160 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
Massive Star cluster formation under the microscope at z=6
We report on a superdense star-forming region with an effective radius (R_e)
smaller than 13 pc identified at z=6.143 and showing a star-formation rate
density \Sigma_SFR~1000 Msun/yr/kpc2 (or conservatively >300 Msun/yr/kpc2).
Such a dense region is detected with S/N>40 hosted by a dwarf extending over
440 pc, dubbed D1 (Vanzella et al. 2017b). D1 is magnified by a factor
17.4+/-5.0 behind the Hubble Frontier Field galaxy cluster MACS~J0416 and
elongated tangentially by a factor 13.2+/-4.0 (including the systematic
errors). The lens model accurately reproduces the positions of the confirmed
multiple images with a r.m.s. of 0.35", and the tangential stretch is well
depicted by a giant multiply-imaged Lya arc. D1 is part of an interacting
star-forming complex extending over 800 pc. The SED-fitting, the very blue
ultraviolet slope (\beta ~ -2.5, F(\lambda) ~ \lambda^\beta) and the prominent
Lya emission of the stellar complex imply that very young (< 10-100 Myr),
moderately dust-attenuated (E(B-V)<0.15) stellar populations are present and
organised in dense subcomponents. We argue that D1 (with a stellar mass of 2 x
10^7 Msun) might contain a young massive star cluster of M < 10^6 Msun and
Muv~-15.6 (or m_uv=31.1), confined within a region of 13 pc, and not dissimilar
from some local super star clusters (SSCs). The ultraviolet appearance of D1 is
also consistent with a simulated local dwarf hosting a SSC placed at z=6 and
lensed back to the observer. This compact system fits into some popular
globular cluster formation scenarios. We show that future high spatial
resolution imaging (e.g., E-ELT/MAORY-MICADO and VLT/MAVIS) will allow us to
spatially resolve light profiles of 2-8 pc.Comment: 21 pages, 14 figures, 1 table, MNRAS accepte
Ionising the Intergalactic Medium by Star Clusters: The first empirical evidence
We present a VLT/X-Shooter spectroscopy of the Lyman continuum (LyC) emitting
galaxy 'Ion2' at z=3.2121 and compare it to that of the recently discovered
strongly lensed LyC-emitter at z=2.37, known as the 'Sunburst' arc. Three main
results emerge from the X-Shooter spectrum: (a) the Lya has three distinct
peaks with the central one at the systemic redshift, indicating a ionised
tunnel through which both Lya and LyC radiation escape; (b) the large O32
oxygen index ([OIII]4959-5007 / [OII]3727-3729) of 9.18(-1.32/+1.82) is
compatible to those measured in local (z~0.4) LyC leakers; (c) there are narrow
nebular high-ionisation metal lines with \sigma_v < 20 km/s, which confirms the
presence of young hot, massive stars. The HeII1640 appears broad, consistent
with a young stellar component including Wolf-Rayet stars. Similarly, the
Sunburst LyC-emitter shows a triple-peaked Lya profile and from VLT/MUSE
spectroscopy the presence of spectral features arising from young hot and
massive stars. The strong lensing magnification, (\mu > 20), suggests that this
exceptional object is a gravitationally-bound star cluster observed at a
cosmological distance, with a stellar mass M <~ 10^7 Msun and an effective
radius smaller than 20 pc. Intriguingly, sources like Sunburst but without
lensing magnification might appear as Ion2-like galaxies, in which unresolved
massive star clusters dominate the ultraviolet emission. This work supports the
idea that dense young star clusters can contribute to the ionisation of the IGM
through holes created by stellar feedback.Comment: 13 pages, 9 figures and 1 table, MNRAS accepted. Some typos fixe
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
Direct Lyman continuum and Lyman-alpha escape observed at redshift 4
We report on the serendipitous discovery of a z=4.0, M1500=-22.20
star-forming galaxy (Ion3) showing copious Lyman continuum (LyC) leakage (~60%
escaping), a remarkable multiple peaked Lya emission, and significant Lya
radiation directly emerging at the resonance frequency. This is the highest
redshift confirmed LyC emitter in which the ionising and Lya radiation possibly
share a common ionised cavity (with N_HI<10^17.2 cm^-2). Ion3 is spatially
resolved, it shows clear stellar winds signatures like the P-Cygni NV1240
profile, and has blue ultraviolet continuum (\beta = -2.5 +/- 0.25, F_\lambda~
\lambda^\beta) with weak low-ionisation interstellar metal lines. Deep
VLT/HAWKI Ks and Spitzer/IRAC 3.6um and 4.5um imaging show a clear photometric
signature of the Halpha line with equivalent width of 1000A rest-frame emerging
over a flat continuum (Ks-4.5um ~ 0). From the SED fitting we derive a stellar
mass of 1.5x10^9 Msun, SFR of 140 Msun/yr and age of ~10 Myr, with a low dust
extinction, E(B-V)< 0.1, placing the source in the starburst region of the
SFR-M^* plane. Ion3 shows similar properties of another LyC emitter previously
discovered (z=3.21, Ion2, Vanzella et al. 2016). Ion3 (and Ion2) represents
ideal high-redshift reference cases to guide the search for reionising sources
at z>6.5 with JWST.Comment: Accepted for publication in MNRAS. 5 pages, 4 figures, 1 tabl
Strongly star-forming rotating disks in a complex merging system at z = 4,7 as revealed by ALMA
We performed a kinematical analysis of the [CII] line emission of the BR
1202-0725 system at z~4,7 using ALMA observations. The most prominent sources
of this system are a quasar and a submillimeter galaxy, separated by a
projected distance of about 24 kpc and characterized by very high SFR, higher
than 1000 Msun/yr. However, the ALMA observations reveal that these galaxies
apparently have undisturbed rotating disks, which is at variance with the
commonly accepted scenario in which strong star formation activity is induced
by a major merger. We also detected faint components which, after spectral
deblending, were spatially resolved from the main QSO and SMG emissions. The
relative velocities and positions of these components are compatible with
orbital motions within the gravitational potentials generated by the QSO host
galaxy and the SMG, suggesting that they are smaller galaxies in interaction or
gas clouds in accretion flows of tidal streams. We did not find any clear
spectral evidence for outflows caused by AGN or stellar feedback. This suggests
that the high star formation rates might be induced by interactions or minor
mergers with these companions, which do not affect the large-scale kinematics
of the disks, however. Our kinematical analysis also indicates that the QSO and
the SMG have similar Mdyn, mostly in the form of molecular gas, and that the
QSO host galaxy and the SMG are seen close to face-on with slightly different
disk inclinations: the QSO host galaxy is seen almost face-on (i~15), while the
SMG is seen at higher inclinations (i~25). Finally, the ratio between the black
hole mass of the QSO, obtained from XShooter spectroscopy, and the Mdyn of the
host galaxy is similar to value found in very massive local galaxies,
suggesting that the evolution of black hole galaxy relations is probably better
studied with dynamical than with stellar host galaxy masses.Comment: Accepted for publication in Astronomy and Astrophysic
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
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
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