462 research outputs found
The standard model of low-mass star formation applied to massive stars : a multi-wavelength picture of AFGL 2591
No full textContext. While it is currently unclear from a theoretical standpoint which forces and processes dominate the formation of high-mass stars, and hence determine the mode in which they form, much of the recent observational evidence suggests that massive stars are born in a similar manner to their low-mass counterparts. Aims. This paper aims to investigate the hypothesis that the embedded luminous star AFGL 2591-VLA 3 (2.3 × 105 L⊙ at 3.33 kpc) is forming according to a scaled-up version of a low-mass star formation scenario. Methods. We present multi-configuration Very Large Array (VLA) 3.6 cm and 7 mm, as well as Combined Array for Research in Millimeter Astronomy C18O and 3 mm continuum observations to investigate the morphology and kinematics of the ionized gas, dust, and molecular gas around AFGL 2591. We also compare our results to ancillary Gemini North near-IR images, and model the near-IR to sub-mm spectral energy distribution (SED) and Two Micron All Sky Survey (2MASS) image profiles of AFGL 2591 using a Monte-Carlo dust continuum radiative transfer code. Results. The observed 3.6 cm images uncover for the first time that the central powering source AFGL 2591-VLA 3 has a compact core plus collimated jet morphology, extending 4000 AU eastward from the central source with an opening angle of <10° at this radius. However, at 7 mm VLA 3 does not show a jet morphology, but instead compact (<500 AU) emission, some of which (<0.57 mJy of 2.9 mJy) is estimated to be from dust emission. The spectral index of AFGL 2591-VLA 3 between 3.6 cm and 7 mm was found to be between 0.4 and 0.5, similar to that of an ionized wind. If the 3.6 cm emission is modelled as an ionized jet, the jet has almost enough momentum to drive the larger-scale flow. However, assuming a shock efficiency of 10%, the momentum rate of the jet is not sufficient to ionize itself via only shocks, and thus a significant portion of the emission is instead likely created in a photoionized wind. The C18O emission uncovers dense entrained material in the outflow(s) from these young stars. The main features of the SED and 2MASS images of AFGL 2591-VLA 3 are also reproduced by our model dust geometry of a rotationally flattened envelope with and without a disk. Conclusions. The above results are consistent with a picture of massive star formation similar to that seen for low-mass protostars. However, within its envelope, AFGL 2591-VLA 3 contains at least four other young stars, constituting a small cluster. Therefore it appears that AFGL 2591-VLA 3 may be able to source its accreting material from a shared gas reservoir while still exhibiting the phenomena expected during the formation of low-mass stars.Peer reviewe
A set of synthetic spectral energy distributions for "diskless", intermediate-mass young stars
No full text<p>These models were published in conjunction with <em>The Duration of Star Formation in Galactic Giant Molecular Clouds. I. The Great Nebula in Carina, </em>by <a href="https://arxiv.org/abs/1906.01730">Povich et al. (2019)</a>. They will also be used in subsequent papers in that series.</p>
<p>The format of these models conforms with the standards of <a href="https://doi.org/10.1051/0004-6361/201425486">Robitaille (2017)</a>, so they are compatible with the <a href="http://sedfitter.readthedocs.io/en/stable/installation.html">python implementation</a> of the <a href="https://doi.org/10.1086/512039">Robitaille et al. (2007)</a> SED fitting tool. We have pre-convolved these models with a number of useful filters, including Johnson/Bessel <em>UB</em><em>VRI, </em>UKIRT <em>ZYJHK, </em>VISTA <em>ZYJHK<sub>S</sub></em>, 2MASS <em>JHK<sub>S</sub></em>, <em>Spitzer/</em>IRAC and MIPS. </p>
<p>A software pipeline implementing these models to constrain the age and mass distributions of young stellar populations is also <a href="https://doi.org/10.5281/zenodo.3234101">publicly available</a>.</p>
<p><strong>Limitations of these models</strong></p>
<p><em>These models produce the best results for stars more massive than the Sun and older than about 0.5 Myr. </em>They employ the pre-main-sequence evolutionary tracks of <a href="https://arxiv.org/abs/astro-ph/0003477">Siess et al. (2000)</a> and <a href="https://ui.adsabs.harvard.edu/abs/1996A&A...307..829B/abstract">Bernasconi & Maeder (1996)</a>. Numerous modern tracks offer significant improvement in the treatment of subsolar-mass stars. In addition, the Kurucz stellar atmospheres used in these synthetic SEDs work best for T<sub>eff </sub>> 4,000 K; for cooler temperatures other models, for example the PHOENIX photospheres, may be more appropriate.</p>
<p>Newer evolutionary tracks covering the intermediate-mass range are now available, for example the Geneva pre-MS tracks of <a href="https://doi.org/10.1051/0004-6361/201935051">Haemmerlé et al. (2019)</a>. The principal innovation of these modern models is the treatment of accretion and location of the intermediate-mass stellar birthline. The coolest, most luminous models in this set are likely <em>unphysical</em>, representing fully-convective stars of >2 solar masses and <0.5 Myr isochronal age.</p>
An updated modular set of synthetic spectral energy distributions for young stellar objects
No full text<p>These are the models released with the following publication:</p>
<p><strong><em>An updated modular set of synthetic spectral energy distributions for young stellar objects</em></strong> (Richardson et al. 2024).</p>
<p>This is a set of young stellar object (YSO) models with associated spectral energy distributions (SEDs) calculated through radiative transfer. It is a significant update to the data published alongside Robitaille (2017, R17). It contains the parameters shaping each model and adds the newly calculated parameters of envelope mass, average dust temperature, disk stability, and line-of-sight extinction. It also makes explicit quantities, such as source luminosity, that were left implicit in the previous release. This set also convolves the SEDs with several new filters, primarily those on the James Webb Space Telescope, and adds a script to facilitate convolution of these models with additional filters as desired by users. All data included in Version 1.1 of the R17 set (the most recent) are included here.</p>
<p>Like their predecessors, these models are versioned. Updates will be released as more models are completed or other changes are made.</p>
<p>Files unzip to r+24_models-{version}/{geometry}. "files.tar.gz" contains scripts for SED convolution and main sequence comparison, the opacity to absorption of dust used in the radiative transfer calculations, main sequence T/L values used for results in the accompanying work, and reference material for the contents of the dataset and latest version.</p>
Star formation across the galaxy : observations and modelling of the spectral energy distributions of young stars
No full textIn the last few decades, the emergence of large-scale infrared surveys has led to a revolution in the study of star formation. In particular, NASA’s Spitzer Space Telescope has recently carried out mid- and far-infrared observations of numerous star formation regions with unprecedented resolution and sensitivity, and has uncovered thousands of forming stars. In combination with present and future large-scale near-infrared and sub-mm surveys, spectral energy distributions from near-infrared to mm wavelengths will be available for these thousands of young stars. Never before has there been such a wealth of multi-wavelength data for so many young stars.
Traditional techniques for studying the physical properties of young stars through their spectral energy distributions have usually focused either on the analysis of many sources using simple observational diagnostics such as colours or spectral indices, or on the analysis of a few sources through the detailed modelling of their full spectral energy distributions.
The work presented in the first part of this thesis aims to bridge these two techniques through the efficient modelling of the spectral energy distributions of many young stars. In particular, the technique developed for this work makes it straightforward to find out how well different physical parameters are constrained, whether any parameters are degenerate, and whether additional data would resolve the degeneracies.
In the second part of this thesis, a census of intrinsically red sources observed by Spitzer in the Galactic plane is presented, including a catalogue of over 11,000 likely young stellar objects. This sample of sources is the largest uniformly selected sample of young stars to date, and effectively provides a map of the sites of star formation in the mid-plane of the Milky-Way. In parallel, this census has uncovered over 7,000 candidate asymptotic giant branch stars, of which over 1,000 are variable at 4.5 or 8.0 microns
Investigating Physical Activity Type, Frequency of Physical Activity, Motives on Physical Activity, and Social Physique Anxiety Among Undergraduate Females
No full textPhysical activity has multiple health benefits, however, physical activity can also lead to the development of excessive exercise, disturbed eating patterns, negative body image and social physique anxiety. This study investigated the relationship between social physique anxiety (SPA), physical activity type, frequency of physical activity, and motives to exercise. Participants were female undergraduates (N=108) enrolled at the University of Victoria. Female students were recruited from Exercise, Physical and Health Education classes that were open to all students from different faculties. A cross-sectional survey assessed social physique anxiety, frequency of physical activity and motives for exercise as well as dividing students according to physical activity type (Varsity, Intramural, High Strenuous Exerciser, Low Strenuous Exerciser). An analysis of variance revealed no association between SPA and physical activity type. Additionally, correlations showed that SPA and frequency of physical activity was not significant. Interestingly, SPA was significant for all motives that include Interest (r=-.371, p<.01), Competence (r=-.330, p<.01), Appearance (r=.430), Fitness (r=-.215, p<.05), and Social (r=-.406, p<.01). A linear regression revealed that only Appearance and Social motives predicted [email protected]
L\u27inventaire for Flora North America (revised), by M. Jurdant, J. L. B\ue9lair, V. Gerardin, and J. P. Ducruc [Review]
No full textVolume: 93Start Page: 216End Page: 21
Human rapid acquisition with relaxation enhancement imaging at 8 T without specific absorption rate violation
No full textA standard fast imaging sequence, rapid acquisition with relaxation enhancement (RARE), has been applied to human magnetic resonance at 8 T. RARE is known for its speed, good contrast and high RF power content. HighlyT 2 weighted images, the hallmark of RARE imaging, were acquired from the human brain. It is demonstrated that whileT 2 values may be reduced at 8 T, high quality RARE images could still be acquired at this field strength. Most importantly however, it is demonstrated that RARE images could be acquired without violating specific absorption rate (SAR) guidelines. Since it is well known thatT 2 weighted images are of significant value in clinical diagnosis, the implementation of RARE at this field strength will provide ultra high field MRI (UHFMRI) with a valuable imaging protocol at this field strength without exceeding SAR limitations
HYPERION: an open-source parallelized three-dimensional dust continuum radiative transfer code
No full textHyperio
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