiance on 10~s timescales over the cou…
Particle acceleration in solar flares remains an outstanding problem in solar physics. It is yet unclear which of the acceleration mechanisms dominates and how exactly is the excessive magnetic energy transferred to the nonthermal and other forms of energy. We emphasize, that the ultimate acceleration mechanism must be capable of efficiently working in the most extreme conditions, such as the shortest detected time scales and the highest acceleration efficiency. Here we focus on detailed multiwavelength analysis of a very initial phase of the SOL2011-08-04 flare, which demonstrated prominent s…
Multi-epoch observations have revealed the variability of pre-main sequence stars and/or their environment. Moreover, structures in orbital motion around the central star, resulting from planet-disk interaction, are predicted to cause temporal variations in the brightness distributions of protoplanetary disks. Through repeated observations with the Very Large Telescope Interferometer (VLTI) over nearly two decades, the ESO Archive has become a treasure chest containing high-resolution multi-epoch near- and mid-infrared observations of the potential planet-forming regions in protoplanetary disk…
We report on synoptic observations at 3.6 and 4.5micron of young stellar objects in IC 348 with 38 epochs covering 40 days. We find that among the detected cluster members, 338 at [3.6] and 269 at both [3.6] and [4.5], many are variable on daily to weekly timescales with typical fluctuations of ~0.1 mag. The fraction of variables ranges from 20% for the diskless pre-main sequence stars to 60% for the stars still surrounded by infalling envelopes. We also find that stars in the exposed cluster core are less variable than the stars in the dense, slightly younger, south-western ridge. This trend …
Investigating Variability in Long-lived Protoplanetary Disks (>10 Myr) using NEOWISE
Indian Institute of Science Education and Research Tirupati（印度科学教育研究所提鲁帕蒂分校）；Instituto de Astronomía, Universidad Nacional Autónoma de México（墨西哥国立自治大学天文学研究所）
Ten very low luminosity objects were observed multiple times in the 8.5 GHz continuum in search of protostellar magnetic activities. A radio outburst of IRAM 04191+1522 IRS was detected, and the variability timescale was about 20 days or shorter. The results of this survey and archival observations suggest that IRAM 04191+1522 IRS is in active states about half the time. Archival data show that L1014 IRS and L1148 IRS were detectable previously and suggest that at least 20%-30% of very low luminosity protostars are radio variables. Considering the variability timescale and flux level of IRAM 0…
We present the results of a search for white light flares on the ~23,000 cool dwarfs in the Kepler Quarter 1 long cadence data. We have identified 373 flaring stars, some of which flare multiple times during the observation period. We calculate relative flare energies, flare rates and durations, and compare these with the quiescent photometric variability of our sample. We find that M dwarfs tend to flare more frequently but for shorter durations than K dwarfs, and that they emit more energy relative to their quiescent luminosity in a given flare than K dwarfs. Stars that are more photometrica…
The Chinese Small Telescope ARray (CSTAR) carried out high-cadence time-series observations of 20.1 square degrees centered on the South Celestial Pole during the 2008, 2009 & 2010 winter seasons from Dome A in Antarctica. The nearly-continuous 6 months of dark conditions during each observing season allowed for >10^6 images to be collected through gri and clear filters, resulting in the detection of >10^4 sources over the course of 3 years of operation. The nearly space-like conditions in the Antarctic plateau are an ideal testbed for the suitability of very small-aperture (<20 cm) telescopes…
Solar flares may be the best-known examples of the explosive conversion of magnetic energy into bulk motion, plasma heating, and particle acceleration via magnetic reconnection. The energy source for all flares is the highly sheared magnetic field of a filament channel above a polarity inversion line (PIL). During the flare, this shear field becomes the so-called reconnection guide field (i.e., the non-reconnecting component), which has been shown to play a major role in determining key properties of the reconnection including the efficiency of particle acceleration. We present new high-resolu…
TESS flares and chromospheric variability in the Pleiades, the Hyades, and Praesepe
Konkoly Observatory, HUN-REN Research Centre for Astronomy and Earth Sciences（孔科利天文台，匈牙利研究与创新学院地球科学与天文学研究中心）；CSFK, MTA Centre of Excellence（CSFK，匈牙利科学院卓越中心）；Eötvös Loránd University, Institute of Physics and Astronomy, Department of Astronomy（罗兰大学，物理与天文学研究所，天文学系）
We performed simple spot-model calculations for quasi-periodic brightness variations of solar-type stars showing superflares, by using Kepler photometric data. Most of superflare stars show quasi-periodic brightness modulations with the typical period of one to a few tens of days. Our results indicate that these brightness variations of superflare stars can be explained by the rotation of the star with fairly large starspots. Using the result of the period analysis, we investigated the relation between the energy and frequency of superflares and the rotation period. Stars with relatively slowe…
We investigated, through numerical models, the flaring variability that may arise from the rotation of maser clouds of approximately spheroidal geometry, ranging from strongly oblate to strongly prolate examples. Inversion solutions were obtained for each of these examples over a range of saturation levels from unsaturated to highly saturated. Formal solutions were computed for rotating clouds with many randomly chosen rotation axes, and corresponding averaged maser light curves plotted with statistical information. The dependence of results on the level of saturation and on the degree of defo…
Time-resolved databases with large spatial coverage are quickly becoming a standard tool for all types of astronomical studies. We report preliminary results from our search for stellar flares in the 2MASS calibration fields. A sample of 4343 M dwarfs, spatially matched between the SDSS and the 2MASS calibration fields, each with hundreds to thousands of epochs in near infrared bandpasses, is analyzed using a modified Welch-Stetson index to characterize the variability. A Monte Carlo model was used to assess the noise of the variability index. We find significnat residuals above the noise with…
We have carried out a new photometric V,Rc study of 12 protoplanetary nebulae, objects in the short-lived transition between the AGB and PN phases of stellar evolution. These had been the subjects of an earlier study, using data from 1994-2007, that found that all 12 varied periodically, with pulsation periods in the range of ~38 to ~150 days. They are all carbon-rich, with F-G spectral types. We combined our new (2008-2018) data with publicly-available ASAS-SN data and determined new periods for their variability. The older and newer period values were compared to investigate evidence of peri…
We present new Jansky Very Large Array (VLA) radio images of the Crab Nebula at 5.5 GHz, taken at two epochs separated by 6 days about two months after a gamma-ray flare in 2012 July. We find no significant change in the Crab's radio emission localized to a region of <2 light-months in radius, either over the 6-day interval between our present observations or between the present observations and ones from 2001. Any radio counterpart to the flare has a radio luminosity of <~ $2 \times 10^{-4}$ times that of the nebula. Comparing our images to one from 2001, we do however find changes in radio b…
We present the results of a monitoring campaign using the KAT-7 and HartRAO 26m telescopes, of hydroxyl, methanol and water vapour masers associated with the high-mass star forming region G9.62+0.20E. Periodic flaring of the main line hydroxyl masers were found, similar to that seen in the 6.7 and 12.2 GHz methanol masers. The 1667 MHz flares are characterized by a rapid decrease in flux density which is coincident with the start of the 12.2 GHz methanol maser flare. The decrease in the OH maser flux density is followed by a slow increase till a maximum is reached after which the maser decays …
We simulate maser flares by varying either the pump rate or the background level of radiation in a 3D model of a maser cloud. We investigate the effect of different cloud shapes, saturation levels and viewpoints. Results are considered for clouds with both uniform and internally variable unsaturated inversion. Pumping and background variations are represented by several different driving functions, some of which are light curves drawn from observations. We summarise the pumping variability results in terms of three observable parameters, the maximum flux density achieved, a variability index a…
I present recent results studying flare emission in magnetars. Strong quasi-periodic oscillations observed in the tail of giant magnetar flares are frequently interpreted as evidence for global seismic oscillations. I demonstrate that such a global oscillation is not directly observable in the lightcurve. New work suggests the amplitude for the strongest QPO stays nearly constant in the rotation phases where it is observed, which I argue suggests it is produced by an additional emission process from the star.
Calcium-Aluminum-rich Inclusions (CAIs), the oldest known solids of the solar system, show evidence for the past presence of short-lived radionuclide beryllium-10, which was likely produced by spallation during protosolar flares. While such $^{10}$Be production has hitherto been modeled at the inner edge of the protoplanetary disk, I calculate here that spallation at the disk surface may reproduce the measured $^{10}$Be/$^9$Be ratios at larger heliocentric distances. Beryllium-10 production in the gas prior to CAI formation would dominate that in the solid. Interestingly, provided the Sun's pr…
This work is part of a multi-wavelength program to study the effects of X-ray/UV/optical stellar radiation on the chemistry of the circumbinary disk around the young high-eccentricity binary DQ Tau. ALMA observations for near/around December 5, 2021 periastron were postponed due to bad weather, but supporting Swift-XRT-UVOT TOO observations were successful. These Swift observations along with previous X-ray-optical-mm data show that DQ Tau keeps exhibiting powerful flares near periastron, offering a unique laboratory for studies of flare effects on the gas-phase ion chemistry in protoplanetary…
The evolution of protoplanetary disks is believed to be driven largely by angular momentum transport resulting from magnetized disk winds and turbulent viscosity. The ionization of the disk that is essential for these processes has been thought due to host star coronal X-rays but could also arise from energetic particles produced by coronal flares or by travelling shock waves and advected by the stellar wind. We have performed test-particle numerical simulations of energetic protons propagating into a realistic T~Tauri stellar wind, including a superposed small-scale magnetostatic turbulence. …
The characterization of exoplanets, their formation, evolution, and chemical changes is tightly linked to our knowledge of their host stars. In particular, stellar X-rays and UV emission have a strong impact on the dynamical and chemical evolution of planetary atmospheres. We analyzed 25 XMM-Newton observations encompassing about eight years and totaling about 958 ks in order to study the X-ray emission of HD 189733 A. We find that the corona of HD 189733 A has an average temperature of 0.4 keV and it is only during flares that the mean temperature increases to 0.9 keV. Apart from the flares, …
Dust evolution in protoplanetary disks from small dust grains to pebbles is key to the planet formation process. The gas in protoplanetary disks should influence the vertical distribution of small dust grains ($\sim$1 $μm$) in the disk.Utilizing archival near-infrared polarized light and millimeter observations, we can measure the scale height and the flare parameter $β$ of the small dust grain scattering surface and $^{12}$CO gas emission surface for three protoplanetary disks IM Lup, HD 163296, and HD 97048 (CU Cha). For two systems, IM Lup and HD 163296, the $^{12}$CO gas and small dust gra…
Chromospheric activity cycles are common in late-type stars; however, only a handful of coronal activity cycles have been discovered. \ihor\ is the most active and youngest star with known coronal cycles. It is also a young solar analog, and we are likely facing the earliest cycles in the evolution of solar-like stars, at an age (~600 Myr) when life appeared on Earth. Our aim is to confirm the ~1.6 yr coronal cycle and characterize its stability over time. We use X-ray observations of iota Hor to study the corona of a star representing the solar past through variability, thermal structure, and…
Methanol and water masers indicate young stellar objects. They often exhibit flares, and a fraction shows periodic activity. Several mechanisms might explain this behavior but the lack of concurrent infrared (IR) data complicates to identify the cause. Recently, 6.7 GHz methanol maser flares were observed, triggered by accretion bursts of high-mass YSOs which confirmed the IR-pumping of these masers. This suggests that regular IR changes might lead to maser periodicity. Hence, we scrutinized space-based IR imaging of YSOs associated with periodic methanol masers. We succeeded to extract the IR…
We aim at estimating the dust scale height of protoplanetary disks from millimeter continuum observations. First, we present a general expression of intensity of a ring in a protoplanetary disk, and show that we can constrain the dust scale height by the azimuthal intensity variation. Then, we apply the presented methodology to the two distinct rings at 68 au and at 100 au of the protoplanetary disk around HD 163296. We constrain the dust scale height by comparing the DSHARP high-resolution millimeter dust continuum image with radiative transfer simulations using RADMC-3D. We find that h_d/h_g…
Getman et al. (2021) reports the discovery, energetics, frequencies, and effects on environs of $>1000$ X-ray super-flares with X-ray energies $E_X \sim 10^{34}-10^{38}$~erg from pre-main sequence (PMS) stars identified in the $Chandra$ MYStIX and SFiNCs surveys. Here we perform detailed plasma evolution modeling of $55$ bright MYStIX/SFiNCs super-flares from these events. They constitute a large sample of the most powerful stellar flares analyzed in a uniform fashion. They are compared with published X-ray super-flares from young stars in the Orion Nebula Cluster, older active stars, and the …
We used the Aladin Virtual Obsrvatory tool and High Resolution Imager ROSAT archival data to search for X-ray variability in scale of days in 23 young stars in the sigma Orionis cluster and a background galaxy. Five stars displayed unambiguous flares and had probabilities p_var >> 99% of being actual variables. Two of the detected flares were violent and long-lasting, with maximum duration of six days and amplitude of eight times above the quiescent level. We classified another four stars as possible X-ray variables, including the binary system formed by the B2Vp star sigma Ori E and its close…
Short-duration flares at millimeter wavelengths provide unique insights into the strongest magnetic reconnection events in stellar coronae, and combine with longer-term variability to introduce complications to next-generation cosmology surveys. We analyze 5.5 years of JCMT Transient Survey 850 micron submillimeter monitoring observations toward eight Gould Belt star-forming regions to search for evidence of transient events or long-duration variability from faint sources. The eight regions (30 arcmin diameter fields), including ~1200 infrared-selected YSOs, have been observed on average 47 ti…
