356 research outputs found
Beate Sigriddaughter’s Story of Sigrid
Get PDFdeath of childNorth VancouverteachingWorld War II1910’sEurop
An extensive VLT/X-shooter library of photospheric templates of pre-main sequence stars
Get PDFContext. Studies of the formation and evolution of young stars and their disks rely on knowledge of the stellar parameters of the young stars. The derivation of these parameters is commonly based on comparison with photospheric template spectra. Furthermore, chromospheric emission in young active stars impacts the measurement of mass accretion rates, a key quantity for studying disk evolution.
Aims. Here we derive stellar properties of low-mass (M⋆≲ 2 M⊙) pre-main sequence stars without disks, which represent ideal photospheric templates for studies of young stars. We also use these spectra to constrain the impact of chromospheric emission on the measurements of mass accretion rates. The spectra are reduced, flux-calibrated, and corrected for telluric absorption, and are made available to the community.
Methods. We derive the spectral type for our targets by analyzing the photospheric molecular features present in their VLT/X-shooter spectra by means of spectral indices and comparison of the relative strength of photospheric absorption features. We also measure effective temperature, gravity, projected rotational velocity, and radial velocity from our spectra by fitting them with synthetic spectra with the ROTFIT tool. The targets have negligible extinction (AV< 0.5 mag) and spectral type from G5 to K6, and from M6.5 to M8. They thus complement the library of photospheric templates presented in our previous publication. We perform synthetic photometry on the spectra to derive the typical colors of young stars in different filters. We measure the luminosity of the emission lines present in the spectra and estimate the noise due to chromospheric emission in the measurements of accretion luminosity in accreting stars.
Results. We provide a calibration of the photospheric colors of young pre-main sequence stars as a function of their spectral type in a set of standard broad-band optical and near-infrared filters. The logarithm of the noise on the accretion luminosity normalized to the stellar luminosity is roughly constant and equal to ~−2.3 for targets with masses larger than 1 solar mass, and decreases with decreasing temperatures for lower-mass stars. For stars with masses of ~ 1.5 M⊙ and ages of ~ 1−5 Myr, the chromospheric noise converts to a limit of measurable mass accretion rates of ~ 3 × 10-10M⊙/yr. The limit on the mass accretion rate set by the chromospheric noise is of the order of the lowest measured values of mass accretion rates in Class II objects
Unveiling the Star-Disk Interaction in Young Stars with High-Resolution Spectrophotometric Observations
No full textNewly formed stars accrete material from their parent cloud, which, due to conservation of angular
momentum, collapses into a disk-like structure around the young star. After approximately one
million years, the envelope around the star has completely dissipated and the system becomes
observable at optical wavelengths, making it possible to study the interaction between the star and
the disk.
Low-mass stars at this stage, known as Classical T Tauri Stars (CTTSs), have strong magnetic
fields that disrupt the accretion disk at a distance of a few stellar radii from the star. Gas flows from
the disk to the star along the magnetic field lines in a process called magnetospheric accretion,
ending in a shock at the stellar surface, which heats the photosphere and creates a hot spot.
Magnetospheric accretion is a complex process involving the exchange of matter, angular
momentum, and energy between the disk and the star. This process is accompanied by mass loss
via stellar and disk winds, which are driven by open magnetic field lines. These outflows play a
critical role in angular momentum extraction, preventing the star from spinning up.
In this PhD dissertation, I will discuss the main results of my research, which is based on high-
resolution spectroscopic data from the Echelle SPectrograph for Rocky Exoplanet and Stable
Spectroscopic Observations (ESPRESSO), the latest-generation echelle spectrograph at the
European Southern Observatory's Very Large Telescope (VLT), and photometric data from the
Transiting Exoplanet Survey Satellite (TESS).
High-resolution spectroscopy can be used to study the star-disk interaction and investigate the
structure of the magnetosphere and its temperature stratification, as well as the kinematics of CTTS
outflows and their complex, layered structure.
High-cadence photometric data from TESS allow to test magnetohydrodynamic simulations by
analyzing the rotational modulation of the hot spot. The analysis of the frequency spectrum of
CTTSs light curves enables the determination of the radius at which the disk is truncated by the
stellar magnetic field.
When combined together, these two techniques permit the determination of the amount of angular
momentum per unit time transferred from the star to the disk, allowing to constrain the mechanisms
that prevent the stellar spin-up in CTTSs
Mixing Methods: Practical Insights from the Humanities in the Digital Age
Get PDFDigitality is a cause and a consequence of different data cultures. It applies to the 10 research projects that are included in this volume. They are rooted in various humanities disciplines such as art history, philosophy, musicology, religious studies, architectural history, media studies, and literature studies. As diverse as the disciplines are the objects and their formats, which are the subject of this book. The cultural data of the projects include recordings of music and spoken word, photographs and other types of images, handwriting, typoscripts and maps. The oldest material dates back to 500 BCE, followed by medieval times, the 18th and 19th centuries, early 20th century and the present. All projects share that they study their material with digital methods, although digitality comes into play at different moments and layers in each of the projects. Hardly readable manuscripts from the 18th century have to be treated with specialized OCR-methods while Plato’s texts are already available in digital form, and therefore open up other affordances for analysis. Special analysis possibilities had to be developed for certain image sources. For all projects, however, it is equally true that only the digitization of the objects makes them accessible to the methods that are the subject of this book.History, Form & Aesthetic
The first evidence of tidally induced activity in a brown dwarf-M dwarf pair: a Chandra study of the NLTT 41135/41136 system
No full textThe Sydney Radio Star Catalogue: Properties of radio stars at megahertz to gigahertz frequencies
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X-ray variability of the triplet star system LTT1445 and evaporation history of the planets around its A component
No full textFar beyond the Sun - II. Probing the stellar magnetism of the young Sun iota Horologii from the photosphere to its corona
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