- ID:
- ivo://CDS.VizieR/J/ApJ/765/126
- Title:
- Star spot models for M-dwarfs in NGC 2516
- Short Name:
- J/ApJ/765/126
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- By combining rotation periods with spectroscopic determinations of projected rotation velocity, Jackson et al. (2009MNRAS.399L..89J) have found that the mean radii for low-mass M-dwarfs in the young, open cluster NGC 2516 are larger than model predictions at a given absolute I magnitude or I-K color and also larger than measured radii of magnetically inactive M-dwarfs. The relative radius difference is correlated with magnitude, increasing from a few percent at M_I_=7 to greater than 50% for the lowest luminosity stars in their sample at M_I_~9.5. Jackson et al. have suggested that a two-temperature star spot model is capable of explaining the observations, but their model requires spot coverage fractions of at least 50% in rapidly rotating M-dwarfs. Here we examine these results in terms of stellar models that include the inhibiting effects of magnetic fields on convective energy transport, with and without the effects of star spots. We find that a pure spot model is inconsistent with the color-magnitude diagram. The observations of radii versus color and radii versus absolute magnitude in NGC 2516 are consistent with models which include only magnetic inhibition or a combination of magnetic inhibition and spots. At a given mass we find a large dispersion in the strength of the vertical component of the magnetic field in the stellar photosphere but the general trend is that the vertical field increases with decreasing mass from a few hundred Gauss at 0.65M_{sun}_ to 600-900G, depending on spot coverage, in the lowest mass stars in the sample at 0.25M_{sun}_.
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- ID:
- ivo://CDS.VizieR/J/A+A/622/A85
- Title:
- Starspot rotation rates vs. activity cycle phase
- Short Name:
- J/A+A/622/A85
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- During the solar magnetic activity cycle the emergence latitudes of sunspots change, leading to the well-known butterfly diagram. This phenomenon is poorly understood for other stars since starspot latitudes are generally unknown. The related changes in starspot rotation rates caused by latitudinal differential rotation can however be measured. Using the set of 3093 Kepler stars with activity cycles identified by Reinhold et al. (2017A&A...603A..52R, Cat. J/A+A/603/A52), we aim to study the temporal change in starspot rotation rates over magnetic activity cycles, and how this relates to the activity level, the mean rotation rate of the star, and its effective temperature. We measured the photometric variability as a proxy for the magnetic activity and the spot rotation rate in each quarter over the duration of the Kepler mission. We phase-fold these measurements with the cycle period. To reduce random errors we perform averages over stars with comparable mean rotation rates and effective temperature at fixed activity-cycle phases. We detect a clear correlation between the variation of activity level and the variation of the starspot rotation rate. The sign and amplitude of this correlation depends on the mean stellar rotation and -- to a lesser extent -- on the effective temperature. For slowly rotating stars (rotation periods between 15-28 days) the starspot rotation rates are clearly anti-correlated with the level of activity during the activity cycles. A transition is observed around rotation periods of 10-15 days, where stars with effective temperature above 4200K instead show positive correlation. Our measurements can be interpreted in terms of a stellar "butterfly diagram", but these appear different from the Sun's since the starspot rotation rates are either in phase or anti-phase with the activity level. Alternatively, the activity cycle periods observed by Kepler are short (around 2.5 years) and may therefore be secondary cycles, perhaps analogous to the solar quasi-biennial oscillations.
- ID:
- ivo://CDS.VizieR/J/ApJ/743/48
- Title:
- Stars with rotation periods and X-ray luminosities
- Short Name:
- J/ApJ/743/48
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a sample of 824 solar and late-type stars with X-ray luminosities and rotation periods. This is used to study the relationship between rotation and stellar activity and derive a new estimate of the convective turnover time. From an unbiased subset of this sample the power law slope of the unsaturated regime, L_X_/L_{bol}_{propto}Ro^{beta}^, is fit as {beta}=-2.70+/-0.13. This is inconsistent with the canonical {beta}=-2 slope to a confidence of 5{sigma}, and argues for an additional term in the dynamo number equation. From a simple scaling analysis this implies {Delta}{Omega}/{Omega}{propto}{Omega}^0.7^, i.e. the differential rotation of solar-type stars gradually declines as they spin down. Super-saturation is observed for the fastest rotators in our sample and its parametric dependencies are explored. Significant correlations are found with both the corotation radius and the excess polar updraft, the latter theory providing a stronger dependence and being supported by other observations. We estimate mass-dependent empirical thresholds for saturation and super- saturation and map out three regimes of coronal emission. Late F-type stars are shown never to pass through the saturated regime, passing straight from super-saturated to unsaturated X-ray emission. The theoretical threshold for coronal stripping is shown to be significantly different from the empirical saturation threshold (Ro<0.13), suggesting it is not responsible. Instead we suggest that a different dynamo configuration is at work in stars with saturated coronal emission. This is supported by a correlation between the empirical saturation threshold and the time when stars transition between convective and interface sequences in rotational spin-down models.
- ID:
- ivo://CDS.VizieR/J/ApJ/851/91
- Title:
- Statistical studies of solar white-light flares
- Short Name:
- J/ApJ/851/91
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Recently, many superflares on solar-type stars have been discovered as white- light flares (WLFs). The statistical study found a correlation between their energies (E) and durations ({tau}): {tau}{propto}E^0.39^, similar to those of solar hard/soft X-ray flares, {tau}{propto}E^0.2-0.33^. This indicates a universal mechanism of energy release on solar and stellar flares, i.e., magnetic reconnection. We here carried out statistical research on 50 solar WLFs observed with Solar Dynamics Observatory/HMI and examined the correlation between the energies and durations. As a result, the E-{tau} relation on solar WLFs ({tau}{propto}E^0.38^) is quite similar to that on stellar superflares ({tau}{propto}E^0.39^). However, the durations of stellar superflares are one order of magnitude shorter than those expected from solar WLFs. We present the following two interpretations for the discrepancy: (1) in solar flares, the cooling timescale of WLFs may be longer than the reconnection one, and the decay time of solar WLFs can be elongated by the cooling effect; (2) the distribution can be understood by applying a scaling law ({tau}{propto}E^1/3^B^-5/3^) derived from the magnetic reconnection theory. In the latter case, the observed superflares are expected to have 2-4 times stronger magnetic field strength than solar flares.
- ID:
- ivo://CDS.VizieR/J/MNRAS/394/1338
- Title:
- Stellar effective magnetic fields. II.
- Short Name:
- J/MNRAS/394/1338
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the second catalog of averaged quadratic effective magnetic fields <B_e_> of 1212 main sequence and giant stars, and 11 white dwarfs. Our sample includes a subset of 610 chemically peculiar early-type stars. Some stars in the sample are members of several open clusters. The catalog was derived from measurements of the longitudinal magnetic field strength B_e_ for stars, which were scattered in the published sources.
- ID:
- ivo://CDS.VizieR/J/A+A/430/1143
- Title:
- Stellar magnetic rotational phase curves
- Short Name:
- J/A+A/430/1143
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Magnetized stars usually exhibit periodic variations of the effective (longitudinal) magnetic field B_e_ caused by their rotation. We present a catalog of magnetic rotational phase curves, B_e_ vs. the rotational phase phi, and tables of their parameters for 136 stars on the main sequence and above it. Phase curves were obtained by the least squares fitting of sine wave or double wave functions to the available B_e_ measurements, which were compiled from the existing literature. Most of the catalogued objects are chemically peculiar A and B type stars (127 stars). For some stars we also improved or determined periods of their rotation. We discuss the distribution of parameters describing magnetic rotational phase curves in our sample.
- ID:
- ivo://CDS.VizieR/J/A+A/652/A31
- Title:
- Stellar magnetic rotational phase curves. II
- Short Name:
- J/A+A/652/A31
- Date:
- 22 Feb 2022
- Publisher:
- CDS
- Description:
- Some stars show periodic variability of the longitudinal (effective) global magnetic field B_e_ mainly due to rotation. We present the second version of the catalog of average magnetic phase curves, which are functions of B_e_ versus the rotational phase phi, tables of their parameters. The curves were obtained from literary data and as the result of observations from SAO RAS telescopes. A total of 357 stars of various spectral types were studied, data on which was published before the end of August 2020. Magnetic Ap and Bp stars make up the most numerous a subset in the catalog (224 objects). Phase curves were obtained by least squares fit of a simple sine wave or a double sine wave to the available B_e measurements. For some stars we also improved or determined periods of their rotation. Phase curves also are presented in 514 figures. In some cases several phase curves presented here correspond to the same star. This occurred when different observational techniques produced significantly different series of B_e_ values for that star. In a few cases the rotational phase curves of the surface field, B_s_, are given for stars where longitudinal magnetic field data were not available.
- ID:
- ivo://CDS.VizieR/J/MNRAS/441/2361
- Title:
- Stellar magnetism, age and rotation
- Short Name:
- J/MNRAS/441/2361
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We investigate how the observed large-scale surface magnetic fields of low-mass stars (~0.1-2M_{sun}_), reconstructed through Zeeman-Doppler imaging, vary with age t, rotation and X-ray emission. Our sample consists of 104 magnetic maps of 73 stars, from accreting pre-main sequence to main-sequence objects (1Myr<~t<~10Gyr). For non-accreting dwarfs we empirically find that the unsigned average large-scale surface field is related to age as t^-0.655+/-0.045^. This relation has a similar dependence to that identified by Skumanich, used as the basis for gyrochronology. Likewise, our relation could be used as an age-dating method ('magnetochronology'). The trends with rotation we find for the large-scale stellar magnetism are consistent with the trends found from Zeeman broadening measurements (sensitive to large- and small-scale fields). These similarities indicate that the fields recovered from both techniques are coupled to each other, suggesting that small- and large-scale fields could share the same dynamo field generation processes. For the accreting objects, fewer statistically significant relations are found, with one being a correlation between the unsigned magnetic flux and rotation period. We attribute this to a signature of star-disc interaction, rather than being driven by the dynamo.
- ID:
- ivo://CDS.VizieR/J/A+A/618/A163
- Title:
- Stokes param. of Bok globules background stars
- Short Name:
- J/A+A/618/A163
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The role of magnetic fields in the process of star formation is a matter of continuous debate. Clear observational proof of the general influence of magnetic fields on the early phase of cloud collapse is still pending. In an earlier study on Bok globules with simple structures, we find strong indications of dominant magnetic fields across large spatial scales. The aim of this study is to test the magnetic field influence across Bok globules with more complex density structures. We apply near-infrared polarimetry to trace the magnetic field structure on scales of 10^4^-10^5^au in selected Bok globules. The combination of these measurements with archival data in the optical and sub-mm wavelength range allows us to characterize the magnetic field on scales of 10^3^-10^6^au. We present polarimetric data in the near-infrared wavelength range for the three Bok globules CB34, CB56, and [OMK2002] 18, combined with archival polarimetric data in the optical wavelength range for CB34 and CB56, and in the sub millimeter wavelength range for CB34 and [OMK2002] 18. We find a strong polarization signal (P>2%) in the near-infrared for all three globules. For CB34, we detect a connection between the structure on scales of 10^4^-10^5^au to 10^5^-10^6^au. For CB56, we trace aligned polarization segments in both the near-infrared and optical data, suggesting a connection of the magnetic field structure across the whole globule. In the case of [OMK2002]18, we find ordered polarization structures on scales of 10^4^-10^5^au. We find strongly aligned polarization segments on large scales which indicate dominant magnetic fields across Bok globules with complex density structures. To reconcile our findings in globules, the lowest mass clouds known, and the results on intermediate (e.g. Taurus) and more massive (e.g. Orion) clouds, we postulate a mass dependent role of magnetic fields, whereby magnetic fields appear to be dominant on low and high mass but rather sub dominant on intermediate mass clouds.
- ID:
- ivo://CDS.VizieR/J/A+A/545/A119
- Title:
- Stromgren v photometry of {sigma} Lup
- Short Name:
- J/A+A/545/A119
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Magnetic early B-type stars are rare. Indirect indicators are needed to identify them before investing in time-intensive spectropolarimetric observations. We use the strongest indirect indicator of a magnetic field in B stars, which is periodic variability of ultraviolet (UV) stellar wind lines occurring symmetric about the approximate rest wavelength. Our aim is to identify probable magnetic candidates which would become targets for follow-up spectropolarimetry to search for a magnetic field. From the UV wind line variability the B1/B2V star sigma Lupi emerged as a new magnetic candidate star. AAT spectropolarimetric measurements with SEMPOL were obtained. The longitudinal component of the magnetic field integrated over the visible surface of the star was determined with the least- squares deconvolution method. The UV line variations of sigma Lupi are similar to what is known in magnetic B stars, but no periodicity could be determined. We detected a varying longitudinal magnetic field with amplitude of about 100G with error bars of typically 20G, which supports an oblique magnetic-rotator configuration. The equivalent width variations of the UV lines, the magnetic and the optical-line variations are consistent with the photometric period of 3.02d, which we identify with the rotation period of the star. Additional observations with ESPaDOnS attached to the CFHT confirmed this discovery, and allowed the determination of a precise magnetic period. Analysis revealed that sigma Lupi is a helium-strong star, with an enhanced nitrogen abundance and an underabundance of carbon, and has a chemically spotted surface. Conclusions. sigma Lupi is a magnetic oblique rotator, and is a He-strong star. Like in other magnetic B stars the UV wind emission appears to originate close to the magnetic equatorial plane, with maximum emission occurring when a magnetic pole points towards the Earth. The 3.01972+/-0.00043d magnetic rotation period is consistent with the photometric period, with maximum light corresponding to maximum magnetic field.
