108 research outputs found
The ultracool dwarf DENIS-P J104814.7-395606. Chromospheres and coronae at the low-mass end of the main-sequence
We have obtained an XMM-Newton observation and a broad-band spectrum from the
ultraviolet to the near infrared with X-Shooter for one of the nearest M9
dwarfs, DENIS-P J1048-3956 (4pc). We integrate these data by a compilation of
activity parameters for ultracool dwarfs from the literature with the aim to
advance our understanding of these objects by comparing them to early-M type
dwarf stars and the Sun.
Our deep XMM-Newton observation has led to the first X-ray detection of
DENIS-P J1048-3956 (log Lx = 25.1) as well as the first measurement of its V
band brightness (V = 17.35mag). Flux-flux relations between X-ray and
chromospheric activity indicators are here for the first time extended into the
regime of the ultracool dwarfs. The approximate agreement of DENIS-P J1048-3956
and other ultracool dwarfs with flux-flux relations for early-M dwarfs suggests
that the same heating mechanisms work in the atmospheres of ultracool dwarfs,
albeit weaker as judged from their lower fluxes. The observed Balmer decrements
of DENIS-P J1048-3956 are compatible with optically thick plasma in LTE at low,
nearly photospheric temperature or optically thin LTE plasma at 20000K.
Describing the decrements with CaseB recombination requires different emitting
regions for Halpha and the higher Balmer lines. The high observed Halpha/Hbeta
flux ratio is also poorly fitted by the optically thin models. We derive a
similarly high value for the Halpha/Hbeta ratio of vB10 and LHS2065 and
conclude that this may be a characteristic of ultracool dwarfs. We add DENIS-P
J1048-3956 to the list of ultracool dwarfs detected in both the radio and the
X-ray band. The Benz-Guedel relation between radio and X-ray luminosity of
late-type stars is well-known to be violated by ultracool dwarfs. We speculate
on the presence of two types of ultracool dwarfs with distinct radio and X-ray
behavior.Comment: accepted for publication in Astronomy & Astrophysic
Chemical abundances of damped Lyman alpha systems in the XQ-100 survey
The XQ-100 survey has provided high signal-noise spectra of 100 redshift
3-4.5 quasars with the X-Shooter spectrograph. The metal abundances for 13
elements in the 41 damped Lyman alpha systems (DLAs) identified in the XQ-100
sample are presented, and an investigation into abundances of a variety of DLA
classes is conducted. The XQ-100 DLA sample contains five DLAs within 5000 km/s
of their host quasar (proximate DLAs; PDLAs) as well as three sightlines which
contain two DLAs within 10,000 km/s of each other along the same line-of-sight
(multiple DLAs; MDLAs). Combined with previous observations in the literature,
we demonstrate that PDLAs with logN(HI)<21.0 show lower [S/H] and [Fe/H]
(relative to intervening systems with similar redshift and N(HI)), whilst
higher [S/H] and [Si/H] are seen in PDLAs with logN(HI)>21.0. These abundance
discrepancies are independent of their line-of-sight velocity separation from
the host quasar, and the velocity width of the metal lines (v90). Contrary to
previous studies, MDLAs show no difference in [alpha/Fe] relative to single
DLAs matched in metallicity and redshift. In addition, we present follow-up
UVES data of J0034+1639, a sightline containing three DLAs, including a
metal-poor DLA with [Fe/H]=-2.82 (the third lowest [Fe/H] in DLAs identified to
date) at z=4.25. Lastly we study the dust-corrected [Zn/Fe], emphasizing that
near-IR coverage of X-Shooter provides unprecedented access to MgII, CaII and
TiII lines (at redshifts 3-4) to provide additional evidence for subsolar
[Zn/Fe] ratio in DLAs.Comment: Accepted to MNRAS. 19 pages plus Appendix material (102 pages total
X-Shooter spectroscopy of young stellar objects: II. Impact of chromospheric emission on accretion rate estimates
Context. The lack of knowledge of photospheric parameters and the level of
chromospheric activity in young low-mass pre-main sequence stars introduces
uncertainties when measuring mass accretion rates in accreting (Class II) Young
Stellar Objects. A detailed investigation of the effect of chromospheric
emission on the estimates of mass accretion rate in young low-mass stars is
still missing. This can be undertaken using samples of young diskless (Class
III) K and M-type stars. Aims. Our goal is to measure the chromospheric
activity of Class III pre main sequence stars to determine its effect on the
estimates of accretion luminosity (Lacc) and mass accretion rate (Macc) in
young stellar objects with disks. Methods. Using VLT/X-Shooter spectra we have
analyzed a sample of 24 non-accreting young stellar objects of spectral type
between K5 and M9.5. We identify the main emission lines normally used as
tracers of accretion in Class II objects, and we determine their fluxes in
order to estimate the contribution of the chromospheric activity to the line
luminosity. Results. We have used the relationships between line luminosity and
accretion luminosity derived in the literature for Class II objects to evaluate
the impact of chromospheric activity on the accretion rate measurements. We
find that the typical chromospheric activity would bias the derived accretion
luminosity by Lacc,noise< 10-3Lsun, with a strong dependence with the Teff of
the objects. The noise on Macc depends on stellar mass and age, and the typical
values of log(Macc,noise) range between -9.2 to -11.6Msun/yr. Conclusions.
Values of Lacc< 10-3Lsun obtained in accreting low-mass pre main sequence stars
through line luminosity should be treated with caution as the line emission may
be dominated by the contribution of chromospheric activity.Comment: accepted for publication in Astronomy & Astrophysic
X-Shooter spectroscopy of young stellar objects: IV -- Accretion in low-mass stars and sub-stellar objects in Lupus
We present X-Shooter/VLT observations of a sample of 36 accreting low-mass
stellar and sub-stellar objects (YSOs) in the Lupus star forming region,
spanning a range in mass from ~0.03 to ~1.2Msun, but mostly with 0.1Msun <
Mstar < 0.5Msun. Our aim is twofold: firstly, analyse the relationship between
excess-continuum and line emission accretion diagnostics, and, secondly, to
investigate the accretion properties in terms of the physical properties of the
central object. The accretion luminosity (Lacc), and from it the accretion rate
(Macc), is derived by modelling the excess emission, from the UV to the
near-IR, as the continuum emission of a slab of hydrogen. The flux and
luminosity (Ll) of a large number of emission lines of H, He, CaII, etc.,
observed simultaneously in the range from ~330nm to 2500nm, were computed. The
luminosity of all the lines is well correlated with Lacc. We provide empirical
relationships between Lacc and the luminosity of 39 emission lines, which have
a lower dispersion as compared to previous relationships in the literature. Our
measurements extend the Pab and Brg relationships to Lacc values about two
orders of magnitude lower than those reported in previous studies. We confirm
that different methodologies to measure Lacc and Macc yield significantly
different results: Ha line profile modelling may underestimate Macc by 0.6 to
0.8dex with respect to Macc derived from continuum-excess measures. Such
differences may explain the likely spurious bi-modal relationships between Macc
and other YSOs properties reported in the literature. We derive Macc in the
range 2e-12 -- 4e-8 Msun/yr and conclude that Macc is proportional to
Mstar^1.8(+/-0.2), with a dispersion lower by a factor of about 2 than in
previous studies. A number of properties indicate that the physical conditions
of the accreting gas are similar over more than 5 orders of magnitude in Macc
Volume intensified dilution of a ring-closing metathesis in ethyl acetate by means of a membrane-assisted process in solvent recycle
BACKGROUND Ring-closing metathesis (RCM) for the synthesis of macrocycles has been used more and more often over recent years, including some interesting applications on industrial scale. However, like all macrocyclization strategies RCM is plagued by the traditional issue of low volume efficiency. To-date this is typically addressed in a molecule specific manner with varying degrees of success. Here we report a process intensification method of metathesis macrocyclization that reduces the solvent load required for the reaction significantly. RESULTS Metathesis macrocylizations were successfully carried out in a solvent volume of upto 82% lower than an equivalent batch reaction, with only minimal impact upon the reaction outcome. A switch of reaction solvent to ethyl acetate renders the process more benign and applicable to large scale. CONCLUSION A membrane-assisted processing method that relies upon organic solvent nanofiltration permitting an internal solvent recycling and concumittent in situ product removal. The method is also designed to be applicable to a wide range of metathesis cyclizations. (c) 2019 Society of Chemical Industr
X-Shooter spectroscopy of young stellar objects III. Photospheric and chromospheric properties of Class III objects
We analyzed X-Shooter/VLT spectra of 24 ClassIII sources from three nearby
star-forming regions (sigmaOrionis, LupusIII, and TWHya). We determined the
effective temperature, surface gravity, rotational velocity, and radial
velocity by comparing the observed spectra with synthetic BT-Settl model
spectra. We investigated in detail the emission lines emerging from the stellar
chromospheres and combined these data with archival X-ray data to allow for a
comparison between chromospheric and coronal emissions. Both X-ray and Halpha
luminosity as measured in terms of the bolometric luminosity are independent of
the effective temperature for early-M stars but decline toward the end of the
spectral M sequence. For the saturated early-M stars the average emission level
is almost one dex higher for X-rays than for Halpha: log(L_x/L_bol) = -2.85 +-
0.36 vs. log(L_Halpha/L_bol) = -3.72 +- 0.21. When all chromospheric emission
lines (including the Balmer series up to H11, CaII HK, the CaII infrared
triplet, and several HeI lines) are summed up the coronal flux still dominates
that of the chromosphere, typically by a factor 2-5. Flux-flux relations
between activity diagnostics that probe different atmospheric layers (from the
lower chromosphere to the corona) separate our sample of active pre-main
sequence stars from the bulk of field M dwarfs studied in the literature. Flux
ratios between individual optical emission lines show a smooth dependence on
the effective temperature. The Balmer decrements can roughly be reproduced by
an NLTE radiative transfer model devised for another young star of similar age.
Future, more complete chromospheric model grids can be tested against this data
set.Comment: accepted for publication in Astronomy & Astrophysic
When two become one: an apparent QSO pair turns out to be a single quasar
We report on our serendipitous discovery that the objects Q 01323-4037 and Q
0132-4037, listed in the V\'eron-Cetty & V\'eron catalog (2006) as two
different quasars, are actually a quasar and a star. We briefly discuss the
origin of the misidentification, and provide a refined measurement of the
quasar redshift.Comment: 3 pages, 2 figures; accepted for publication in A&
Nature and statistical properties of quasar associated absorption systems in the XQ-100 Legacy Survey
We statistically study the physical properties of a sample of narrow
absorption line (NAL) systems looking for empirical evidences to distinguish
between intrinsic and intervening NALs without taking into account any a priori
definition or velocity cut-off. We analyze the spectra of 100 quasars with 3.5
< z < 4.5, observed with X-shooter/VLT in the context of the XQ-100
Legacy Survey. We detect a 8 excess in the number density of
absorbers within 10,000 km/s of the quasar emission redshift with respect to
the random occurrence of NALs. This excess does not show a dependence on the
quasar bolometric luminosity and it is not due to the redshift evolution of
NALs. It extends far beyond the standard 5000 km/s cut-off traditionally
defined for associated absorption lines. We propose to modify this definition,
extending the threshold to 10,000 km/s when also weak absorbers (equivalent
width < 0.2 \AA) are considered. We infer NV is the ion that better traces the
effects of the quasar ionization field, offering the best statistical tool to
identify intrinsic systems. Following this criterion we estimate that the
fraction of quasars in our sample hosting an intrinsic NAL system is 33
percent. Lastly, we compare the properties of the material along the quasar
line of sight, derived from our sample, with results based on close quasar
pairs investigating the transverse direction. We find a deficiency of cool gas
(traced by CII) along the line of sight associated with the quasar host galaxy,
in contrast with what is observed in the transverse direction.Comment: 18 pages, 13 figures, 5 table
GRB host galaxies with VLT/X-Shooter: properties at 0.8 < z < 1.3
Long gamma-ray bursts (LGRBs) are associated with the death of massive stars.
Their host galaxies therefore represent a unique class of objects tracing star
formation across the observable Universe. Indeed, recently accumulated evidence
shows that GRB hosts do not differ substantially from general population of
galaxies at high (z > 2) redshifts. However, it has been long recognised that
the properties of z < 1.5 hosts, compared to general star-forming population,
are unusual. To better understand the reasons for the supposed difference in
LGRB hosts properties at z < 1.5, we obtained VLT/X- Shooter spectra of six
hosts lying in the redshift range of 0.8 < z < 1.3. Some of these hosts have
been observed before, yet we still lack well constrained information on their
characteristics such as metallicity, dust extinction and star formation rate.
We search for emission lines in the VLT/X-Shooter spectra of the hosts and
measure their fluxes. We perform a detailed analysis, estimating host average
extinction, star-formation rates, metallicities and electron densities where
possible. Measured quantities of our hosts are compared to a larger sample of
previously observed GRB hosts at z < 2. Star-formation rates and metallicities
are measured for all the hosts analyzed in this paper and metallicities are
well determined for 4 hosts. The mass-metallicity relation, the fundamental
metallicity relation and SFRs derived from our hosts occupy similar parameter
space as other host galaxies investigated so-far at the same redshift. We
therefore conclude that GRB hosts in our sample support the found discrepancy
between the properties of low-redshift GRB hosts and the general population of
star- forming galaxies.Comment: 13 pages, 6 figures, accepted for publication in MNRA
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