In this paper, we study the temperature and density properties of multiple structural components of coronal mass ejections (CMEs) using differential emission measure (DEM) analysis. The DEM analysis ...is based on the six-passband EUV observations of solar corona from the Atmospheric Imaging Assembly on board the Solar Dynamic Observatory. The structural components studied include the hot channel in the core region (presumably the magnetic flux rope of the CME), the bright loop-like leading front (LF), and coronal dimming in the wake of the CME. We find that the presumed flux rope has the highest average temperature (>8 MK) and density (~1.0 x 10 super(9) cm super(-3)), resulting in an enhanced emission measure over a broad temperature range (3 < or =, slant T(MK) < or =, slant 20). On the other hand, the CME LF has a relatively cool temperature (~2 MK) and a narrow temperature distribution similar to the pre-emption coronal temperature (1 < or =, slant T(MK) < or =, slant 3). The density in the LF, however, is increased by 2%-32% compared with that of the pre-emption corona, depending on the event and location. In coronal dimmings, the temperature is more broadly distributed (1 < or =, slant T(MK) < or =, slant 4), but the density decreases by ~35%-~40%. These observational results show that: (1) CME core regions are significantly heated, presumably through magnetic reconnection; (2) CME LFs are a consequence of compression of ambient plasma caused by the expansion of the CME core region; and (3) the dimmings are largely caused by the plasma rarefaction associated with the eruption.
Stars of stellar type later than about M3.5 are believed to be fully convective and therefore unable to support magnetic dynamos like the one that produces the 11-yr solar cycle. Because of their ...intrinsic faintness, very few late M stars have undergone long-term monitoring to test this prediction, which is critical to our understanding of magnetic field generation in such stars. Magnetic activity is also of interest as the driver of UV and X-ray radiation, as well as energetic particles and stellar winds, that affects the atmospheres of close-in planets that lie within habitable zones, such as the recently discovered Proxima b. We report here on several years of optical, UV, and X-ray observations of Proxima Centauri (GJ 551; dM5.5e): 15 yr of All Sky Automated Survey photometry in the V band (1085 nights) and 3 yr in the I band (196 nights), 4 yr of Swift X-Ray Telescope and UV/Optical Telescope observations (more than 120 exposures), and nine sets of X-ray observations from other X-ray missions (ASCA, XMM-Newton, and three Chandra instruments) spanning 22 yr. We confirm previous reports of an 83-d rotational period and find strong evidence for a 7-yr stellar cycle, along with indications of differential rotation at about the solar level. X-ray/UV intensity is anticorrelated with optical V-band brightness for both rotational and cyclical variations. From comparison with other stars observed to have X-ray cycles, we deduce a simple empirical relationship between X-ray cyclic modulation and Rossby number, and we also present Swift UV grism spectra covering 2300-6000 A.
Abstract
A major obstacle to detecting and characterizing long-period, low-mass exoplanets is the intrinsic radial-velocity (RV) variability of host stars. To better understand RV variability, we ...estimate disk-averaged RV variations of the Sun over its magnetic cycle, from the Fe
i
line observed by SDO/HMI, using a physical model for rotationally modulated magnetic activity that was previously validated against HARPS-N solar observations. We estimate the unsigned magnetic flux and show that a linear fit to it reduces the
rms
of RV variations by 62%, i.e., a factor of 2.6. We additionally apply the
FF
′ method, which predicts RV variations based on a star’s photometric variations. At cycle maximum, we find that additional processes must be at play beyond suppression of convective blueshift and velocity imbalances resulting from brightness inhomogeneities, in agreement with recent studies of RV variations. By modeling RV variations over the magnetic cycle using a linear fit to the unsigned magnetic flux, we recover injected planets at a period of ≈300 days with RV semi-amplitudes down to 0.3 m s
−1
. To reach 0.1 m s
−1
, we will need to identify and model additional phenomena that are not well traced by
∣
B
ˆ
obs
∣
or
FF
′. This study motivates ongoing and future efforts to develop observation and analysis techniques to measure the unsigned magnetic flux at high precision in slowly rotating, relatively inactive stars like the Sun. We conclude that the unsigned magnetic flux is an excellent proxy for rotationally modulated, activity-induced RV variations, and could become key to confirming and characterizing Earth analogs.
Manifestations of stellar activity (such as star-spots, plage/faculae, and convective flows) are well-known to induce spectroscopic signals often referred to as astrophysical noise by exoplanet ...hunters. For example, setting an ultimate goal of detecting true Earth analogs demands reaching radial velocity (RV) precisions of ∼9 cm s−1. While this is becoming technically feasible with the latest generation of highly stabilized spectrographs, it is astrophysical noise that sets the true fundamental barrier on attainable RV precisions. In this paper, we parameterize the impact of solar surface magneto-convection on absorption line profiles, and extend the analysis from the solar disk center (Paper I) to the solar limb. Off disk-center, the plasma flows orthogonal to the granule tops begin to lie along the line of sight, and those parallel to the granule tops are no longer completely aligned with the observer. Moreover, the granulation is corrugated and the granules can block other granules, as well as the intergranular lane components. Overall, the visible plasma flows and geometry of the corrugated surface significantly impact the resultant line profiles and induce center-to-limb variations in shape and net position. We detail these herein, and compare to various solar observations. We find our granulation parameterization can recreate realistic line profiles and induced radial velocity shifts, across the stellar disk, indicative of both those found in computationally heavy radiative 3D magnetohydrodynamical simulations and empirical solar observations.
IC 10 X-1 is a bright (Lx = 10 super(38) ergs s super(-1)) variable X-ray source in the Local Group starburst galaxy IC 10. The most plausible optical counterpart is a luminous Wolf-Rayet star, ...making IC 10 X-1 a rare example of a Wolf-Rayet X-ray binary. In this Letter, we report on the detection of an X-ray orbital period for IC 10 X-1 of 34.4 hr. This result, combined with a reexamination of optical spectra, allows us to determine a mass function for the system of f(M) = 7.8 M unk and a probable mass for the compact object of 24-33 M unk. If this analysis is correct, the compact object is the most massive stellar-mass black hole known. We further show that the observed period is inconsistent with Roche lobe overflow, suggesting that the binary is detached and that the black hole is accreting the wind of the Wolf-Rayet star. The observed mass-loss rate of MAC92 17A is sufficient to power the X-ray luminosity of IC 10 X-1.
Abstract
The temporal analysis of stellar activity evolution is usually dominated by a complex trade-off between model complexity and interpretability, often by neglecting the nonstationary nature of ...the process. Recent studies appear to indicate that the presence of multiple coexisting cycles in a single star is more common than previously thought. The correct identification of physically meaningful cyclic components in spectroscopic time series is therefore a crucial task, which cannot overlook local behaviors. Here we propose a decomposition technique that adaptively recovers amplitude- and frequency-varying components. We present our results for the solar activity as measured both by the sunspot number and the
K
-line emission index, and we consistently recover the Schwabe and Gleissberg cycles as well as the Gnevyshev–Ohl pattern probably related to the Hale cycle. We also recover the known 8 yr cycle for 61 Cygni A, in addition to evidence of a three-cycles-long pattern reminiscent of the Gnevyshev–Ohl rule. This is particularly interesting as we cannot discard the possibility of a relationship between the measured field polarity reversals and this Hale-like periodicity.
Context. Coronal bright points (CBP) are ubiquitous small brightenings in the solar corona associated with small magnetic bipoles. Aims. We derive the solar differential rotation profile by tracing ...the motions of CBPs detected by the Atmospheric Imaging Assembly (AIA) instrument aboard the Solar Dynamics Observatory (SDO). We also investigate problems related to the detection of CBPs resulting from instrument and detection algorithm limitations. Methods. To determine the positions and identification of CBPs we used a segmentation algorithm. A linear fit of their central meridian distance and latitude vs time was used to derive velocities. Results. We obtained 906 velocity measurements in a time interval of only 2 days. The differential rotation profile can be expressed as ωrot = (14.◦47 ± 0.◦10 + (0.◦6 ± 1.◦0) sin2(b) = (-4.◦7 ± 1.◦7) sin4(b)) d-1. Our result is in agreement with other work and it comes with reasonable errors in spite of the very short time interval used. This was made possible by the higher sensitivity and resolution of the AIA instrument compared to similar equipment as well as high cadence. The segmentation algorithm also played a crucial role by detecting so many CBPs, which reduced the errors to a reasonable level. Conclusions. Data and methods presented in this paper show a great potential for obtaining very accurate velocity profiles, both for rotation and meridional motion and, consequently, Reynolds stresses. The amount of CBP data that could be obtained from this instrument should also provide a great opportunity to study changes of velocity patterns with a temporal resolution of only a few months. Other possibilities are studies of evolution of CBPs and proper motions of magnetic elements on the Sun.
Abstract
Characterizing the cyclic magnetic activity of stars that are close approximations of our Sun offers our best hope for understanding our Sun’s current and past magnetism, the space weather ...around solar-type stars, and more generally, the dynamos of other cool stars. The nearest current approximation to the Sun is the solar twin 18 Scorpii, a naked-eye Sun-like star of spectral type G2 Va. However, while 18 Scorpii’s physical parameters closely match those of the Sun, its activity cycle is about 7 yr, and shorter than the solar cycle. We report the measurement of a periodicity of 15 yr that corresponds to a longer activity cycle for 18 Scorpii based on observations extending to the last three decades. The global magnetic geometry of 18 Scorpii changes with this 15 yr cycle and appears to be equivalent to the solar 22 yr magnetic polarity cycle. These results suggest that 18 Scorpii is also a magnetic proxy for a younger Sun, adding an important new datum for testing dynamo theory and magnetic evolution of low-mass stars. The results perturb our understanding of the relationship between cycle and rotation, constrain the Sun’s magnetism and the Sun–Earth connection over the past billion years, and suggest that solar Schwabe and Hale cycle periods have increased over that time span.
Context. Understanding stellar activity in solar-type stars is crucial for the physics of stellar atmospheres as well as for ongoing exoplanet programmes. Aims. We aim to test how well we understand ...stellar activity using our own star, the Sun, as a test case. Methods. We performed a detailed study of the main optical activity indicators (Ca II H & K, Balmer lines, Na I D1 D2, and He I D3) measured for the Sun using the data provided by the HARPS-N solar-telescope feed at the Telescopio Nazionale Galileo. We made use of periodogram analyses to study solar rotation, and we used the pool variance technique to study the temporal evolution of active regions. The correlations between the different activity indicators as well as the correlations between activity indexes and the derived parameters from the cross-correlation technique are analysed. We also study the temporal evolution of these correlations and their possible relationship with indicators of inhomogeneities in the solar photosphere like sunspot number or radio flux values. Results. The value of the solar rotation period is found in all the activity indicators, with the only exception being Hδ. The derived values vary from 26.29 days (Hγ line) to 31.23 days (He I). From an analysis of sliding periodograms we find that in most of the activity indicators the spectral power is split into several “bands” of periods around 26 and 30 days. They might be explained by the migration of active regions between the equator and a latitude of ∼30°, spot evolution, or a combination of both effects. A typical lifetime of active regions of approximately ten rotation periods is inferred from the pooled variance diagrams, which is in agreement with previous works. We find that Hα, Hβ, Hγ, Hϵ, and He I show a significant correlation with the S index. Significant correlations between the contrast, bisector span, and the heliocentric radial velocity with the activity indexes are also found. We show that the full width at half maximum, the bisector, and the disc-integrated magnetic field correlate with the radial velocity variations. The correlation of the S index and Hα changes with time, increasing with larger sun spot numbers and solar irradiance. A similar tendency with the S index and radial velocity correlation is also present in the data. Conclusions. Our results are consistent with a scenario in which higher activity favours the correlation between the S index and the Hα activity indicators and between the S index and radial velocity variations.
A major obstacle to interpreting the rotation period distribution for main-sequence stars from Kepler mission data has been the lack of a precise evolutionary status for these objects. We address ...this by investigating the evolutionary status based on Gaia Data Release 2 parallaxes and photometry for more than 30,000 Kepler stars with rotation period measurements. Many of these are subgiants and should be excluded in future work on dwarfs. We particularly investigate a 193-star sample of solar analogs and report newly determined rotation periods for 125 of these. These include 54 stars from a prior sample, of which we can confirm the periods for 50. The remainder are new, and 10 of them longer than a solar rotation period, suggesting that Sun-like stars continue to spin down on the main sequence past solar age. Our sample of solar analogs could potentially serve as a benchmark for future missions, such as PLAnetary Transits and Oscillations of stars, and emphasizes the need for additional astrometric, photometric, and spectroscopic information before interpreting the stellar populations and results from time series surveys.
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