14 research outputs found
Surface abundances of light elements for a large sample of early B-type stars - IV. The magnesium abundance in 52 stars - a test of metallicity
From high-resolution spectra a non-LTE analysis of the MgII 4481.2 A feature
is implemented for 52 early and medium local B stars on the main sequence (MS).
The influence of the neighbouring line AlIII 4479.9 A is considered. The
magnesium abundance is determined; it is found that log e(Mg) = 7.67 +- 0.21 on
average. It is shown that uncertainties in the microturbulent parameter Vt are
the main source of errors in log e(Mg). When using 36 stars with the most
reliable Vt values derived from OII and NII lines, we obtain the mean abundance
log e(Mg) = 7.59 +- 0.15. The latter value is precisely confirmed for several
hot B stars from an analysis of the MgII 7877 A weak line. The derived
abundance log e(Mg) = 7.59 +- 0.15 is in excellent agreement with the solar
magnesium abundance log e_sun(Mg) = 7.55 +- 0.02, as well as with the proto-Sun
abundance log e_ps(Mg) = 7.62 +- 0.02. Thus, it is confirmed that the Sun and
the B-type MS stars in our neighbourhood have the same metallicity.Comment: 9 pages, 6 figures. Has been accepted for publication at MNRA
Accurate Fundamental Parameters or A, F, and G-type Supergiants in the Solar Neighbourhood
The following parameters are determined for 63 Galactic supergiants in the
solar neighbourhood: effective temperature Teff, surface gravity log g, iron
abundance log e(Fe), microturbulent parameter Vt, mass M/Msun, age t and
distance d. A significant improvement in the accuracy of the determination of
log g and, all parameters dependent on it, is obtained through application of
van Leeuwens (2007) re-reduction of the Hipparcos parallaxes. The typical error
in the log g values is now +-0.06 dex for supergiants with distances d < 300 pc
and +-0.12 dex for supergiants with d between 300 and 700 pc; the mean error in
Teff for these stars is +-120 K. For supergiants with d > 700 pc parallaxes are
uncertain or unmeasurable, so typical errors in their log g values are 0.2-0.3
dex.
A new Teff scale for A5-G5 stars of luminosity classes Ib-II is presented.
Spectral subtypes and luminosity classes of several stars are corrected.
Combining the Teff and log g with evolutionary tracks, stellar masses and ages
are determined; a majority of the sample has masses between 4 Msun and 15 Msun
and, hence, their progenitors were early to middle B-type main sequence stars.
Using Fe ii lines, which are insensitive to departures from LTE, the
microturbulent parameter Vt and the iron abundance log e(Fe) are determined
from high-resolution spectra. The parameter Vt is correlated with gravity: Vt
increases with decreasing log g. The mean iron abundance for the 48 supergiants
with distances d < 700 pc is log e(Fe)=7.48+-0.09, a value close to the solar
value of 7.45+-0.05, and thus the local supergiants and the Sun have the same
metallicity.Comment: 12 pages, 9 figures. Will be published at MNRA
Nitrogen Enrichment in Atmospheres of A- and F- Type Supergiants
Using new accurate fundamental parameters of 30 Galactic A and F supergiants,
namely their effective temperatures Teff and surface gravities log g, we
implemented a non-LTE analysis of the nitrogen abundance in their atmospheres.
It is shown that the non-LTE corrections to the N abundances increase with
Teff. The nitrogen overabundance as a general feature of this type of stars is
confirmed. A majority of the stars has a nitrogen excess [N/Fe] between 0.2 and
0.9 dex with the maximum position of the star's distribution on [N/Fe] between
0.4 and 0.7 dex. The N excesses are discussed in light of predictions for
B-type main sequence (MS) stars with rotationally induced mixing and for their
next evolutionary phase, i.e. A- and F-type supergiants that have experienced
the first dredge-up. Rotationally induced mixing in the MS progenitors of the
supergiants may be a significant cause of the nitrogen excesses. When comparing
our results with predictions of the theory developed for stars with the mixing,
we find that the bulk of the supergiants (28 of 30) show the N enrichment that
can be expected (i) either after the MS phase for stars with the initial
rotational velocities v0 = 200-400 km s-1, (ii) or after the first dredge-up
for stars with v0 = 50-400 km s-1. The latter possibility is preferred on
account of the longer lifetime for stars on red-blue loops following the first
dredge-up. Two supergiants without a discernible N enrichment, namely HR 825
and HR 7876, may be post-MS objects with the relatively low initial rotational
velocity of about 100 km s-1. The suggested range for v0 is approximately
consistent with inferences from the observed projected rotational velocities of
B-type MS stars, progenitors of A and F supergiants.Comment: 14 pages, 13 figure
VIRUS: status and performance of the massively replicated fiber integral field spectrograph for the upgraded Hobby-Eberly Telescope
The Visible Integral-field Replicable Unit Spectrograph (VIRUS) consists of 156 identical spectrographs (arrayed as 78 pairs, each with a pair of spectrographs) fed by 35,000 fibers, each 1.5 arcsec diameter, at the focus of the upgraded 10 m Hobby-Eberly Telescope (HET). VIRUS has a fixed bandpass of 350-550 nm and resolving power R~750. The fibers are grouped into 78 integral field units, each with 448 fibers and 20 m average length. VIRUS is the first example of large-scale replication applied to optical astronomy and is capable of surveying large areas of sky, spectrally. The VIRUS concept offers significant savings of engineering effort and cost when compared to traditional instruments.
The main motivator for VIRUS is to map the evolution of dark energy for the Hobby-Eberly Telescope Dark Energy Experiment (HETDEXâĄ), using 0.8M Lyman-alpha emitting galaxies as tracers. The VIRUS array has been undergoing staged deployment starting in late 2015. Currently, more than half of the array has been populated and the HETDEX survey started in 2017 December. It will provide a powerful new facility instrument for the HET, well suited to the survey niche of the telescope, and will open up large spectroscopic surveys of the emission line universe for the first time. We will review the current state of production, lessons learned in sustaining volume production, characterization, deployment, and commissioning of this massive instrument.</p
VIRUS: status and performance of the massively-replicated fiber integral field spectrograph for the upgraded Hobby-Eberly Telescope
The Visible Integral-field Replicable Unit Spectrograph (VIRUS) consists of 156 identical spectrographs (arrayed as 78 pairs, each with a pair of spectrographs) fed by 35,000 fibers, each 1.5 arcsec diameter, at the focus of the upgraded 10 m Hobby-Eberly Telescope (HET). VIRUS has a fixed bandpass of 350-550 nm and resolving power R~750. The fibers are grouped into 78 integral field units, each with 448 fibers and 20 m average length. VIRUS is the first example of large-scale replication applied to optical astronomy and is capable of surveying large areas of sky, spectrally. The VIRUS concept offers significant savings of engineering effort and cost when compared to traditional instruments. The main motivator for VIRUS is to map the evolution of dark energy for the Hobby-Eberly Telescope Dark Energy Experiment (HETDEXâĄ), using 0.8M Lyman-alpha emitting galaxies as tracers. The VIRUS array has been undergoing staged deployment starting in late 2015. Currently, more than half of the array has been populated and the HETDEX survey started in 2017 December. It will provide a powerful new facility instrument for the HET, well suited to the survey niche of the telescope, and will open up large spectroscopic surveys of the emission line universe for the first time. We will review the current state of production, lessons learned in sustaining volume production, characterization, deployment, and commissioning of this massive instrument.</p
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The HETDEX Instrumentation: Hobby-Eberly Telescope Wide-field Upgrade and VIRUS
The Hobby-Eberly Telescope (HET) Dark Energy Experiment (HETDEX) is undertaking a blind wide-field low-resolution spectroscopic survey of 540 deg2 of sky to identify and derive redshifts for a million Lyα-emitting galaxies in the redshift range 1.9 < z < 3.5. The ultimate goal is to measure the expansion rate of the universe at this epoch, to sharply constrain cosmological parameters and thus the nature of dark energy. A major multiyear Wide-Field Upgrade (WFU) of the HET was completed in 2016 that substantially increased the field of view to 22âČ diameter and the pupil to 10 m, by replacing the optical corrector, tracker, and Prime Focus Instrument Package and by developing a new telescope control system. The new, wide-field HET now feeds the Visible Integral-field Replicable Unit Spectrograph (VIRUS), a new low-resolution integral-field spectrograph (LRS2), and the Habitable Zone Planet Finder, a precision near-infrared radial velocity spectrograph. VIRUS consists of 156 identical spectrographs fed by almost 35,000 fibers in 78 integral-field units arrayed at the focus of the upgraded HET. VIRUS operates in a bandpass of 3500-5500 Ă
with resolving power R â 800. VIRUS is the first example of large-scale replication applied to instrumentation in optical astronomy to achieve spectroscopic surveys of very large areas of sky. This paper presents technical details of the HET WFU and VIRUS, as flowed down from the HETDEX science requirements, along with experience from commissioning this major telescope upgrade and the innovative instrumentation suite for HETDEX