Stars Of Spectral Types Research Articles

Variations in the Brightness Variability Amplitude of A–M Stars

We study relations of the parameter Rvar (amplitude of the brightness variability, stellar activity indicator) with respect to the effective temperature and the period of rotation of stars belonging to the A–M spectral types. A common dataset gives an idea of variations in this parameter for stars with convection. It was confirmed that the limit of applicability for the gyrochronological relation PtM does not correspond to the limiting value (B - V )0 0.47, and the relationship between a rotation period and color is likewise visible up to groups of stars with (B - V )0 = 0.2. It is shown that the value of the parameter Rvar for stars with effective temperatures of 6500–7500 K are about an order of magnitude lower than that for stars, which indicates a nonmonotonic change of the parameter and, therefore, does not allow us to conclude that there is a single dependence of this parameter in the entire temperature range. In the diagram Rvar–P (the rotation period), A-type stars fall in the sequence of fast-rotating active objects, whereas stars with similar temperatures and belonging to the F spectral type (including objects from the cluster NGC 6866) lie in a different sequence corresponding to fast-rotating objects with a small amplitude in the brightness variability. The existence of a unified relationship between the properties of rotational activity in stars of spectral types A, F, and later, as well as possible physical processes leading to the implementation of a standard gyrochronological ratio and activity in A stars are discussed.

Simulations of starspot anomalies within TESS exoplanetary transit light curves

Context. The primary targets of the NASA Transiting Exoplanet Survey Satellite (TESS) are K and M dwarf stars within our solar neighbourhood. Young K and M dwarf stars are known to exhibit a high starspot coverage (≈50%), however, older stars are known to show fewer starspots. This implies that TESS transit light curves at 2 min cadence may contain starspot anomalies, and if so, will require transit-starspot models to determine accurately the properties of the system. Aims. The goals are to determine if starspot anomalies can manifest in TESS transit light curves, to determine the detection limits of the starspot anomalies, and to examine the relationship between the change in flux caused by the starspot anomaly and the planetary transit. Methods. We conducted 20 573 simulations of planetary transits around spotted stars using the transit-starspot model, PRISM. In total 3888 different scenarios were considered using three different host star spectral types, M4V, M1V, and K5V. The mean amplitude of the starspot anomaly was measured and compared to the photometric precision of the light curve to determine if the characteristic “blip” of the starspot anomaly was noticeable in the light curve. Results. The simulations show that starspot anomalies are observable in TESS 2 min cadence data. The smallest starspot detectable in TESS transit light curves has a radius of ≈ 1900 km. The starspot detection limits for the three host stars are 4900 ± 1700 km (M4V), 13 800 ± 6000 km (M1V), and 15 900 ± 6800 km (K5V). The smallest change in flux of the starspot (ΔFspot = 0.00015 ± 0.00001) can be detected when the ratio of planetary to stellar radii k = 0.082 ± 0.004. Conclusions. The results confirm known dependencies between the amplitude of the starspot anomaly and the photometric parameters of the light curve. The results facilitated the characterisation of the relationship between the change in flux of the starspot anomaly and the change in flux of the planetary transit for TESS transit light curves.

Surface Rotation and Photometric Activity for Kepler Targets. I. M and K Main-sequence Stars

Brightness variations due to dark spots on the stellar surface encode information about stellar surface rotation and magnetic activity. In this work, we analyze the Kepler long-cadence data of 26,521 main-sequence stars of spectral types M and K in order to measure their surface rotation and photometric activity level. Rotation-period estimates are obtained by the combination of a wavelet analysis and autocorrelation function of the light curves. Reliable rotation estimates are determined by comparing the results from the different rotation diagnostics and four data sets. We also measure the photometric activity proxy Sph using the amplitude of the flux variations on an appropriate timescale. We report rotation periods and photometric activity proxies for about 60 per cent of the sample, including 4,431 targets for which McQuillan et al. (2013a,2014) did not report a rotation period. For the common targets with rotation estimates in this study and in McQuillan et al. (2013a,2014), our rotation periods agree within 99 per cent. In this work, we also identify potential polluters, such as misclassified red giants and classical pulsator candidates. Within the parameter range we study, there is a mild tendency for hotter stars to have shorter rotation periods. The photometric activity proxy spans a wider range of values with increasing effective temperature. The rotation period and photometric activity proxy are also related, with Sph being larger for fast rotators. Similar to McQuillan et al. (2013a,2014), we find a bimodal distribution of rotation periods.

Probing CO and N2 Snow Surfaces in Protoplanetary Disks with N2H+ Emission

Abstract Snowlines of major volatiles regulate the gas and solid C/N/O ratios in the planet-forming midplanes of protoplanetary disks. Snow surfaces are the 2D extensions of snowlines in the outer disk regions, where radiative heating results in an increasing temperature with disk height. CO and N2 are two of the most abundant carriers of C, N, and O. N2H+ can be used to probe the snow surfaces of both molecules, because it is destroyed by CO and formed from N2. Here we present Atacama Large Millimeter/submillimeter Array (ALMA) observations of N2H+ at ∼0.″2–0.″4 resolution in the disks around LkCa 15, GM Aur, DM Tau, V4046 Sgr, AS 209, and IM Lup. We find two distinctive emission morphologies: N2H+ is either present in a bright, narrow ring surrounded by extended tenuous emission, or in a broad ring. These emission patterns can be explained by two different kinds of vertical temperature structures. Bright, narrow N2H+ rings are expected in disks with a thick Vertically Isothermal Region above the Midplane (VIRaM) layer (LkCa 15, GM Aur, DM Tau) where the N2H+ emission peaks between the CO and N2 snowlines. Broad N2H+ rings come from disks with a thin VIRaM layer (V4046 Sgr, AS 209, IM Lup). We use a simple model to extract the first sets of CO and N2 snowline pairs and corresponding freeze-out temperatures toward the disks with a thick VIRaM layer. The results reveal a range of N2 and CO snowline radii toward stars of similar spectral type, demonstrating the need for empirically determined snowlines in disks.

Activity time series of old stars from late F to early K

Context. A number of high-precision time series have recently become available for many stars as a result of data from CoRoT, Kepler, and TESS. These data have been widely used to study stellar activity. Photometry provides information that is integrated over the stellar disk. Therefore, there are many degeneracies between spots and plages or sizes and contrasts. In addition, it is important to relate activity indicators, derived from photometric light curves, to other indicators (Log R′HK and radial velocities). Aims. Our aim is to understand how to relate photometric variability to physical parameters in order to help the interpretation of these observations. Methods. We used a large number of synthetic time series of brightness variations for old main sequence stars within the F6-K4 range. Simultaneously, we computed using consistent modeling for radial velocity, astrometry, and chromospheric emission. We analyzed these time series to study the effect of the star spectral type on brightness variability, the relationship between brightness variability and chromospheric emission, and the interpretation of brightness variability as a function of spot and plage properties. We then studied spot-dominated or plage-dominated regimes. Results. We find that within our range of activity levels, the brightness variability increases toward low-mass stars, as suggested by Kepler results. However, many elements can create an interpretation bias. Brightness variability roughly correlates to Log R′HK level. There is, however, a large dispersion in this relationship, mostly caused by spot contrast and inclination. It is also directly related to the number of structures, and we show that it can not be interpreted solely in terms of spot sizes. Finally, a detailed analysis of its relation with Log R′HK shows that in the activity range of old main-sequence stars, we can obtain both spot or plage dominated regimes, as was shown by observations in previous works. The same star can also be observed in both regimes depending on inclination. Furthermore, only strong correlations between chromospheric emission and brightness variability are significant. Conclusions. Our realistic time series proves to be extremely useful when interpreting observations and understanding their limitations, most notably in terms of activity interpretation. Inclination is crucial and affects many properties, such as amplitudes and the respective role of spots and plages.

Follow-up observations of X-ray emitting hot subdwarf stars: the compact He-poor sdO star Feige 34

We report on results obtained with theXMM-Newtonobservation of Feige 34 carried out in April 2018. This is the first spectroscopic X-ray observation of a compact and helium-poor hot subdwarf star. The source was detected at a flux levelfX = 3.4 × 10−14erg cm−2s−1in the energy range 0.2–3 keV, which implies an X-ray-to-bolometric flux ratiofX/fbol ≃ 10−6.5. The source spectrum can be described with the sum of two thermal-plasma components with subsolar abundances at temperatures of ≃0.3 and 1.1 keV. These properties are similar to what is observed in early-type main-sequence stars, where the X-ray emission is attributed to turbulence and shocks in the stellar wind. Therefore, the same phenomenon could explain the X-ray properties of Feige 34. However, it is not possible to reproduce the observed spectrum with a thermal-plasma model if the elemental abundances are fixed at the values obtained from the optical and UV spectroscopy. Moreover, we show that the X-ray luminosity and spectrum are consistent with those expected from a young main-sequence star of late spectral type. Therefore, we discuss the possibility that the observed X-ray emission is due to the companion star of M0 spectral type, whose presence is suggested by the IR excess in the spectral energy distribution of Feige 34.

A VLT/FLAMES survey for massive binaries in Westerlund 1

Context. X-ray emission from massive stars was first reported four decades ago, but the precise physics governing its formation as a function of stellar properties and binarity remains not fully understood. With the recent suggestion that such objects may be important sites of cosmic ray production, a better understanding of their high-energy properties is particularly timely. Aims. The young massive cluster Westerlund 1 provides an ideal testbed for understanding this emission, with over 50 cluster members detected in historical X-ray observations. In the decade since these data were obtained, significant new multi-epoch observations of the cluster have been made, allowing a fundamental reappraisal of the nature of both X-ray bright and dark stars. Methods. Optical spectroscopy permits accurate classification of cluster members, while multi-epoch observations of a sub-set allow identification and characterisation of the binary population. Results. A total of 45 X-ray sources within Wd1 now have precise spectral classifications. Of these, 16 have been identified as candidate or confirmed massive binaries. X-ray emission is confined to O9-B0.5 supergiants, Wolf-Rayets and a small group of highly luminous interacting/post-interaction OB+OB binaries. Despite their presence in large numbers, no emission is seen from earlier, less evolved O stars or later, cooler B super-/hypergiants. A total of 22 stars have X-ray properties that are suggestive of a contribution from emission originating in a wind collision zone. Conclusions. We suppose that the lack of X-ray emission from O giants is due to their comparatively low intrinsic bolometric luminosity if, as expected, they follow the canonical LX/Lbol relation for hot stars. The transition away from X-ray emission for OB supergiants occurs at the location of the bistability jump; we speculate that below this limit, stellar wind velocities are insufficient for internal, X-ray emitting shocks to form. Our results are consistent with recent findings that massive binaries are not uniformly brighter than single stars of comparable luminosity or spectral type, although it is noteworthy that the brightest and hardest stellar X-ray sources within Wd1 are all either confirmed or candidate massive, interacting/post-interaction binaries.

Data-driven Spectroscopy of Cool Stars at High Spectral Resolution

Abstract The advent of large-scale spectroscopic surveys underscores the need to develop robust techniques for determining stellar properties (“labels,” i.e., physical parameters and elemental abundances). However, traditional spectroscopic methods that utilize stellar models struggle to reproduce cool (<4700 K) stellar atmospheres due to an abundance of unconstrained molecular transitions, making modeling via synthetic spectral libraries difficult. Because small, cool stars such as K and M dwarfs are both common and good targets for finding small, cool planets, establishing precise spectral modeling techniques for these stars is of high priority. To address this, we apply The Cannon, a data-driven method of determining stellar labels, to Keck High Resolution Echelle Spectrometer spectra of 141 cool (<5200 K) stars from the California Planet Search. Our implementation is capable of predicting labels for small (<1 R ⊙) stars of spectral types K and later with accuracies of 68 K in effective temperature (T eff), 5% in stellar radius (R *), and 0.08 dex in bulk metallicity ([Fe/H]), and maintains this performance at low spectral resolutions (R < 5000). As M dwarfs are the focus of many future planet-detection surveys, this work can aid efforts to better characterize the cool star population and uncover correlations between cool star abundances and planet occurrence for constraining planet formation theories.

The Influence of Host Star Spectral Type on Ultra-hot Jupiter Atmospheres

Abstract Ultra-hot Jupiters are the most highly irradiated gas giant planets, with equilibrium temperatures from 2000 to over 4000 K. Ultra-hot Jupiters are amenable to characterization due to their high temperatures, inflated radii, and short periods, but their atmospheres are atypical for planets in that the photosphere possesses large concentrations of atoms and ions relative to molecules. Here we evaluate how the atmospheres of these planets respond to irradiation by stars of different spectral type. We find that ultra-hot Jupiters exhibit temperature inversions that are sensitive to the spectral type of the host star. The slope and temperature range across the inversion both increase as the host star effective temperature increases due to enhanced absorption at short wavelengths and low pressures. The steep temperature inversions in ultra-hot Jupiters around hot stars result in increased thermal dissociation and ionization compared to similar planets around cooler stars. The resulting increase in H− opacity leads to a transit spectrum that has muted absorption features. The emission spectrum, however, exhibits a large contrast in brightness temperature, a signature that will be detectable with both secondary eclipse observations and high-dispersion spectroscopy. We also find that the departures from local thermodynamic equilibrium in the stellar atmosphere can affect the degree of heating caused by atomic metals in the planet’s upper atmosphere. Additionally, we further quantify the significance of heating by different opacity sources in ultra-hot Jupiter atmospheres.

Testing massive star evolution, star formation history, and feedback at low metallicity

Stars that start their lives with spectral types O and early B are the progenitors of core-collapse supernovae, long gamma-ray bursts, neutron stars, and black holes. These massive stars are the primary sources of stellar feedback in star-forming galaxies. At low metallicities, the properties of massive stars and their evolution are not yet fully explored. Here we report a spectroscopic study of 320 massive stars of spectral types O (23 stars) and B (297 stars) in the Wing of the Small Magellanic Cloud (SMC). The spectra, which we obtained with the ESO Very Large Telescope, were analyzed using state-of-the-art stellar atmosphere models, and the stellar parameters were determined. We find that the stellar winds of our sample stars are generally much weaker than theoretically expected. The stellar rotation rates show broad, tentatively bimodal distributions. The upper Hertzsprung–Russell diagram (HRD) is well populated by the stars of our sample from a specific field in the SMC Wing. A few very luminous O stars are found close to the main sequence, while all other, slightly evolved stars obey a strict luminosity limit. Considering additional massive stars in evolved stages, with published parameters and located all over the SMC, essentially confirms this picture. The comparison with single-star evolutionary tracks suggests a dichotomy in the fate of massive stars in the SMC. Only stars with an initial mass below ∼30 M⊙ seem to evolve from the main sequence to the cool side of the HRD to become a red supergiant and to explode as type II-P supernova. In contrast, stars with initially more than ∼30 M⊙ appear to stay always hot and might evolve quasi chemically homogeneously, finally collapsing to relatively massive black holes. However, we find no indication that chemical mixing is correlated with rapid rotation. We measured the key parameters of stellar feedback and established the links between the rates of star formation and supernovae. Our study demonstrates that in metal-poor environments stellar feedback is dominated by core-collapse supernovae in combination with winds and ionizing radiation supplied by a few of the most massive stars. We found indications of the stochastic mode of massive star formation, where the resulting stellar population is fully capable of producing large-scale structures such as the supergiant shell SMC-SGS 1 in the Wing. The low level of feedback in metal-poor stellar populations allows star formation episodes to persist over long timescales.

Variation of the magnetic field of the Ap star HD 50169 over its 29-year rotation period

Context. The Ap stars that rotate extremely slowly, with periods of decades to centuries, represent one of the keys to the understanding of the processes leading to the differentiation of stellar rotation. Aims. We characterise the variations of the magnetic field of the Ap star HD 50169 and derive constraints about its structure. Methods. We combined published measurements of the mean longitudinal field ⟨Bz⟩ of HD 50169 with new determinations of this field moment from circular spectropolarimetry obtained at the 6m telescope BTA of the Special Astrophysical Observatory of the Russian Academy of Sciences. For the mean magnetic field modulus ⟨B⟩, literature data were complemented by the analysis of ESO spectra, both newly acquired and from the archive. Radial velocities were also obtained from these spectra. Results. We present the first determination of the rotation period of HD 50169, Prot = 29.04 ± 0.82 yr. HD 50169 is currently the longest-period Ap star for which magnetic field measurements have been obtained over more than a full cycle. The variation curves of both ⟨Bz⟩ and ⟨B⟩ have a significant degree of anharmonicity, and there is a definite phase shift between their respective extrema. We confirm that HD 50169 is a wide spectroscopic binary, refine its orbital elements, and suggest that the secondary is probably a dwarf star of spectral type M. Conclusions. The shapes and mutual phase shifts of the derived magnetic variation curves unquestionably indicate that the magnetic field of HD 50169 is not symmetric about an axis passing through its centre. Overall, HD 50169 appears similar to the bulk of the long-period Ap stars.

The Belgian repository of fundamental atomic data and stellar spectra (BRASS)

Context. Fundamental atomic transition parameters, such as oscillator strengths and rest wavelengths, play a key role in modelling and understanding the chemical composition of stars in the universe. Despite the significant work under way to produce these parameters for many astrophysically important ions, uncertainties in these parameters remain large and can limit the accuracy of chemical abundance determinations.Aims. The Belgian repository of fundamental atomic data and stellar spectra (BRASS) aims to provide a large systematic and homogeneous quality assessment of the atomic data available for quantitative spectroscopy. BRASS shall compare synthetic spectra against extremely high-quality observed spectra, at a resolution of ∼85 000 and signal-noise ratios of ∼1000, for approximately 20 bright BAFGK spectral-type stars, in order to critically evaluate the atomic data available for over a thousand potentially useful spectral lines.Methods. A large-scale homogeneous selection of atomic lines is performed by synthesising theoretical spectra of literature atomic lines for FGK-type stars including the Sun, resulting in a selection of 1091 theoretically deep and unblended lines in the wavelength range 4200–6800 Å, which may be suitable for quality assessment. Astrophysical log(g f) values are determined for the 1091 transitions using two commonly employed methods. The agreement of these log(g f) values are used to select well-behaved lines for quality assessment.Results. We found 845 atomic lines to be suitable for quality assessment, of which 408 were found to be robust against systematic differences between analysis methods. Around 53% of the quality-assessed lines were found to have at least one literature log(g f) value in agreement with our derived values, though the remaining values can disagree by as much as 0.5 dex. Only ∼38% of Fe Ilines were found to have sufficiently accurate log(g f) values, increasing to ∼70–75% for the remaining Fe-group lines.

Analytic solutions to the maximum and average exoplanet transit depth for common stellar limb darkening laws

Context. The depth of an exoplanetary transit in the light curve of a distant star is commonly approximated as the squared planet-to-star radius ratio, (Rp/Rs)2. Stellar limb darkening, however, can result in significantly deeper transits. An analytic solution would be worthwhile to illustrate the principles of the problem and predict the actual transit signal required for the planning of transit observations with certain signal-to-noise requirements without the need of computer-based transit simulations. Aims. We calculate the overshoot of the mid-transit depth caused by stellar limb darkening compared to the (Rp/Rs)2 estimate for arbitrary transit impact parameters. In turn, this allows us to compute the true planet-to-star radius ratio from the transit depth for a given parameterization of a limb darkening law and for a known transit impact parameter. Methods. We compute the maximum emerging specific stellar intensity covered by the planet in transit and derive analytic solutions for the transit depth overshoot. Solutions are presented for the linear, quadratic, square-root, logarithmic, and nonlinear stellar limb darkening with arbitrary transit impact parameters. We also derive formulae to calculate the average intensity along the transit chord, which allows us to estimate the actual transit depth (and therefore Rp∕Rs) from the mean in-transit flux. Results. The transit depth overshoot of exoplanets compared to the (Rp/Rs)2 estimate increases from about 15% for main-sequence stars of spectral type A to roughly 20% for sun-like stars and some 30% for K and M stars. The error in our analytical solutions for Rp∕Rs from the small planet approximation is orders of magnitude smaller than the uncertainties arising from typical noise in real light curves and from the uncertain limb darkening. Conclusions. Our equations can be used to predict with high accuracy the expected transit depth of extrasolar planets. The actual planet radius can be calculated from the measured transit depth or from the mean in-transit flux if the stellar limb darkening can be properly parameterized and if the transit impact parameter is known. Light curve fitting is not required.

High-resolution spectroscopy and high contrast imaging with the ELT: looking for O2 in Proxima b

We research the requirements of High Contrast Imaging when combined with the cross correlation (CC) of high resolution spectra with known spectroscopic templates for detecting and characterising exoplanets in reflected light. We simulate applying the technique to a potentially habitable Proxima b-like planet and show that the O$_2$ A-band spectral feature could feasibly be detected on nearby rocky exoplanets using future instruments on the ELT . The technique is then more widely analysed showing that detections of planets and O$_2$ with signal to noise in the CC function (SNR$_{CC}$) > 3 can be obtained when the signal to noise of the simulated planet spectrum (SNR$_{spec}$) is from 0.25 to 1.2. We place constraints on the spectral resolution, instrument contrast, point spread function (PSF), exposure times and systematic error in stellar light subtraction for making such detections. We find that accurate stellar light subtraction (with 99.99% removal) and PSFs with high spatial resolutions are key to making detections. Lastly a further investigation suggests the ELT could potentially discover and characterise planets of all sizes around different spectral type stars, as well as detecting O$_2$ on Super-Earths with habitable zone orbits around nearby M stars.

Magnetic activities on active solar-type stars

Indian Institute of Astrophysics, II Block Koramangala, Bangalore 560034, India We present results obtained from the studies of magnetic activities on four solar-type stars (F-type star KIC 6791060, K-type star LO Peg, and two M-type planet-hosting stars K2–33 and EPIC 211901114) by using optical observations from several ground- and space-based telescopes. In this study, we investigate magnetic activities such as spot-topographic evolution and flaring events in these stars. We compare the results obtained from this study with that of the Sun. In the surface temperature maps, one active longitude has been detected in KIC 6791060, whereas in each of the other three cases two active longitudes are seen. The spottedness was found to vary in the range of 0.07–0.44%, 9–26%, 3.6–4.2%, and 4.5–5.3% for KIC 6791060, LO Peg, K2–33, and EPIC 211901114, respectively. Several flaring events have been identified in each star. An increasing trend in flaring frequency per stellar rotation has been found in the stars with change in spectral type from F to M. These findings indicate the increase in magnetic activities with the spectral type of stars. This can be explained due to increasing the ratio of the thickness of the convection zone to the radiation zone from F-type star to the M-type stars.

The population of hot subdwarf stars studied with Gaia

Based on data from the ESA Gaia Data Release 2 (DR2) and several ground-based, multi-band photometry surveys we have compiled an all-sky catalogue of 39 800 hot subluminous star candidates selected in Gaia DR2 by means of colour, absolute magnitude, and reduced proper motion cuts. We expect the majority of the candidates to be hot subdwarf stars of spectral type B and O, followed by blue horizontal branch stars of late B-type (HBB), hot post-AGB stars, and central stars of planetary nebulae. The contamination by cooler stars should be about 10%. The catalogue is magnitude limited to Gaia G < 19 mag and covers the whole sky. Except within the Galactic plane and LMC/SMC regions, we expect the catalogue to be almost complete up to about 1.5 kpc. The main purpose of this catalogue is to serve as input target list for the large-scale photometric and spectroscopic surveys which are ongoing or scheduled to start in the coming years. In the long run, securing a statistically significant sample of spectroscopically confirmed hot subluminous stars is key to advance towards a more detailed understanding of the latest stages of stellar evolution for single and binary stars.

Far-infrared emission of massive stars

We present results of the analysis of a sample of 22 stars of spectral types from O7 to B5 and luminosity classes I–V for which spectra from the Infrared Spectrograph (IRS) of Spitzer are available. The IRS spectra of these stars are examined for signs of excess infrared (IR) emission by comparison with stellar atmospheric spectra. We find that the spectra of half of the studied stars are dominated by excess emission in the far-IR, including all six super- and bright giants. In order to examine the origin of the far-IR excess, we supplement the Spitzer data with optical high-resolution echelle spectroscopy (λ∕Δλ ~ 105), near-IR high-contrast coronagraphic imaging taken with the SPHERE instrument at VLT with a spatial resolution of 0.′′05, and WISE and Herschel photometry. In the optical region, we detect various absorption and emission lines (H α, C III, and N III) irrespective of the far-IR excess. Pfund α and Humphrey α lines are observed at the same time as the far-IR excess. These lines are stronger in stars with far-IR excess than in stars without excess. A scattered-light disk in the central r ≲ 2.5′′ region of the far-IR excess stars HD 149404, HD 151804, and HD 154368 can be excluded from H band imaging down to a 1σ contrast of F(r)∕F∗~ 10−6. The far-IR excess is fit either by a free–free component from ionized gas as for the winds of hot stars or a large (1 pc) circumstellar dust shell. The putative dust envelopes required to explain the excess have a visual extinction as low as a few hundred μ-mag.

New Features of Parenago’s Discontinuity from Gaia DR1 Data

The velocity field of main-sequence stars and red giants from the TGAS catalogue with heliocentric distances up to 1.5 kpc has been analyzed for various spectral types. To estimate the influence of a low accuracy of stellar parallax measurements on the results of a kinematic analysis of distant stars, first we have studied in detail how the kinematic parameters derived with 1/π distances are shifted when these distances are replaced by three other versions of distances from Astraatmadja et al. (2016b). We have obtained detailed tables in which the ranges of these shifts in the Ogorodnikov–Milne and Bottlinger model parameters are shown for the stars of each spectral type. We have the smallest shifts in the case of determining the Oort coefficients A and B, for which there are 10% shifts only for main-sequence stars of spectral type B. In the remaining cases, these shifts are 0–3%. For the remaining parameters the shifts do not exceed 30%. Thus, we have shown that using the 1/π distance scale in estimating the Ogorodnikov–Milne and Bottlingermodel parameters (except for the parameter Ω″0) yields reliable results even when using parallaxes with large relative errors (up to 60%). To study Parenago’s discontinuity, we have investigated the dependence of the Ogorodnikov–Milne and Bottlinger model parameters on color for 1 260 071 mainsequence stars and 534 387 red giants. As far as we know, such a data set is used for the first time to investigate Parenago’s discontinuity. The main result is the detection of maximum points at B − V = 0.75 after which the solar velocity component V and the Oort coefficient B decrease when moving from blue stars to red ones. This fact is a new feature of Parenago’s discontinuity, because the component V does not change in the classical case at B − V >0.6. We have made an attempt to represent the well-known Parenago’s discontinuity as a special case of the more complex effect of a gradual change in a number of kinematic parameters as the mean age and composition of the group of stars under study changes.

An atlas of cool supergiants from the Magellanic Clouds and typical interlopers

We present an atlas composed of more than 1500 spectra of late-type stars (spectral types from G to M) observed simultaneously in the optical and calcium triplet spectral ranges. These spectra were obtained as part of a survey to search for cool supergiants in the Magellanic Clouds and were taken over four epochs. We provide the spectral and luminosity classification for each spectrum (71% are supergiants, 13% are giants or luminous giants, 4% are carbon or S stars, and the remaining 12% are foreground stars of lesser luminosities). We also provide a detailed guide for the spectral classification of luminous late-type stars, the result of the extensive classification work done for the atlas. Although this guide is based on classical criteria, we have put them together and re-elaborated them for modern CCD-spectra as these criteria were scattered among many different works and mainly conceived for use with photographic plate spectra. The result is a systematic, well-tested process for identifying and classifying luminous late-type stars, illustrated with CCD spectra of standard stars and the classifications of our own catalogue.

Resolving faint structures in the debris disk around TWA 7

Context. Debris disks are the intrinsic by-products of the star and planet formation processes. Most likely due to instrumental limitations and their natural faintness, little is known about debris disks around low mass stars, especially when it comes to spatially resolved observations. Aims. We present new VLT/SPHERE IRDIS dual-polarization imaging (DPI) observations in which we detect the dust ring around the M2 spectral type star TWA 7. Combined with additional angular differential imaging observations we aim at a fine characterization of the debris disk and setting constraints on the presence of low-mass planets. Methods. We modeled the SPHERE DPI observations and constrain the location of the small dust grains, as well as the spectral energy distribution of the debris disk, using the results inferred from the observations, and performed simple N-body simulations. Results. We find that the dust density distribution peaks at ~0.72′′ (25 au), with a very shallow outer power-law slope, and that the disk has an inclination of ~13° with a position angle of ~91° east of north. We also report low signal-to-noise ratio detections of an outer belt at a distance of ~1.5′′ (~52 au) from the star, of a spiral arm in the southern side of the star, and of a possible dusty clump at 0.11′′. These findings seem to persist over timescales of at least a year. Using the intensity images, we do not detect any planets in the close vicinity of the star, but the sensitivity reaches Jovian planet mass upper limits. We find that the SED is best reproduced with an inner disk at ~0.2′′ (~7 au) and another belt at 0.72′′ (25 au). Conclusions. We report the detections of several unexpected features in the disk around TWA 7. A yet undetected 100M⊕ planet with a semi-major axis at 20−30 au could possibly explain the outer belt as well as the spiral arm. We conclude that stellar winds are unlikely to be responsible for the spiral arm.