Context. While the near-infrared wavelength regime is becoming more and more important for astrophysics there is a marked lack of spectrophotometric standard star data that would allow the flux ...calibration of such data. Furthermore, flux calibrating medium- to high-resolution échelle spectroscopy data is challenging even in the optical wavelength range, because the available flux standard data are often too coarsely sampled. Aims. We will provide standard star reference data that allow users to derive response curves from 300 nm to 2500 nm for spectroscopic data of medium to high resolution, including those taken with échelle spectrographs. In addition we describe a method to correct for moderate telluric absorption without the need of observing telluric standard stars. Methods. As reference data for the flux standard stars we use theoretical spectra derived from stellar model atmospheres. We verify that they provide an appropriate description of the observed standard star spectra by checking for residuals in line cores and line overlap regions in the ratios of observed (X-shooter) spectra to model spectra. The finally selected model spectra are then corrected for remaining mismatches and photometrically calibrated using independent observations. The correction of telluric absorption is performed with the help of telluric model spectra. Results. We provide new, finely sampled reference spectra without telluric absorption for six southern flux standard stars that allow the users to flux calibrate their data from 300 nm to 2500 nm, and a method to correct for telluric absorption using atmospheric models.
Context. Airglow emission lines, which dominate the optical-to-near-infrared sky radiation, show strong, line-dependent variability on time scales from minutes to decades. Therefore, the subtraction ...of the sky background in the affected wavelength regime becomes a problem if plain-sky spectra have to be taken at a different time from the astronomical data. Aims. A solution of this problem is the physically motivated scaling of the airglow lines in the plain-sky data to fit the sky lines in the object spectrum. We have developed a corresponding instrument-independent approach based on one-dimensional spectra. Methods. Our code skycorr separates sky lines and sky/object continuum by an iterative approach involving a line finder and airglow line data. The sky lines, which mainly belong to OH and O2 bands, are grouped according to their expected variability. The line groups in the sky data are then scaled to fit the sky in the science data. Required pixel-specific weights for overlapping groups are taken from a comprehensive airglow model. Deviations in the wavelength calibration are corrected for by fitting Chebyshev polynomials and rebinning via asymmetric damped sinc kernels. The scaled sky lines and the sky continuum are subtracted separately. Results. ESO-VLT X-shooter data covering 2.5 h with a good time resolution were selected to illustrate the performance. Data taken six nights and about one year before were also used as reference sky data. The variation of the sky-subtraction quality as a function of time difference between the object and sky data depends on changes in the airglow intensity, atmospheric transparency, and instrument calibration. Except for short time intervals of a few minutes, the sky line residuals were between 2.1 and 5.5 times weaker than for sky subtraction without fitting. Additional tests showed that skycorr performs consistently better than the method of Davies (2007, MNRAS, 375, 1099) developed for ESO-VLT SINFONI data.
Revisiting Proxima with ESPRESSO Suárez Mascareño, A.; Faria, J. P.; Figueira, P. ...
Astronomy and astrophysics (Berlin),
07/2020, Letnik:
639
Journal Article
Recenzirano
Odprti dostop
Context.
The discovery of Proxima b marked one of the most important milestones in exoplanetary science in recent years. Yet the limited precision of the available radial velocity data and the ...difficulty in modelling the stellar activity calls for a confirmation of the Earth-mass planet.
Aims.
We aim to confirm the presence of Proxima b using independent measurements obtained with the new ESPRESSO spectrograph, and refine the planetary parameters taking advantage of its improved precision.
Methods.
We analysed 63 spectroscopic ESPRESSO observations of Proxima (Gl 551) taken during 2019. We obtained radial velocity measurements with a typical radial velocity photon noise of 26 cm s
−1
. We combined these data with archival spectroscopic observations and newly obtained photometric measurements to model the stellar activity signals and disentangle them from planetary signals in the radial velocity (RV) data. We ran a joint Markov chain Monte Carlo analysis on the time series of the RV and full width half maximum of the cross-correlation function to model the planetary and stellar signals present in the data, applying Gaussian process regression to deal with the stellar activity signals.
Results.
We confirm the presence of Proxima b independently in the ESPRESSO data and in the combined ESPRESSO+ HARPS+UVES dataset. The ESPRESSO data on its own shows Proxima b at a period of 11.218 ± 0.029 days, with a minimum mass of 1.29 ± 0.13
M
⊕
. In the combined dataset we measure a period of 11.18427 ± 0.00070 days with a minimum mass of 1.173 ± 0.086
M
⊕
. We get a clear measurement of the stellar rotation period (87 ± 12 d) and its induced RV signal, but no evidence of stellar activity as a potential cause for the 11.2 days signal. We find some evidence for the presence of a second short-period signal, at 5.15 days with a semi-amplitude of only 40 cm s
−1
. If caused by a planetary companion, it would correspond to a minimum mass of 0.29 ± 0.08
M
⊕
. We find that forthe case of Proxima, the full width half maximum of the cross-correlation function can be used as a proxy for the brightness changes and that its gradient with time can be used to successfully detrend the RV data from part of the influence of stellar activity. The activity-induced RV signal in the ESPRESSO data shows a trend in amplitude towards redder wavelengths. Velocities measured using the red end of the spectrograph are less affected by activity, suggesting that the stellar activity is spot dominated. This could be used to create differential RVs that are activity dominated and can be used to disentangle activity-induced and planetary-induced signals. The data collected excludes the presence of extra companions with masses above 0.6
M
⊕
at periods shorter than 50 days.
Aims.
We report on ESPRESSO high-resolution transmission spectroscopic observations of two primary transits of the highly irradiated, ultra-hot Jupiter-sized planet, WASP-76b. We investigated the ...presence of several key atomic and molecular features of interest that may reveal the atmospheric properties of the planet.
Methods.
We extracted two transmission spectra of WASP-76b with
R
≈ 140 000 using a procedure that allowed us to process the full ESPRESSO wavelength range (3800–7880 Å) simultaneously. We observed that at a high signal-to-noise ratio, the continuum of ESPRESSO spectra shows ‘wiggles’, which are likely caused by an interference pattern outside the spectrograph. To search for the planetary features, we visually analysed the extracted transmission spectra and cross-correlated the observations against theoretical spectra of different atomic and molecular species.
Results.
The following atomic features are detected: Li
I
, Na
I
, Mg
I
, Ca
II
, Mn
I
, K
I
, and Fe
I
. All are detected with a confidence level between 9.2
σ
(Na
I
) and 2.8
σ
(Mg
I
). We did not detect the following species: Ti
I
, Cr
I
, Ni
I
, TiO, VO, and ZrO. We impose the following 1
σ
upper limits on their detectability: 60, 77, 122, 6, 8, and 8 ppm, respectively.
Conclusions.
We report the detection of Li
I
on WASP-76b for the first time. In addition, we confirm the presence of Na
I
and Fe
I
as previously reported in the literature. We show that the procedure employed in this work can detect features down to the level of ~0.1% in the transmission spectrum and ~10 ppm by means of a cross-correlation method. We discuss the presence of neutral and singly ionised features in the atmosphere of WASP-76b.
We present the first detailed analysis of the rest-frame ultraviolet spectrum of the gravitationally lensed Lyman break galaxy (LBG), the “8 o'clock arc”, obtained with the intermediate-resolution ...X-shooter spectrograph recently commissioned on the ESO Very Large Telescope. Besides MS 1512–cB58, the Cosmic Horseshoe, and the Cosmic Eye, three other lensed LBGs at comparable redshifts, this is the fourth of such a study, usually unfeasible at high redshifts. The spectrum of the 8 o'clock arc is rich in stellar and interstellar features, and presents several similarities to the well-known MS 1512–cB58 LBG. The stellar photospheric absorption lines allowed us to constrain the systemic redshift, $z_{\rm sys} = 2.7350$ ± 0.0003, of the galaxy, and derive its stellar metallicity, $Z=0.82~Z_{\odot}$, which is in excellent agreement with the metallicity determined from nebular emission lines. With a total stellar mass of ${\sim}4.2$ $\times$ $10^{11}~M_{\odot}$, the 8 o'clock arc agrees with the mass-metallicity relation found for z>2 star-forming galaxies, although being located near the upper end of the distribution given its high mass and high metallicity. Broad HeII λ1640 emission is found, indicative of the presence of Wolf-Rayet stars formed in an intense period of star formation. The 31 interstellar absorption lines detected led to the abundance measurements of 9 elements. The metallicity of the interstellar medium (ISM), $Z=0.65~Z_{\odot}$ (Si), is very comparable to the metallicity of stars and ionized gas, and suggests that the ISM of the 8 o'clock arc has been rapidly polluted and enriched by ejecta of OB stars. The ISM lines extend over a very large velocity range, $\Delta \upsilon \sim 1000$ km s-1, from about -800 to +300 km s-1 relative to the systemic redshift, and have their peak optical depth blueshifted relative to the stars, implying gas outflows of $\upsilon_{\rm ISM} \simeq -120$ km s-1. The zero residual intensity in the strongest lines indicates a nearly complete coverage of the UV continuum by the ISM. The Lyα line is dominated by a damped absorption profile on top of which is superposed a weak emission, redshifted relative to the ISM lines by about +690 km s-1 and resulting from multiply backscattered Lyα photons emitted in the HII region surrounded by the cold, expanding ISM shell. A homogeneous spherical shell model with a constant outflow velocity, determined by the observations, is able to reproduce the observed Lyα line profile. Furthermore, the required dust content, $E(B-V) \approx 0.3$, is in good agreement with the attenuation measured from the Balmer decrement. These results obtained from the radiation transfer modeling of the Lyα line in the 8 o'clock arc fully support the scenario proposed earlier, where the diversity of Lyα line profiles in Lyman break galaxies and Lyα emitters, from absorption to emission, is mostly due to variations of HI column density and dust content.
X-shooter is the first 2nd generation instrument of the ESO Very Large Telescope (VLT). It is a very efficient, single-target, intermediate-resolution spectrograph that was installed at the ...Cassegrain focus of UT2 in 2009. The instrument covers, in a single exposure, the spectral range from 300 to 2500 nm. It is designed to maximize the sensitivity in this spectral range through dichroic splitting in three arms with optimized optics, coatings, dispersive elements and detectors. It operates at intermediate spectral resolution (R ~ 4000−17 000, depending on wavelength and slit width) with fixed échelle spectral format (prism cross-dispersers) in the three arms. It includes a 1.8″ × 4″ integral field unit as an alternative to the 11′′ long slits. A dedicated data reduction package delivers fully calibrated two-dimensional and extracted spectra over the full wavelength range. We describe the main characteristics of the instrument and present its performance as measured during commissioning, science verification and the first months of science operations.
Context.
The bright star
π
Men was chosen as the first target for a radial velocity follow-up to test the performance of ESPRESSO, the new high-resolution spectrograph at the European Southern ...Observatory’s Very Large Telescope. The star hosts a multi-planet system (a transiting 4
M
⊕
planet at ~0.07 au and a sub-stellar companion on a ~2100-day eccentric orbit), which is particularly suitable for a precise multi-technique characterization.
Aims.
With the new ESPRESSO observations, which cover a time span of 200 days, we aim to improve the precision and accuracy of the planet parameters and search for additional low-mass companions. We also take advantage of the new photometric transits of
π
Men c observed by TESS over a time span that overlaps with that of the ESPRESSO follow-up campaign.
Methods.
We analysed the enlarged spectroscopic and photometric datasets and compared the results to those in the literature. We further characterized the system by means of absolute astrometry with H
IPPARCOS
and
Gaia
. We used the high-resolution spectra of ESPRESSO for an independent determination of the stellar fundamental parameters.
Results.
We present a precise characterization of the planetary system around
π
Men. The ESPRESSO radial velocities alone (37 nightly binned data with typical uncertainty of 10 cm s
−1
) allow for a precise retrieval of the Doppler signal induced by
π
Men c. The residuals show a root mean square of 1.2 m s
−1
, which is half that of the HARPS data; based on the residuals, we put limits on the presence of additional low-mass planets (e.g. we can exclude companions with a minimum mass less than ~2
M
⊕
within the orbit of
π
Men c). We improve the ephemeris of
π
Men c using 18 additional TESS transits, and, in combination with the astrometric measurements, we determine the inclination of the orbital plane of
π
Men b with high precision (
i
b
=45.8
−1.1
+1.4
deg). This leads to the precise measurement of its absolute mass
m
b
=14.1
−0.4
+0.5
M
Jup
, indicating that
π
Men b can be classified as a brown dwarf.
Conclusions.
The
π
Men system represents a nice example of the extreme precision radial velocities that can be obtained with ESPRESSO for bright targets. Our determination of the 3D architecture of the
π
Men planetary system and the high relative misalignment of the planetary orbital planes put constraints on and challenge the theories of the formation and dynamical evolution of planetary systems. The accurate measurement of the mass of
π
Men b contributes to make the brown dwarf desert a bit greener.
ABSTRACT
With the equivalent area of a 16-m telescope, ESPRESSO in 4UT mode allows to inaugurate high resolution spectroscopy for solar-type stars belonging to extragalactic globular clusters. We ...determine the chemical composition of an extragalactic blue straggler (BS). The star has a G magnitude of 19.01 and belongs to the globular cluster Pal12, that is associated to the Sagittarius dwarf galaxy. Abundances are computed by using high resolution spectroscopy and LTE analysis. Two 50 min ESPRESSO spectra, co-added, provide a signal-to-noise ratio of ∼25 with a resolving power R∼70 000. This allows us to measure with good precision abundance of several (13) elements. Li could help to distinguish between formation models of BS; we are able to set a 3σ upper limit of Li = 3.1, which is still too high to discriminate between competing models. The abundances we retrieve for the BS are compatible with those of giant stars of Pal 12 published in literature, re-analysed by us using the same procedure and line list. Small differences are present that can be ascribed to NLTE effects, but for Mg the BS shows a large under-abundance. The most likely explanation is that the BS atmosphere is dominated by gas processed through the Mg-Al cycle, but we have no suitable Al or Na lines to confirm this hypothesis. We show that ESPRESSO with 4UT can be used to derive precise abundances for solar-type stars fainter than magnitude 19. At these magnitudes a proper sky subtraction is needed and in crowded field the targets must be chosen with utmost care, to avoid contamination of the sky fibre from nearby stars.
The Gaia-ESO Survey is a large public spectroscopic survey that aims to derive radial velocities and fundamental parameters of about 105 Milky Way stars in the field and in clusters. Observations are ...carried out with the multi-object optical spectrograph FLAMES, using simultaneously the medium-resolution (R ~ 20 000) GIRAFFE spectrograph and the high-resolution (R ~ 47 000) UVES spectrograph. In this paper we describe the methods and the software used for the data reduction, the derivation of the radial velocities, and the quality control of the FLAMES-UVES spectra. Data reduction has been performed using a workflow specifically developed for this project. This workflow runs the ESO public pipeline optimizing the data reduction for the Gaia-ESO Survey, automatically performs sky subtraction, barycentric correction and normalisation, and calculates radial velocities and a first guess of the rotational velocities. The quality control is performed using the output parameters from the ESO pipeline, by a visual inspection of the spectra and by the analysis of the signal-to-noise ratio of the spectra. Using the observations of the first 18 months, specifically targets observed multiple times at different epochs, stars observed with both GIRAFFE and UVES, and observations of radial velocity standards, we estimated the precision and the accuracy of the radial velocities. The statistical error on the radial velocities is σ ~ 0.4 km s-1 and is mainly due to uncertainties in the zero point of the wavelength calibration. However, we found a systematic bias with respect to the GIRAFFE spectra (~0.9 km s-1) and to the radial velocities of the standard stars (~0.5 km s-1) retrieved from the literature. This bias will be corrected in the future data releases, when a common zero point for all the set-ups and instruments used for the survey is be established.
Context. The Li-rich turn-off star recently discovered in the old, metal-poor globular cluster NGC 6397 could represent the smoking gun for some fundamental, but very rare episode of Li enrichment in ...globular clusters and in the early Galaxy. Aims. We aim to understand the nature of the Li enrichment by performing a spectroscopic analysis of the star, in particular of its beryllium (Be) abundance, and by investigating its binary nature. Methods. We used the VLT/UVES spectrograph to observe the near UV region where the Be ii resonance doublet and the NH bands are located. We also re-analyzed an archival Magellan/MIKE spectra for C and O abundance determination. Results. We could not detect the Be ii lines and derive an upper limit of log (Be/H) < −12.2 that is consistent with the Be observed in other stars of the cluster. We could detect a weak G-band, which implies a mild carbon enhancement C/Fe+ 0.4 ± 0.2. We could not detect the UV NH band, showing that this star is less N-enhanced than other stars of the cluster, and we derive an upper limit N/Fe < 0.0. For oxygen we could not convincingly detect any of the near UV OH lines, which implies that oxygen cannot be strongly enhanced in this star. This is consistent with the detection of the strongest line of the O i triplet at 777 nm, which is contaminated by telluric absorptions but is consistent with O/Fe ~ 0.5. Combining the UVES and Mike data, we could not detect any variation in the radial velocity greater than 0.95 kms-1 over 8 years. Conclusions. The chemical composition of the star strongly resembles that of “first generation” NGC 6397 stars, with the huge Li as the only deviating abundance. Not detecting Be rules out two possible explanations of the Li overabundance: capture of a substellar body and spallation caused by a nearby type II SNe. Discrepancies are also found with respect to other accretion scenarios, except for contamination by the ejecta of a star that has undergone the RGB Li-flash. This is at present the most likely possibility for explaining the extraordinary Li enrichment of this star.
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