arXiv subjects

P. C. Schneider

Publications and source records attributed to P. C. Schneider.

At least 37 records · Page 2Linked to original sources

arXiv2023

A multi-wavelength view of the multiple activity cycles of $ε$~Eridani

$ε$ Eridani is a highly active young K2 star with an activity cycle of about three years established using Ca II H & K line index measurements (S_MWO). This relatively short cycle has been demonstrated to be consistent with X-ray and magnetic flux measurements. Recent work suggested a change in the cyclic behaviour. Here we report new X-ray flux and S_MWO measurements and also include S_MWO measurements from the historical Mount Wilson program. This results in an observational time baseline of over 50 years for the S_MWO data and of over 7 years in X-rays. Moreover, we include Ca II infrared triplet (IRT) index measurements (S_IRT) from 2013-2022 in our study. With the extended X-ray data set, we can now detect the short cycle for the first time using a periodogram analysis. Near-simultaneous S_MWO data and X-ray fluxes, which are offset by 20 days at most, are moderately strongly correlated when only the lowest activity state (concerning short-term variability) is considered in both diagnostics. In the S_MWO data, we find strong evidence for a much longer cycle of about 34 years and an 11-year cycle instead of the formerly proposed $12$-year cycle in addition to the known 3-year cycle. The superposition of the three periods naturally explains the recent drop in S_MWO measurements. The two shorter cycles are also detected in the S_IRT data, although the activity cycles exhibit lower amplitudes in the S_IRT than in the S_MWO data. Finally, the rotation period of $ε$Eri can be found more frequently in the S_MWO as well as in the S_IRT data for times near the minimum of the long cycle. This may be explained by a scenario in which the filling factor for magnetically active regions near cycle maximum is too high to allow for notable short-term variations.

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arXiv2022

The He I $λ$10830 $Å$ line as a probe of winds and accretion in young stars in Lupus and Upper Scorpius

The He I 1 micron line is a high excitation line which allows us to probe the innermost regions of protostellar disks, and to trace both accreting and outflowing material. We use X-Shooter observations of a sample of 107 young stars in the Lupus (1-3 Myr) and Upper Scorpius (5-10 Myr) star-forming regions to search for correlations between the line properties, as well as the disk inclination and accretion luminosity. We identified eight distinct profile types in the sample. We fitted Gaussian curves to the line features to measure the maximum velocities traced in absorption, the full-width half-maximum (FWHM) of the line features, and the Gaussian area of the features. We compare the proportion of each profile type in our sample to previous studies in Taurus. We find significant variations between Taurus and Lupus in the proportion of P Cygni and inverse P Cygni profiles, and between Lupus and Upper Scorpius in the number of emission-only and combination profile types. We find that the blue-shifted absorption features appear less blue-shifted at disk inclinations close to edge-on, but no such trend with inclination is observed in sources with only red-shifted features. Higher accretion rates were observed in sources with strong blue-shifted features which, along with the changes in the proportions of each profile type observed in the two regions, indicates that younger sources may drive stronger jets or winds. Overall, we observe variations in the proportion of each profile type and in the line properties which indicates and evolution of accretion and ejection signatures over time, and with source properties. These results confirm past works and models of the He I line, but for a larger sample and for multiple star-forming regions. The work highlights the power of the He I line as a probe of the gas in the innermost regions of the disk.

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arXiv2022

The stellar content of the ROSAT all-sky survey

We present and apply a method to identify the stellar content of the ROSAT all-sky survey (RASS). We performed a crossmatch between the RASS sources and stellar candidates selected from Gaia Early Data Release 3 (EDR3) and estimated stellar probabilities for every RASS source from the geometric properties of the match and additional properties, namely the X-ray to G-band flux ratio and the counterpart distances. A comparison with preliminary detections from the first eROSITA all-sky survey (eRASS1) show that the positional offsets of the RASS sources are larger than expected from the uncertainties given in the RASS catalog. From the RASS sources with reliable positional uncertainties, we identify 28630 (24.9 %) sources as stellar; this is the largest sample of stellar X-ray sources to date. Directly from the stellar probabilities, we estimate the completeness and reliability of the sample to be about 93 % and confirm this value by comparing it to the identification of randomly shifted RASS sources, preliminary stellar eRASS1 identifications, and results from a previous identification of RASS sources. Our stellar RASS sources contain sources of all spectral types and luminosity classes. According to their position in the color-magnitude diagram, many stellar RASS sources are young stars with ages of a few $10^7$ yr or binaries. When plotting the X-ray to bolometric flux ratio as a function of the color, the onset of convection and the saturation limit are clearly visible. We note that later-type stars reach continuously higher $F_X/F_{bol}$ values, which is probably due to more frequent flaring. The color distribution of the stellar RASS sources clearly differs from the unrelated background sources. We present the three-dimensional distribution of the stellar RASS sources that shows a clear increase in the source density near known stellar clusters.

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arXiv2022

Coronal and chromospheric emission in A-type stars

Cool stars on the main sequence generate X-rays from coronal activity, powered by a convective dynamo. With increasing temperature, the convective envelope becomes smaller and X-ray emission fainter. We present Chandra/HRC-I observations of four single stars with early A spectral types. Only the coolest star of this sample, $τ^3$ Eri ($T_\mathrm{eff}\approx8\,,000$ K), is detected with $\log(L_X/L_\mathrm{bol})=-7.6$ while the three hotter stars ($T_\mathrm{eff}\geq8\,,000$ K), namely $δ$ Leo, $β$ Leo, and $ι$ Cen, remain undetected with upper limits $\log(L_X/L_\mathrm{bol})<-8.4$. The drop in X-ray emission thus occurs in a narrow range of effective temperatures around $\sim 8100$ K and matches the drop of activity in the C III and O VI transition region lines.

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arXiv2022

The high energy spectrum of Proxima Centauri simultaneously observed at X-ray and FUV wavelengths

The M dwarf Proxima~Centauri is known to be magnetically active and it hosts a likely Earth-like planet in its habitable zone. Understanding the characteristics of stellar radiation by understanding the properties of the emitting plasma is of paramount importance for a proper assessment of the conditions on Proxima~Centauri~b and exoplanets around M dwarfs in general. We determine the temperature structure of the coronal and transition region plasma of Proxima Centauri from simultaneous X-ray and far-ultraviolet (FUV) observations. The differential emission measure distribution (DEM) was constructed for flaring and quiescent periods by analysing optically thin X-ray and FUV emission lines. Four X-ray observations of Proxima Centauri were conducted by the LETGS instrument on board of the Chandra X-ray Observatory and four FUV observations were carried out using the Hubble STIS spectrograph. From the X-ray light curves, we determined a variation of the quiescent count rate by a factor of two within 20\% of the stellar rotation period. To obtain the DEM, 18 optically thin emission lines were analysed (12 X-ray and six FUV). The flare fluxes differ from the quiescence fluxes by factors of 4-20 (FUV) and 1-30 (X-ray). The temperature structure of the stellar corona and transition region was determined for both the quiescence and flaring state by fitting the DEM(T) with Chebyshev polynomials for a temperature range $\log$T = 4.25 - 8. Compared to quiescence, the emission measure increases during flares for temperatures below 0.3\,MK (FUV dominated region) and beyond 3.6\,MK (X-ray dominated region). The reconstructed DEM shape provides acceptable line flux predictions compared to the measured values. We provide synthetic spectra at 1-1700 Å, which may be considered as representative for the high-energy irradiation of Proxima~Cen~b during quiescent and flare periods.

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arXiv2022

X-ray emission from candidate stellar merger remnant TYC 2597-735-1 and its Blue Ring Nebula

Tight binary or multiple star systems can interact through mass transfer and follow vastly different evolutionary pathways than single stars. The star TYC 2597-735-1 is a candidate for a recent stellar merger remnant resulting from a coalescence of a low-mass companion with a primary star a few thousand years ago. This violent event is evident in a conical outflow ("Blue Ring Nebula") emitting in UV light and surrounded by leading shock filaments observed in H$α$ and UV emission. From Chandra data, we report the detection of X-ray emission from the location of TYC 2597-735-1 with a luminosity $\log(L_\mathrm{X}/L_\mathrm{bol})=-5.5$. Together with a previously reported period around 14~days, this indicates ongoing stellar activity and the presence of strong magnetic fields on TYC 2597-735-1. Supported by stellar evolution models of merger remnants, we interpret the inferred stellar magnetic field as dynamo action associated with a newly formed convection zone in the atmosphere of TYC 2597-735-1, though internal shocks at the base of an accretion-powered jet cannot be ruled out. We speculate that this object will evolve into an FK Com type source, i.e. a class of rapidly spinning magnetically active stars for which a merger origin has been proposed but for which no relic accretion or large-scale nebula remains visible. We also detect likely X-ray emission from two small regions close to the outer shock fronts in the Blue Ring Nebula, which may arise from either inhomogenities in the circumstellar medium or in the mass and velocity distribution in the merger-driven outflow.

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arXiv2022

The ODYSSEUS Survey. Motivation and First Results: Accretion, Ejection, and Disk Irradiation of CVSO 109

The Hubble UV Legacy Library of Young Stars as Essential Standards (ULLYSES) Director's Discretionary Program of low-mass pre-main-sequence stars, coupled with forthcoming data from ALMA and JWST, will provide the foundation to revolutionize our understanding of the relationship between young stars and their protoplanetary disks. A comprehensive evaluation of the physics of disk evolution and planet formation requires understanding the intricate relationships between mass accretion, mass outflow, and disk structure. Here we describe the Outflows and Disks around Young Stars: Synergies for the Exploration of ULLYSES Spectra (ODYSSEUS) Survey and present initial results of the classical T Tauri Star CVSO 109 in Orion OB1b as a demonstration of the science that will result from the survey. ODYSSEUS will analyze the ULLYSES spectral database, ensuring a uniform and systematic approach in order to (1) measure how the accretion flow depends on the accretion rate and magnetic structures, (2) determine where winds and jets are launched and how mass-loss rates compare with accretion, and (3) establish the influence of FUV radiation on the chemistry of the warm inner regions of planet-forming disks. ODYSSEUS will also acquire and provide contemporaneous observations at X-ray, optical, NIR, and millimeter wavelengths to enhance the impact of the ULLYSES data. Our goal is to provide a consistent framework to accurately measure the level and evolution of mass accretion in protoplanetary disks, the properties and magnitudes of inner-disk mass loss, and the influence of UV radiation fields that determine ionization levels and drive disk chemistry.

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arXiv2021

H$α$ and He I absorption in HAT-P-32 b observed with CARMENES -- Detection of Roche lobe overflow and mass loss

We analyze two high-resolution spectral transit time series of the hot Jupiter HAT-P-32 b obtained with the CARMENES spectrograph. Our new XMM-Newton X-ray observations of the system show that the fast-rotating F-type host star exhibits a high X-ray luminosity of 2.3e29~erg/s (5-100 A), corresponding to a flux of 6.9e4 erg/cm**2/s at the planetary orbit, which results in an energy-limited escape estimate of about 1e13 g/s for the planetary mass-loss rate. The spectral time series show significant, time-dependent absorption in the Halpha and He I triplet lines with maximum depths of about 3.3% and 5.3%. The mid-transit absorption signals in the Halpha and He I lines are consistent with results from one-dimensional hydrodynamic modeling, which also yields mass-loss rates on the order of 1e13 g/s. We observe an early ingress of a redshifted component of the transmission signal, which extends into a redshifted absorption component, persisting until about the middle of the optical transit. While a super-rotating wind can explain redshifted ingress absorption, we find that an up-orbit stream, transporting planetary mass in the direction of the star, also provides a plausible explanation for the pre-transit signal. This makes HAT-P-32 a benchmark system for exploring atmospheric dynamics via transmission spectroscopy.

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arXiv2021

UV Fluorescence Traces Gas and LyA Evolution in Protoplanetary Disks

Ultraviolet spectra of protoplanetary disks trace distributions of warm gas at radii where rocky planets form. We combine HST-COS observations of H2 and CO emission from 12 classical T Tauri stars to more extensively map inner disk surface layers, where gas temperature distributions allow radially stratified fluorescence from the two species. We calculate empirical emitting radii for each species under the assumption that the line widths are entirely set by Keplerian broadening, demonstrating that the CO fluorescence originates further from the stars (r ~ 20 AU) than the H2 (r ~ 0.8 AU). This is supported by 2-D radiative transfer models, which show that the peak and outer radii of the CO flux distributions generally extend further into the outer disk than the H2. These results also indicate that additional sources of LyA photons remain unaccounted for, requiring more complex models to fully reproduce the molecular gas emission. As a first step, we confirm that the morphologies of the UV-CO bands and LyA radiation fields are significantly correlated and discover that both trace the degree of dust disk evolution. The UV tracers appear to follow the same sequence of disk evolution as forbidden line emission from jets and winds, as the observed LyA profiles transition between dominant red wing and dominant blue wing shapes when the high-velocity optical emission disappears. Our results suggest a scenario where UV radiation fields, disk winds and jets, and molecular gas evolve in harmony with the dust disks throughout their lifetimes.

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arXiv2021

eROSITA X-ray scan of the eta Chamaeleontis cluster

The nearby young open cluster eta Chamaeleontis has been observed by eROSITA/SRG during its CalPV phase for 150 ks. The eROSITA data were taken in the field-scan mode, an observing mode of Spectrum-Roentgen-Gamma (SRG) that follows a rectangular grid-like pattern, here covering a 5x5 deg field with an exposure depth of about 5 ks. We study the known members in X-rays and search for potential new members of the anticipated dispersed low-mass cluster population. Detected sources were identified by cross-matching X-ray sources with Gaia and 2MASS, and young stars were identified by their X-ray activity, the position in the color-magnitude diagram, and by their astrometric and kinematic properties. X-ray-luminosities, light curves, and spectra of cluster members were obtained and compared with previous X-ray data. Literature results of other member searches were used to verify our new member candidates in the observed field. We determine X-ray properties of virtually all known eta Cha members and identify five additional stellar systems that show basically identical characteristics, but are more dispersed. Four of them were previously proposed as potential members; this status is supported by our X-ray study. Based on their spatial distribution, further members are expected beyond the sky region we surveyed. The identified stellar systems very likely belong to the ejected halo population, which brings the total number of eta Cha cluster members to at least 23.

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arXiv2021

X-raying the Sco-Cen OB association: The low-mass stellar population revealed by eROSITA

We present the results of the first X-ray all-sky survey (eRASS1) performed by the eROSITA instrument on board the Spectrum-Roentgen-Gamma (SRG) observatory of the Sco-Cen OB association. Bona fide Sco-Cen member stars are young and are therefore expected to emit X-rays at the saturation level. The sensitivity limit of eRASS1 makes these stars detectable down to about a tenth of a solar mass. By cross-correlating the eRASS1 source catalog with the Gaia EDR3 catalog, we arrive at a complete identification of the stellar (i.e., coronal) source content of eROSITA\ in the Sco-Cen association, and in particular obtain for the first time a 3D view of the detected stellar X-ray sources. Focusing on the low-mass population and placing the optical counterparts identified in this way in a color-magnitude diagram, we can isolate the young stars out of the detected X-ray sources and obtain age estimates of the various Sco-Cen populations. A joint analysis of the 2D and 3D space motions, the latter being available only for a smaller subset of the detected stellar X-ray sources, reveals that the space motions of the selected population show a high degree of parallelism, but there is also an additional population of young, X-ray emitting and essentially cospatial stars that appears to be more diffuse in velocity space. Its nature is currently unclear. We argue that with our procedures, an identification of almost the whole stellar content of the Sco-Cen association will become possible once the final Gaia and eROSITA catalogs are available by the end of this decade. We furthermore call into question any source population classification scheme that relies on purely kinematic selection criteria.

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arXiv2021

The eROSITA Final Equatorial-Depth Survey (eFEDS). The stellar counterparts of eROSITA sources identified by machine learning and Bayesian algorithms

Stars are ubiquitous X-ray emitters and will be a substantial fraction of the X-ray sources detected in the on-going all-sky survey performed by the eROSITA instrument aboard the Spectrum Roentgen Gamma (SRG) observatory. We use the X-ray sources in the eROSITA Final Equatorial-Depth Survey (eFEDS) field observed during the SRG performance verification phase to investigate different strategies to identify the stars among other source categories. We focus here on Support Vector Machine (SVM) and Bayesian approaches, and our approaches are based on a cross-match with the Gaia catalog, which will eventually contain counterparts to virtually all stellar eROSITA sources. We estimate that 2060 stars are among the eFEDS sources based on the geometric match distance distribution, and we identify the 2060 most likely stellar sources with the SVM and Bayesian methods, the latter being named HamStars in the eROSITA context. Both methods reach completeness and reliability percentages of almost 90%, and the agreement between both methods is, incidentally, also about 90%. Knowing the true number of stellar sources allowed us to derive association probabilities $p_{ij}$ for the SVM method similar to the Bayesian method so that one can construct samples with defined completeness and reliability properties using appropriate cuts in $p_{ij}$. The thus identified stellar sources show the typical characteristics known for magnetically active stars, specifically, they are generally compatible with the saturation level, show a large spread in activity for stars of spectral F to G, and have comparatively high fractional X-ray luminosities for later spectral types.

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arXiv2021

Simultaneous eROSITA and TESS observations of the ultra-active star AB~Doradus

We present simultaneous multiwavelength observations of the ultra-active star AB~Doradus obtained in the X-ray range with the eROSITA instrument on board the Russian--German Spectrum-Roentgen-Gamma mission (SRG), and in the optical range obtained with the Transiting Exoplanet Survey Satellite (TESS). Thanks to its fortuitous location in the vicinity of the southern ecliptic pole, AB~Dor was observed by these missions simultaneously for almost 20 days. With the hitherto obtained data we study the long-term evolution of the X-ray flux from AB~Dor and the relation between this observable and the photospheric activity of its spots. Over the 1.5 years of eROSITA survey observations, the "quiescent" X-ray flux of AB~Dor has not changed, and furthermore it appears unrelated to the photospheric modulations observed by TESS. During the simultaneous eROSITA and TESS coverage, an extremely large flare event with a total energy release of at least 4 $\times$ 10$^{36}$ erg in the optical was observed, the largest ever seen on AB~Dor. We show that the total X-ray output of this flare was far smaller than this, and discuss whether this maybe a general feature of flares on late-type stars.

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arXiv2021

PENELLOPE: the ESO data legacy program to complement the Hubble UV Legacy Library of Young Stars (ULLYSES) I. Survey presentation and accretion properties of Orion OB1 and $σ$-Orionis

The evolution of young stars and disks is driven by the interplay of several processes, notably accretion and ejection of material. Critical to correctly describe the conditions of planet formation, these processes are best probed spectroscopically. About five-hundred orbits of the Hubble Space Telescope (HST) are being devoted in 2020-2022 to the ULLYSES public survey of about 70 low-mass (M<2Msun) young (age<10 Myr) stars at UV wavelengths. Here we present the PENELLOPE Large Program that is being carried out at the ESO Very Large Telescope (VLT) to acquire, contemporaneous to HST, optical ESPRESSO/UVES high-resolution spectra to investigate the kinematics of the emitting gas, and UV-to-NIR X-Shooter medium-resolution flux-calibrated spectra to provide the fundamental parameters that HST data alone cannot provide, such as extinction and stellar properties. The data obtained by PENELLOPE have no proprietary time, and the fully reduced spectra are made available to the whole community. Here, we describe the data and the first scientific analysis of the accretion properties for the sample of thirteen targets located in the Orion OB1 association and in the sigma-Orionis cluster, observed in Nov-Dec 2020. We find that the accretion rates are in line with those observed previously in similarly young star-forming regions, with a variability on a timescale of days of <3. The comparison of the fits to the continuum excess emission obtained with a slab model on the X-Shooter spectra and the HST/STIS spectra shows a shortcoming in the X-Shooter estimates of <10%, well within the assumed uncertainty. Its origin can be either a wrong UV extinction curve or due to the simplicity of this modelling, and will be investigated in the course of the PENELLOPE program. The combined ULLYSES and PENELLOPE data will be key for a better understanding of the accretion/ejection mechanisms in young stars.

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arXiv2020

Proxima Centauri -- the nearest planet host observed simultaneously with AstroSat, Chandra and HST

Our nearest stellar neighbour, Proxima Centauri, is a low mass star with spectral typedM5.5 and hosting an Earth-like planet orbiting within its habitable zone. However, the habitability of the planet depends on the high-energy radiation of the chromo-spheric and coronal activity of the host star. We report the Astrosat, Chandra and HST observation of Proxima Centauri carried out as part of the multi-wavelength simultaneous observational campaign. Using the soft X-ray data, we probe the different activity states of the star. We investigate the coronal temperatures, emission measures and abundance. Finally, we compare our results with earlier observations of Proxima Centauri

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arXiv2020

The X-ray view of the Hyades cluster: updated

We revisit the X-ray properties of the main-sequence Hyades members and the relation between X-ray emission and stellar rotation. As input catalog for Hyades members, we combined three recent membership lists derived from Gaia DR2 data including the Hyades core and its tidal tails. We searched for X-ray detections from the ROSAT all-sky survey (RASS) and pointings from ROSAT, Chandra, and XMM-Newton of the Hyades members and adopted rotation periods derived from Kepler's K2 mission and other resources. We find an X-ray detection for 281 of 1066 bona fide main-sequence Hyades members and provide statistical upper limits for the undetected sources. F- and G-type stars have the highest detection fraction (72 %), while K- and M-type dwarfs have lower detection rates (22 %). The X-ray luminosities of the detected members range from ~$2\times10^{27}$ for late M-type dwarfs to ~$2\times10^{30}$ erg s$^{-1}$ for active binaries. The X-ray luminosity distribution functions formally differ for the members in the core and tidal tails, which is likely caused by a larger fraction of field stars in our Hyades tails sample. Compared to previous studies, our sample is slightly fainter in X-rays due to differences in the used Hyades membership list, furthermore, we extend the X-ray luminosity distribution to fainter luminosities. The X-ray activity of F- and G-type stars is well defined at $F_X/F_bol=10^{-5}$ and increases to later spectral types reaching the saturation limit for members later than spectral type M3. The X-ray flux varies by less than a factor of three between epochs for the 104 Hyades members with multiple epoch data. Rotation periods are found for 204 Hyades members, with about half of them being detected in X-rays. The activity-rotation-relation derived for the coeval Hyades members has properties very similar to those obtained by other authors investigating stars of different ages.

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arXiv2020

Discovery of a jet from the single HAe/Be star HD 100546

Young accreting stars drive outflows that collimate into jets, which can be seen hundreds of au from their driving sources. Accretion and outflow activity cease with system age, and it is believed that magneto-centrifugally launched disk winds are critical agents in regulating accretion through the protoplanetary disk. Protostellar jets are well studied in classical T~Tauri stars ($M_\star\lesssim2\,M_\odot$), while few nearby ($d\lesssim150\,$pc) intermediate-mass stars ($M_\star=2-10\,M_\odot$), known as Herbig Ae/Be stars, have detected jets. We report VLT/MUSE observations of the Herbig~Ae/Be star HD~100546 and the discovery of a protostellar jet. The jet is similar in appearance to jets driven by low-mass stars and compares well with the jet of HD~163296, the only other known optical jet from a nearby Herbig~Ae/Be star. We derive a (one-sided) mass-loss rate in the jet of $\log \dot{M}_{jet} \sim -9.5$ (in $M_\odot$\,yr$^{-1}$) and a ratio of outflow to accretion of roughly $3\times10^{-3}$, which is lower than that of CTTS jets. The discovery of the HD~100546 jet is particularly interesting because the protoplanetary disk around HD~100546 shows a large radial gap, spiral structure, and might host a protoplanetary system. A bar-like structure previously seen in H$α$ with VLT/SPHERE shares the jet position angle, likely represents the base of the jet, and suggests a jet-launching region within about 2\,au. We conclude that the evolution of the disk at radii beyond a few au does not affect the ability of the system to launch jets.

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arXiv2020

The UV Perspective of Low-Mass Star Formation

The formation of low-mass stars in molecular clouds involves accretion disks and jets, which are of broad astrophysical interest. Accreting stars represent the closest examples of these phenomena. Star and planet formation are also intimately connected, setting the starting point for planetary systems like our own. The ultraviolet (UV) spectral range is particularly suited to study star formation, because virtually all relevant processes radiate at temperatures associated with UV emission processes or have strong observational signatures in the UV. In this review, we describe how UV observations provide unique diagnostics for the accretion process, the physical properties of the protoplanetary disk, and jets and outflows.

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