arXiv subjects

S. Wolk

Publications and source records attributed to S. Wolk.

17 recordsLinked to original sources

arXiv2021

Constraints on the mass and atmospheric composition and evolution of the low-density young planet DS Tuc A b

We performed a radial velocity (RV) monitoring of the 40 Myr old star DS Tuc A with HARPS at the ESO-3.6m to determine the planetary mass of its 8.14-days planet, first revealed by TESS. We also observed two planetary transits with HARPS and ESPRESSO at ESO-VLT, to measure the Rossiter-McLaughlin (RM) effect and characterise the planetary atmosphere. We measured the high-energy emission of the host with XMM observations to investigate models for atmospheric evaporation. We employed Gaussian Processes (GP) regression to model the high level of the stellar activity, which is more than 40 times larger than the expected RV planetary signal. We extracted the transmission spectrum of DS Tuc A b from the ESPRESSO data and searched for atmospheric elements/molecules either by single-line retrieval and by performing cross-correlation with a set of theoretical templates. Through a set of simulations, we evaluated different scenarios for the atmospheric photo-evaporation of the planet induced by the strong XUV stellar irradiation. While the stellar activity prevented us from obtaining a clear detection of the planetary signal from the RVs, we set a robust mass upper limit of 14.4 M_e for DS Tuc A b. We also confirm that the planetary system is almost (but not perfectly) aligned. The strong level of stellar activity hampers the detection of any atmospheric compounds, in line with other studies presented in the literature. The expected evolution of DS Tuc A b from our grid of models indicates that the planetary radius after the photo-evaporation phase will fall within the Fulton gap. The comparison of the available parameters of known young transiting planets with the distribution of their mature counterpart confirms that the former are characterised by a low density, with DS Tuc A b being one of the less dense.

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arXiv2021

Great Observatories: The Past and Future of Panchromatic Astrophysics

NASA's Great Observatories have opened up the electromagnetic spectrum from space, providing sustained access to wavelengths not accessible from the ground. Together, Hubble, Compton, Chandra, and Spitzer have provided the scientific community with an agile and powerful suite of telescopes with which to attack broad scientific questions, and react to a rapidly changing scientific landscape. As the existing Great Observatories age, or are decommissioned, community access to these wavelengths will diminish, with an accompanying loss of scientific capability. This report, commissioned by the NASA Cosmic Origins, Physics of the Cosmos and Exoplanet Exploration Program Analysis Groups (PAGs), analyzes the importance of multi-wavelength observations from space during the epoch of the Great Observatories, providing examples that span a broad range of astrophysical investigations.

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arXiv2018

The Dynamics, Structure, and Fate of a Young Cluster During Gas Dispersal: Hectoschelle, Chandra, Spitzer, and Gaia Observations of CepOB3b

We present a study of the kinematics and structure of the Cep OB3b cluster based on new spectra obtained with the Hectoschelle spectrograph on the MMT and data from Spitzer, Chandr}, and Gaia. At a distance of 819+/-16 pc, Cep OB3b is one of the closest examples of a young (~3 - 5 Myr), large (~3000 total members) cluster at the late stages of gas dispersal. The cluster is broken into two sub-clusters surrounded by a lower density halo. We fit the empirical density law of King (1962) to each sub-cluster to constrain their sizes and structure. The richer eastern sub-cluster has circular symmetry, a modest central density, and lacks molecular gas toward its core suggesting it has undergone expansion due to gas dispersal. In contrast, the western sub-cluster deviates from circular symmetry, has a smaller core size, and contains significant molecular gas near its core, suggesting that it is in an earlier phase of gas dispersal. We present posterior probability distributions for the velocity dispersions from the Hectoschelle spectra. The east will continue to expand and likely form a bound cluster with ~35% of stars remaining. The west is undergoing slower gas dispersal and will potentially form a bound cluster with ~75% of stars remaining. If the halo dissipates, this will leave two independent clusters with ~300 members; proper motions suggest that the two sub-clusters are not bound to each other.

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arXiv2010

High-resolution x-ray telescopes

High-energy astrophysics is a relatively young scientific field, made possible by space-borne telescopes. During the half-century history of x-ray astronomy, the sensitivity of focusing x-ray telescopes-through finer angular resolution and increased effective area-has improved by a factor of a 100 million. This technological advance has enabled numerous exciting discoveries and increasingly detailed study of the high-energy universe-including accreting (stellar-mass and super-massive) black holes, accreting and isolated neutron stars, pulsar-wind nebulae, shocked plasma in supernova remnants, and hot thermal plasma in clusters of galaxies. As the largest structures in the universe, galaxy clusters constitute a unique laboratory for measuring the gravitational effects of dark matter and of dark energy. Here, we review the history of high-resolution x-ray telescopes and highlight some of the scientific results enabled by these telescopes. Next, we describe the planned next-generation x-ray-astronomy facility-the International X-ray Observatory (IXO). We conclude with an overview of a concept for the next next-generation facility-Generation X. The scientific objectives of such a mission will require very large areas (about 10000 m2) of highly-nested lightweight grazing-incidence mirrors with exceptional (about 0.1-arcsecond) angular resolution. Achieving this angular resolution with lightweight mirrors will likely require on-orbit adjustment of alignment and figure.

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arXiv2010

Debris Disks of Members of the Blanco 1 Open Cluster

We have used the Spitzer Space Telescope to obtain Multiband Imaging Photometer for Spitzer (MIPS) 24 um photometry for 37 members of the ~100 Myr old open cluster Blanco 1. For the brightest 25 of these stars (where we have 3sigma uncertainties less than 15%), we find significant mid-IR excesses for eight stars, corresponding to a debris disk detection frequency of about 32%. The stars with excesses include two A stars, four F dwarfs and two G dwarfs. The most significant linkage between 24 um excess and any other stellar property for our Blanco 1 sample of stars is with binarity. Blanco 1 members that are photometric binaries show few or no detected 24 um excesses whereas a quarter of the apparently single Blanco 1 members do have excesses. We have examined the MIPS data for two other clusters of similar age to Blanco 1 -- NGC 2547 and the Pleiades. The AFGK photometric binary star members of both of these clusters also show a much lower frequency of 24 um excesses compared to stars that lie near the single-star main sequence. We provide a new determination of the relation between V-Ks color and Ks-[24] color for main sequence photospheres based on Hyades members observed with MIPS. As a result of our analysis of the Hyades data, we identify three low mass Hyades members as candidates for having debris disks near the MIPS detection limit.

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arXiv2010

The Properties of X-ray Luminous Young Stellar Objects in the NGC 1333 and Serpens Embedded Clusters

We present Chandra X-ray data of the NGC 1333 embedded cluster, combining these data with existing Chandra data, Sptizer photometry and ground based spectroscopy of both the NGC 1333 & Serpens North clusters to perform a detailed study of the X-ray properties of two of the nearest embedded clusters to the Sun. In NGC 1333, a total of 95 cluster members are detected in X-rays, of which 54 were previously identified with Spitzer. Of the Spitzer sources, we detect 23% of the Class I protostars, 53% of the Flat Spectrum sources, 52% of the Class II, and 50% of the Transition Disk YSOs. Forty-one Class III members of the cluster are identified, bringing the total identified YSO population to 178. The X-ray Luminosity Functions (XLFs) of the NGC 1333 and Serpens clusters are compared to each other and the Orion Nebula Cluster. Based on this comparison, we obtain a new distance for the Serpens cluster of 360+22/-13 pc. The X-ray luminosity was found to depend on the bolometric luminosity as in previous studies of other clusters, and that Lx depends primarily on the stellar surface area. In the NGC 1333 cluster, the Class III sources have a somewhat higher X-ray luminosity for a given surface area. We also find evidence in NGC 1333 for a jump in the X-ray luminosity between spectral types of M0 and K7, we speculate that this may result from the presence of radiative zones in the K-stars. The gas column density vs. extinction in the NGC 1333 was found to be N_H = 0.89 +/- 0.13 x 10^22 A_K, this is lower than expected of the standard ISM but similar to that found previously in the Serpens Cloud Core.

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arXiv2010

A Deep Chandra X-ray Spectrum of the Accreting Young Star TW Hydrae

We present X-ray spectral analysis of the accreting young star TW Hydrae from a 489 ks observation using the Chandra High Energy Transmission Grating. The spectrum provides a rich set of diagnostics for electron temperature T_e, electron density N_e, hydrogen column density N_H, relative elemental abundances and velocities and reveals its source in 3 distinct regions of the stellar atmosphere: the stellar corona, the accretion shock, and a very large extended volume of warm postshock plasma. The presence of Mg XII, Si XIII, and Si XIV emission lines in the spectrum requires coronal structures at ~10 MK. Lower temperature lines (e.g., from O VIII, Ne IX, and Mg XI) formed at 2.5 MK appear more consistent with emission from an accretion shock. He-like Ne IX line ratio diagnostics indicate that T_e = 2.50 +/- 0.25 MK and N_e = 3.0 +/- 0.2 x 10^(12) cm^(-3) in the shock. These values agree well with standard magnetic accretion models. However, the Chandra observations significantly diverge from current model predictions for the postshock plasma. This gas is expected to cool radiatively, producing O VII as it flows into an increasingly dense stellar atmosphere. Surprisingly, O VII indicates N_e = 5.7 ^(+4.4}_(-1.2) x 10^(11) cm^(-3), five times lower than N_e in the accretion shock itself, and ~7 times lower than the model prediction. We estimate that the postshock region producing O VII has roughly 300 times larger volume, and 30 times more emitting mass than the shock itself. Apparently, the shocked plasma heats the surrounding stellar atmosphere to soft X-ray emitting temperatures and supplies this material to nearby large magnetic structures -- which may be closed magnetic loops or open magnetic field leading to mass outflow. (Abridged)

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arXiv2007

X-ray properties of protostars in the Orion Nebula

The origin and evolution of the X-rays in very young stellar objects (YSOs) are not yet well understood since it is very hard to observe YSOs in the protostellar phase. We study the X-ray properties of Class 0-I objects in the Orion Nebula Cluster (ONC) and compare them with those of the more evolved Class II and III members. Using Chandra Orion Ultradeep Project (COUP) data, we study the X-ray properties of stars in different evolutionary classes: luminosities, NH, temperatures and time variability are compared in order to understand if the interaction between the circumstellar material and the central object can influence the X-ray emission. We have assembled the deepest and most complete photometric catalog of objects in the ONC region from the UV to 8 microns using data from HST, WFI@2.2m ESO and ISPI@4m CTIO telescopes, and Spitzer IRAC. We select high probability candidate Class 0-I protostars, distinguishing between those having a spectral energy distribution which rises from K up to 8 microns (Class 0-Ia) from those where the SED rises from K up to 4.5 microns and decreasing afterwards (Class 0-Ib). We select a sample of bona fide Class II stars and a set of Class III stars with IR emission consistent with normal photospheres. Our principal result is that Class 0-Ia objects are significantly less luminous in X-rays, both in the total and hard bands, than the more evolved Class II stars with mass larger than 0.5 Msun; these latter show X-ray luminosities similar to those of Class 0-Ib stars. This result supports the hypothesis that the onset of X-ray emission occurs at a very early stage of star formation. Temporal variability and spectral properties of Class 0-I stars are similar to those of the more evolved Class II and III objects, except for a larger absorption likely due to gas in the circumstellar material.

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arXiv2007

Cool Stars in Hot Places

During the last three decades, evidence has mounted that star and planet formation is not an isolated process, but is influenced by current and previous generations of stars. Although cool stars form in a range of environments, from isolated globules to rich embedded clusters, the influences of other stars on cool star and planet formation may be most significant in embedded clusters, where hundreds to thousands of cool stars form in close proximity to OB stars. At the cool stars 14 meeting, a splinter session was convened to discuss the role of environment in the formation of cool stars and planetary systems; with an emphasis on the ``hot'' environment found in rich clusters. We review here the basic results, ideas and questions presented at the session. We have organized this contribution into five basic questions: what is the typical environment of cool star formation, what role do hot star play in cool star formation, what role does environment play in planet formation, what is the role of hot star winds and supernovae, and what was the formation environment of the Sun? The intention is to review progress made in addressing each question, and to underscore areas of agreement and contention.

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arXiv2006

Active X-ray Optics for Generation-X, the Next High Resolution X-ray Observatory

X-rays provide one of the few bands through which we can study the epoch of reionization, when the first galaxies, black holes and stars were born. To reach the sensitivity required to image these first discrete objects in the universe needs a major advance in X-ray optics. Generation-X (Gen-X) is currently the only X-ray astronomy mission concept that addresses this goal. Gen-X aims to improve substantially on the Chandra angular resolution and to do so with substantially larger effective area. These two goals can only be met if a mirror technology can be developed that yields high angular resolution at much lower mass/unit area than the Chandra optics, matching that of Constellation-X (Con-X). We describe an approach to this goal based on active X-ray optics that correct the mid-frequency departures from an ideal Wolter optic on-orbit. We concentrate on the problems of sensing figure errors, calculating the corrections required, and applying those corrections. The time needed to make this in-flight calibration is reasonable. A laboratory version of these optics has already been developed by others and is successfully operating at synchrotron light sources. With only a moderate investment in these optics the goals of Gen-X resolution can be realized.

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arXiv2006

The Structure and Evolution of Young Stellar Clusters

We examine the properties of embedded clusters within 1 kiloparsec using new data from the Spitzer Space Telescope, as well as recent results from 2MASS and other ground-based near-infrared surveys. We use surveys of entire molecular clouds to understand the range and distribution of cluster membership, size and surface density. The Spitzer data demonstrate clearly that there is a continuum of star- forming environments, from relative isolation to dense clusters. The number of members of a cluster is correlated with the cluster radius, such that the average surface density of clusters having a few to a thousand members varies by a factor of only a few. The spatial distributions of Spitzer-identified young stellar objects frequently show elongation, low density halos, and sub-clustering. The spatial distributions of protostars resemble the distribution of dense molecular gas, suggesting that their morphologies result directly from the fragmentation of the natal gas. We also examine the effects of the cluster environments on star and planet formation. Although Far-UV and Extreme-UV radiation from massive stars can truncate disks in a few million years, fewer than half of the young stars in our sample (embedded clusters within 1 kpc) are found in regions of strong FUV and EUV fields. Typical volume densities and lifetimes of the observed clusters suggest that dynamical interactions are not an important mechanism for truncating disks on solar system size scales.

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arXiv2005

X-Ray Flares of Sun-Like Young Stellar Objects and Their Effects on Protoplanetary Disks

Astronomical observations of flares from analogs of the early Sun have the potential to give critical insights into the high energy irradiation environment of protoplanetary disks. Solar-mass young stellar objects are significantly more X-ray luminous than the typical low-mass T Tauri star. They undergo frequent strong flaring on a several day time scale. Very powerful flares also occur, but on a longer time frame. The hard X-ray spectrum of these stars become even harder during flaring. The X-rays from these sun-like young stellar objects have the potential to ionize circumstellar material out to large distances. Three specific illustrations are given of the effects of the X-rays: The physics and chemistry of the atmospheres of the inner accretion disks; the ionization level at the disk midplane, important for the viability of the magnetorotational instability; and the nuclear fluence in the irradiation zone just interior to the inner edge of the disk, important in local irradiation scenarios for producing the short-lived radionuclides found in meteorites.

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arXiv2004

Chandra Orion Ultradeep Project: Observations and Source Lists

We present a description of the data reduction methods and the derived catalog of more than 1600 X-ray point sources from the exceptionally deep January 2003 Chandra X-ray Observatory observation of the Orion Nebula Cluster and embedded populations around OMC-1. The observation was obtained with Chandra's Advanced CCD Imaging Spectrometer (ACIS) and has been nicknamed the Chandra Orion Ultradeep Project (COUP). With an 838 ks exposure made over a continuous period of 13.2 days, the COUP observation provides the most uniform and comprehensive dataset on the X-ray emission of normal stars ever obtained in the history of X-ray astronomy.

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arXiv2002

Chandra Spectra of the Soft X-ray Diffuse Background

We present an exploratory Chandra ACIS-S3 study of the diffuse component of the Cosmic X-ray Background in the 0.3-7 keV band for four directions at high Galactic latitudes, with emphasis on details of the ACIS instrumental background modeling. Observations of the dark Moon are used to model the detector background. A comparison of the Moon data and the data obtained with ACIS stowed outside the focal area showed that the dark Moon does not emit significantly in our band. Point sources down to 3 10^-16 erg/s/cm2 in the 0.5-2 keV band are excluded in our two deepest observations. We estimate the contribution of fainter, undetected sources to be less than 20% of the remaining CXB flux in this band in all four pointings. In the 0.3-1 keV band, the diffuse signal varies strongly from field to field and contributes between 55% and 90% of the total CXB signal. It is dominated by emission lines that can be modeled by a kT=0.1-0.4 keV plasma. In particular, the two fields located away from bright Galactic features show a prominent line blend at E=580 eV (O VII + O VIII) and a possible line feature at E~300 eV. The two pointings toward the North Polar Spur exhibit a brighter O blend and additional bright lines at 730-830 eV (Fe XVII). We measure the total 1-2 keV flux of (1.0-1.2 +-0.2) 10^-15 erg/s/cm2/arcmin (mostly resolved), and the 2-7 keV flux of (4.0-4.5 +-1.5) 10^-15 erg/s/cm2/arcmin. At E>2 keV, the diffuse emission is consistent with zero, to an accuracy limited by the short Moon exposure and systematic uncertainties of the S3 background. Assuming Galactic or local origin of the line emission, we put an upper limit of 3 10^-15 erg/s/cm2/arcmin on the 0.3-1 keV extragalactic diffuse flux.

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arXiv2002

The discovery of X-rays from Venus with Chandra

On January 10 and 13, 2001, Venus was observed for the first time with an X-ray astronomy satellite. The observation, performed with the ACIS-I and LETG/ACIS-S instruments on Chandra, yielded data of high spatial, spectral, and temporal resolution. Venus is clearly detected as a half-lit crescent, with considerable brightening on the sunward limb. The morphology agrees well with that expected from fluorescent scattering of solar X-rays in the planetary atmosphere. The radiation is observed at discrete energies, mainly at the O-K_alpha energy of 0.53 keV. Fluorescence radiation is also detected from C-K_alpha at 0.28 keV and, marginally, from N-K_alpha at 0.40 keV. An additional emission line is indicated at 0.29 keV, which might be the signature of the C 1s --> pi* transition in CO2 and CO. Evidence for temporal variability of the X-ray flux was found at the 2.6 sigma level, with fluctuations by factors of a few times indicated on time scales of minutes. All these findings are fully consistent with fluorescent scattering of solar X-rays. No other source of X-ray emission was detected, in particular none from charge exchange interactions between highly charged heavy solar wind ions and atmospheric neutrals, the dominant process for the X-ray emission of comets. This is in agreement with the sensitivity of the observation.

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arXiv2002

Discovery of X-rays from Venus with Chandra

On January 10 and 13, 2001, Venus was observed for the first time with an X-ray astronomy satellite. The observation, performed with the ACIS-I and LETG/ACIS-S instruments on Chandra, yielded data of high spatial, spectral, and temporal resolution. Venus is clearly detected as a half-lit crescent, with considerable brightening on the sunward limb. The morphology agrees well with that expected from fluorescent scattering of solar X-rays in the planetary atmosphere. The radiation is observed at discrete energies, mainly at the O-K_alpha energy of 0.53 keV. Fluorescent radiation is also detected from C-K_alpha at 0.28 keV and, marginally, from N-K_alpha at 0.40 keV. An additional emission line is indicated at 0.29 keV, which might be the signature of the C 1s --> pi* transition in CO_2 and CO. Evidence for temporal variability of the X-ray flux was found at the 2.6 sigma level, with fluctuations by factors of a few times indicated on time scales of minutes. All these findings are fully consistent with fluorescent scattering of solar X-rays. No other source of X-ray emission was detected, in particular none from charge exchange interactions between highly charged heavy solar wind ions and atmospheric neutrals, the dominant process for the X-ray emission of comets. This is in agreement with the sensitivity of the observation.

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arXiv1997

Multiwavelength Monitoring of the BL Lacertae Object PKS 2155-304 in May 1994. I. The Ground-Based Campaign

Optical, near-infrared, and radio observations of the BL Lac object PKS2155-304 were obtained simultaneously with a continuous UV/EUV/X-ray monitoring campaign in 1994 May. Further optical observations were gathered throughout most of 1994. The radio, millimeter, and near-infrared data show no strong correlations with the higher energies. The optical light curves exhibit flickering of 0.2-0.3 mag on timescales of 1-2 days, superimposed on longer timescale variations. Rapid variations of ~0.01 mag/min, which, if real, are the fastest seen to date for any BL Lac object. Small (0.2-0.3 mag) increases in the V and R bands occur simultaneously with a flare seen at higher energies. All optical wavebands (UBVRI) track each other well over the period of observation with no detectable delay. For most of the period the average colors remain relatively constant, although there is a tendency for the colors (in particular B-V) to vary more when the source fades. In polarized light, PKS 2155-304 showed strong color dependence and the highest optical polarization (U = 14.3%) ever observed for this source. The polarization variations trace the flares seen in the ultraviolet flux.

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