67 research outputs found
The Belgian repository of fundamental atomic data and stellar spectra (BRASS). I. Cross-matching atomic databases of astrophysical interest
Fundamental atomic parameters, such as oscillator strengths, play a key role
in modelling and understanding the chemical composition of stars in the
universe. Despite the significant work underway to produce these parameters for
many astrophysically important ions, uncertainties in these parameters remain
large and can propagate throughout the entire field of astronomy. The Belgian
repository of fundamental atomic data and stellar spectra (BRASS) aims to
provide the largest systematic and homogeneous quality assessment of atomic
data to date in terms of wavelength, atomic and stellar parameter coverage. To
prepare for it, we first compiled multiple literature occurrences of many
individual atomic transitions, from several atomic databases of astrophysical
interest, and assessed their agreement. Several atomic repositories were
searched and their data retrieved and formatted in a consistent manner. Data
entries from all repositories were cross-matched against our initial BRASS
atomic line list to find multiple occurrences of the same transition. Where
possible we used a non-parametric cross-match depending only on electronic
configurations and total angular momentum values. We also checked for duplicate
entries of the same physical transition, within each retrieved repository,
using the non-parametric cross-match. We report the cross-matched transitions
for each repository and compare their fundamental atomic parameters. We find
differences in log(gf) values of up to 2 dex or more. We also find and report
that ~2% of our line list and Vienna Atomic Line Database retrievals are
composed of duplicate transitions. Finally we provide a number of examples of
atomic spectral lines with different log(gf) values, and discuss the impact of
these uncertain log(gf) values on quantitative spectroscopy. All cross-matched
atomic data and duplicate transitions are available to download at
brass.sdf.org.Comment: 18 pages, 12 figures, 9 tables. Accepted for publication in A&
Magnetic zinc oxide/silica microbeads for the photocatalytic degradation of azo dyes
Photocatalysis is a promising technique for the complete degradation of azo dyes that are present in wastewater. In this work, a new photocatalyst in the form of ZnO/Fe3O4/SiO2 hybrid microbeads was fabricated for the photocatalytic degradation of methylene blue. The microbeads were synthesised through self-assembly and subsequent agglomeration of nanoparticles within an aqueous phase of a water-in-oil emulsion template. Photocatalytic degradation experiments were conducted through exposure of a methylene blue solution to UV light. The hybrid microbeads performed better than ZnO powder at the same initial dye and ZnO concentration because of higher adsorption and degradation. The initial concentration of dye solution and catalyst dosage have a significant impact on the degradation. Higher degradation is seen with lower initial dye concentration and higher catalyst dosage. The presence of magnetic nanoparticles within the beads allows for their full recovery and reuse for degradation experiments. Complete degradation of dye was achieved using the new ZnO/Fe3O4/SiO2 microbeads
Massive Stars in the Gaia-ESO Survey
The Gaia-ESO Survey (GES) is an ambitious project to study the formation and evolution of the Milky Way and its stellar populations. It is led by Gerry Gilmore and Sofia Randich and includes about 350 Co-Investigators. During 300 nights (spread over 5 years) of order 10^5 Giraffe spectra and 10^4 UVES spectra will be taken. As part of the survey, about 13 clusters will be observed that were chosen specifically for their massive-star content. We report on the preliminary analysis of GES data from two such clusters: NGC 3293 and NGC 6705. We determine stellar parameters for the B-type stars in NGC 3293 and compare the A-type stars population between the two clusters. We also use a repeat observation to study binarity and use the radial velocity information to study cluster membership in NGC 3293. We also list our plans for future observations, which include the Carina nebula region
Instability regions in the upper HR diagram
ABSTRACT The following instability regions for blueward evolving-supergiants are outlined and compared. (1) Areas in the Hertzsprung -Russell (HR) diagram where stars are dynamically unstable. (2) Areas where the effective acceleration in the upper part of the photospheres is negative, hence directed outward. (3) Areas where the sonic points of the stellar winds (where v wind ¼ v sound Þ are situated inside the photospheres, at a level deeper than t Ross ¼ 0:01. We compare the results with the positions of actual stars in the HR diagram and we find evidence that the recent strong contraction of the yellow hypergiant HR 8752 was initiated in a period during which kg eff l , 0, whereupon the star became dynamically unstable. The instability and extreme shells around IRC110420 are suggested to be related to three factors: kg eff l , 0; the sonic point is situated inside the photosphere; and the star is dynamically unstable
X-shooter, NACO, and AMBER observations of the LBV Pistol Star \footnote{Based on ESO runs 85.D-0182A, 085.D-0625AC}
We present multi-instruments and multi-wavelengths observations of the famous
LBV star Pistol Star. These observations are part of a larger program about
early O stars at different metallicities. The Pistol star has been claimed as
the most massive star known, with 250 solar masses. We present the preliminary
results based on X-Shooter spectra, as well as the observations performed with
the VLTI-AMBER and the VLT-NACO adaptive optics. The X-shooter spectrograph
allows to obtain simultaneously a spectrum from the UV to the K-band with a
resolving power of 15000. The preliminary results obtained indicate that
Pistol Star has similar properties of Eta Car, including shells of matter, but
also the binarity. Other objects of the program, here briefly presented, were
selected for their particular nature: early O stars with mass discrepancies
between stellar evolution models and observations, discrepancies with the wind
momentum luminosity relation.Comment: Poster at the 39th LIAC, submitted version of the proceeding
On the CO Near-Infrared Band and the Line-splitting Phenomenon in the Yellow Hypergiant ρ Cassiopeiae
We report on multiepoch optical and near-infrared spectroscopy around the first-overtone rovibrational band of CO in the pulsating yellow hypergiant ρ Cas, one of the most massive stars in the Galaxy and a candidate SN II progenitor. We argue that the double cores of the CO absorption lines, which have previously been attributed to separate circumstellar shells expelled during its recurrent outbursts, result in fact from a superposition of a wide absorption line and a narrow central emission line. The CO line doubling returns over subsequent pulsation cycles, where the superposed line emission assumes its largest intensity near phases of maximum light. We find that the morphology and behavior of the CO band closely resemble the remarkable "line-splitting phenomenon" also observed in optical low-excitation atomic lines. Based on radiative transport calculations, we present a simplified model of the near-infrared CO emission emerging from cooler atmospheric layers in the immediate vicinity of the photosphere. We speculate that the kinetic temperature minimum in our model results from a periodic pulsation-driven shock wave. We further discuss a number of alternative explanations for the origin of the ubiquitous emission-line spectrum, possibly due to a quasi-chromosphere or a steady shock wave at the interface of a fast expanding wind and the interstellar medium. We present a number of interesting spectroscopic similarities between ρ Cas and other types of cool variable supergiants, such as the RV Tau and R CrB stars. We further propose a possibly common mechanism for the enigmatic outburst behavior of these luminous pulsating cool stars
On the CO Near-IR Band and the Line Splitting Phenomenon in the Yellow Hypergiant Rho Cassiopeiae
We report on multi-epoch optical and near-infrared spectroscopy around the
first overtone ro-vibrational band of CO in the pulsating yellow hypergiant Rho
Cas, one of the most massive stars in the Galaxy and a candidate SN II
progenitor. We argue that the double cores of the CO absorption lines, that
have previously been attributed to separate circumstellar shells expelled
during its recurrent outbursts, result in fact from a superposition of a wide
absorption line and a narrow central emission line. The CO line doubling
returns over subsequent pulsation cycles, where the superposed line emission
assumes its largest intensity near phases of maximum light. We find that the
morphology and behavior of the CO band closely resemble the remarkable
"line-splitting phenomenon" also observed in optical low-excitation atomic
lines. Based on radiative transport calculations we present a simplified model
of the near-IR CO emission emerging from cooler atmospheric layers in the
immediate vicinity of the photosphere. We speculate that the kinetic
temperature minimum in our model results from a periodical pulsation-driven
shock wave. We further discuss a number of alternative explanations for the
origin of the ubiquitous emission line spectrum, possibly due to a
quasi-chromosphere or a steady shock wave at the interface of a fast expanding
wind and the ISM. We present a number of interesting spectroscopic similarities
between Rho Cas and other types of cool variable supergiants such as the RV Tau
and R CrB stars. We further propose a possibly common mechanism for the
enigmatic outburst behavior of these luminous pulsating cool stars.Comment: accepted to ApJ; 3 color fig
High-angular resolution observations of the Pistol Star
First results of near-IR adaptive optics (AO)-assisted imaging,
interferometry, and spectroscopy of this Luminous Blue Variable (LBV) are
presented. They suggest that the Pistol Star is at least double. If the
association is physical, it would reinforce questions concerning the importance
of multiplicity for the formation and evolution of extremely massive stars.Comment: poster at IAUS27
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