60 research outputs found
Orbital constraints for young low-mass visual M-dwarf binaries
No full textWe have procured astrometric epochs for 15 low-mass visual M-dwarf binaries for which we are able to place orbital constraints. From the Keplerian motions of the binaries we are able to extract dynamical masses, which will be compared to current state-of-the-art theoretical evolutionary models. The dynamical masses will eventually be used for constructing empirical models, and to improve isochronal dating of young moving groups and associations. Here we present the current status and the preliminary results from the survey
Characterising Emblematic Binaries at the Lowest Stellar and Substellar Masses
No full textStars are involved in most research fields of astronomy, ranging from studies of faraway galaxies, exploding supernovae, to more nearby exoplanets and even our own Sun. As such, it is paramount that our physical interpretation of stars is accurate. By observing stars at different epochs, we can fashion evolutionary models to predict important events that occur at different phases during their life-cycle. Thus, exemplary stars where properties including mass, age and luminosity can be observed become increasingly valuable as benchmarks for calibrating said models with. Sometimes, all of these essential properties can be measured for a single system. For instance, for a binary star which circles a common centre of mass we can from its orbital motion calculate the dynamical mass of the system. If the stellar system also has a well-determined age we may use it as a benchmark for our models, and hence refer to it as an emblematic binary system. In this thesis we are searching for exactly these emblematic binaries, both among lowmass stars and substellar brown dwarfs. We also show how to measure the different characteristics that make the systems into exemplary touchstones. We provide an overview over the different types of stellar binaries, how mass and age estimates are performed, as well as discuss the implications multiplicity has for the formation and evolution of stars and brown dwarfs. In Paper I we present the results from an orbital fit we constrained for a low-mass binary with a known age, making into a valuable and relatively rare benchmark. We also show in Paper II how long baseline astrometry can be exploited in order to place better constraints for orbital fits and dynamical masses for low-mass companions to stars by measuring the perturbation in proper motion over time. The dynamical masses are sequentially tested against evolutionary models, which at these low masses display several discrepancies compared to the observables, and are thus questioned. We explore more uncharted mass-regimes in Paper III, where we employ laser guide star assisted adaptive optics to search for multiplicity among faint substellar objects in young moving groups, detecting 3 new young brown dwarf binary systems. These new binaries will prove to be highly valuable systems for future research of brown dwarfs, and will be able to be studied further with for instance the Extremely Large Telescope or James Webb Space Telescope, which also makes them into prominent benchmarks for substellar evolutionary models. Furthermore, age estimation typically dominates the error budget for low-mass stars and brown dwarfs, requiring several different approaches for a robust assessment. In Paper IV we test and compare different techniques for age determination of 7 low-mass binary stars. These binaries have had their orbital motion monitored for a longer time, and will soon be constrained well enough that dynamical masses may be procured. As such, these low-mass binaries will extend the so far scarce number of exemplary systems where both mass, luminosity and age can be determined, to later be used to calibrate theoretical evolutionary models.At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Submitted. </p
Characterising young visual M-dwarf binaries with near-infrared integral field spectra
No full textWe present the results from an integral field spectroscopy study of seven close visual binary pairs of young M-dwarf multiple systems. The target systems are part of the astrometric monitoring AstraLux programme, surveying hundreds of M-dwarf systems for multiplicity and obtaining astrometric epochs for orbital constraints. Our new VLT/SINFONI data provides resolved spectral type classifications in the J, H, and K bands for seven of these low-mass M-dwarf binaries, which we determine by comparing them to empirical templates and examining the strength of water absorption in the K band. The medium resolution K-band spectra also allows us to derive effective temperatures for the individual components. All targets in the survey display several signs of youth, and some have kinematics similar to young moving groups, or low surface gravities which we determined from measuring equivalent widths of gravity sensitive alkali lines in the J band. Resolved photometry from our targets is also compared with isochrones from theoretical evolutionary models, further implying young ages. Dynamical masses will be provided from continued monitoring of these systems, which can be seen as emblematic binary benchmarks that may be used to calibrate evolutionary models for low-mass stars in the future
Improving dynamical mass constraints for intermediate-period substellar companions using
No full textThe relationship between luminosity and mass is of fundamental importance for direct imaging studies of brown dwarf and planetary companions to stars. In principle this can be inferred from theoretical mass-luminosity models; however, these relations have not yet been thoroughly calibrated, since there is a lack of substellar companions for which both the brightness and mass have been directly measured. One notable exception is GJ 758 B, a brown dwarf companion in a ~20 AU orbit around a nearby Sun-like star, which has been both directly imaged and dynamically detected through a radial velocity trend in the primary. This has enabled a mass constraint for GJ 758 B of . Here, we note that Gaia is ideally suited for further constraining the mass of intermediate-separation companions such as GJ 758 B. A study of the differential proper motion, Δμ, with regards to HIPPARCO
Astrophysical properties of 600 bonafide single stars in the Hyades open cluster
No full textThe determination of the astrophysical properties of stars remains
challenging, and frequently relies on the application of stellar models.
Stellar sequences in nearby open clusters provide some of the best means to
test and calibrate stellar evolutionary models and isochrones, and to use these
models to assign astrophysical properties consistently to a large sample of
stars. We aim at updating the single star sequence of members of the Hyades
cluster, identifying the best-fitting isochrones, and determining the
astrophysical properties of the stars. The Gaia Catalogue of Nearby Stars
provides a comprehensive sample of high-probability members of the Hyades
cluster. We apply a multi-step method to flag photometric outliers, and to
identify bonafide single stars and likely binary and multiple systems. The
single stars define a tight sequence, which in the mass range 0.12 to 2.2 Msun
is well-fitted by PARSEC isochrones for a supersolar metallicity of [M/H] =
+0.18 +- 0.03 and an age of 775 +- 25 Myr. The isochrones enable us to assign
mass, effective temperature, luminosity, and surface gravity to each of the 600
bonafide single main-sequence stars. The observed sequence validates the PARSEC
isochrones. The derived stellar properties can serve as benchmarks for
atmospheric and evolutionary models, and for all-sky catalogs of stellar
astrophysical properties. The stellar properties are also relevant for studies
of exoplanet properties among Hyades exoplanet hosts.Comment: Accepted for publication in AJ, 8 pages, 5 figures, full Table 1 will
be available in machine readable format (mrt
Detection of new strongly variable brown dwarfs in the L/T transition
No full textContext. Brown dwarfs in the spectral range L9–T3.5, within the so called L/T transition, have been shown to be variable at higher amplitudes and with greater frequency than other field dwarfs. This strong variability allows for the probing of their atmospheric structure in 3D through multi-wavelength observations for studying the underlying physical mechanisms responsible for the variability. The few known strongly variable dwarfs in this range have been extensively studied. Now, more variables such as these need to be discovered and studied to better constrain atmospheric models. This is also critical to better understand giant exoplanets and to shed light on a number of possible correlations between brown dwarf characteristics and variability.
Aims. Previous studies suggest an occurrence rate for strong variability (peak-to-peak amplitudes > 2%) of up to ∼39% among brown dwarfs within the L/T transition. In this work, we aim to discover new strong variables in this spectral range by targeting ten previously unsurveyed brown dwarfs.
Methods. We used the NOTCam at the Nordic Optical Telescope to observe 11 targets, with spectral types ranging from L9.5 to T3.5, in the J-band during October 2017 and September 2018. Using differential aperture photometry, we then analysed the light curves for significant variability using Lomb-Scargle periodogram algorithms and least squares fitting.
Results. We report first discoveries of strong and significant variability in four out of the ten targets (false alarm probability < 0.1%), measuring peak-to-peak amplitudes up to 10.7 ± 0.4% in J for the T1 dwarf 2MASS J22153705+2110554, for which we observe significant light curve evolution between the 2017 and 2018 epochs. We also report a marginally significant detection of strong variability, and confirm that the well known 2MASS J01365662+0933473 is still strongly variable three years after the last reported epoch. Finally, we present an extensive multi-epoch catalogue of strong variables reported in the literature and discuss possible correlations that are identifiable from the catalogue.
Conclusions. We significantly add to the number of known strong variables, and through Poisson statistics infer an occurrence rate for strong variability among L9–T3.5 brown dwarfs of %, which is in agreement with previous estimates. The new variables identified in this work are also excellently suited for extensive multi-wavelength observations dedicated to probing the 3D structure of brown dwarf atmospheres
Benchmarking Gaia DR3 Apsis with the Hyades and Pleiades open clusters
No full textThe Gaia astrophysical parameters inference system (Apsis) provides
astrophysical parameter estimates for several to 100s of millions of stars. We
aim to benchmark Gaia DR3 Apsis. We have compiled about 1500 bona fide single
stars in the Hyades and Pleiades open clusters for validation of PARSEC
isochrones, and for comparison with Apsis estimates. PARSEC stellar isochrones
in the Gaia photometric system enable us to assign average ages and
metallicities to the clusters, and mass, effective temperature, luminosity, and
surface gravity to the individual stars. Apsis does not recover the single-age,
single-metallicity characteristic of the cluster populations. Ages assigned to
cluster members seemingly follow the input template for Galactic populations,
with earlier-type stars systematically being assigned younger ages than
later-type stars. Cluster metallicities are underestimated by 0.1 to 0.2 dex.
Effective temperature estimates are in general reliable. Surface gravity
estimates reveal strong systematics for specific ranges of Gaia BP-RP colours.
We caution that Gaia DR3 Apsis estimates can be subject to significant
systematics. Some of the Apsis estimates, like metallicity, might only be
meaningful for statistical studies of the time-averaged Galactic stellar
population, but are not recommended to be used for individual stars.Comment: Accepted for publication in A&A. Table 2 in its entirety can be
requested from the authors in machine-readable format (mrt), and will become
available via CDS. 8 pages, 6 figures. v2: Figure 6, right, updated based on
Gaia DR3 Apsis GSP-Spec quality flag
High-contrast, high-angular resolution view of the GJ 367 exoplanet system
No full textInternational audienceWe search for additional companions in the GJ 367 exoplanet system and aim to better constrain its age and evolutionary status. We analyse high-contrast direct imaging observations obtained with HST/NICMOS, VLT/NACO, and VLT/SPHERE. We investigate and critically discuss conflicting age indicators based on theoretical isochrones and models for Galactic dynamics. A comparison of GAIA EDR3 parallax and photometric measurements with theoretical isochrones suggests a young age ≤60 Myr for GJ 367. The star's Galactic kinematics exclude membership to any nearby young moving group or stellar stream. Its highly eccentric Galactic orbit, however, is atypical for a young star. Age estimates considering Galactic dynamical evolution are most consistent with an age of 1-8 Gyr. We find no evidence for a significant mid-infrared excess in the WISE bands, suggesting the absence of warm dust in the GJ 367 system. The direct imaging data provide significantly improved detection limits compared to previous studies. At 530 mas (5 au) separation, the SPHERE data achieve a 5σ contrast of 2.6 × 10-6. The data exclude the presence of a stellar companion at projected separations ≥0.4 au. At projected separations ≥5 au we can exclude substellar companions with a mass ≥1.5 MJup for an age of 50 Myr, and ≥20 MJup for an age of 5 Gyr. By applying the stellar parameters corresponding to the 50 Myr isochrone, we derive a bulk density of ρplanet = 6.2 g cm-3 for GJ 367 b, which is 25 per cent smaller than a previous estimate
Characterising Emblematic Binaries at the Lowest Stellar and Substellar Masses [Elektronisk resurs]
No full textStars are involved in most research fields of astronomy, ranging from studies of faraway galaxies, exploding supernovae, to more nearby exoplanets and even our own Sun. As such, it is paramount that our physical interpretation of stars is accurate. By observing stars at different epochs, we can fashion evolutionary models to predict important events that occur at different phases during their life-cycle. Thus, exemplary stars where properties including mass, age and luminosity can be observed become increasingly valuable as benchmarks for calibrating said models with. Sometimes, all of these essential properties can be measured for a single system. For instance, for a binary star which circles a common centre of mass we can from its orbital motion calculate the dynamical mass of the system. If the stellar system also has a well-determined age we may use it as a benchmark for our models, and hence refer to it as an emblematic binary system.In this thesis we are searching for exactly these emblematic binaries, both among lowmass stars and substellar brown dwarfs. We also show how to measure the different characteristics that make the systems into exemplary touchstones. We provide an overview over the different types of stellar binaries, how mass and age estimates are performed, as well as discuss the implications multiplicity has for the formation and evolution of stars and brown dwarfs. In Paper I we present the results from an orbital fit we constrained for a low-mass binary with a known age, making into a valuable and relatively rare benchmark. We also show in Paper II how long baseline astrometry can be exploited in order to place better constraints for orbital fits and dynamical masses for low-mass companions to stars by measuring the perturbation in proper motion over time. The dynamical masses are sequentially tested against evolutionary models, which at these low masses display several discrepancies compared to the observables, and are thus questioned. We explore more uncharted mass-regimes in Paper III, where we employ laser guide star assisted adaptive optics to search for multiplicity among faint substellar objects in young moving groups, detecting 3 new young brown dwarf binary systems. These new binaries will prove to be highly valuable systems for future research of brown dwarfs, and will be able to be studied further with for instance the Extremely Large Telescope or James Webb Space Telescope, which also makes them into prominent benchmarks for substellar evolutionary models. Furthermore, age estimation typically dominates the error budget for low-mass stars and brown dwarfs, requiring several different approaches for a robust assessment. In Paper IV we test and compare different techniques for age determination of 7 low-mass binary stars. These binaries have had their orbital motion monitored for a longer time, and will soon be constrained well enough that dynamical masses may be procured. As such, these low-mass binaries will extend the so far scarce number of exemplary systems where both mass, luminosity and age can be determined, to later be used to calibrate theoretical evolutionary models.</p
Characterizing Young Visual M-dwarf Binaries with Near IR Integral Field Spectra
No full textWe present the results from an integral field spectroscopy study of seven close visual binary pairs of young M-dwarf multiple systems. The target systems are part of the astrometric monitoring AstraLux program, surveying hundreds of M-dwarf systems for multiplicity and obtaining astrometric epochs for orbital constraints. Our new VLT/SINFONI data provides resolved spectral type classification in the J, H and K bands for seven of these low-mass M-dwarf binaries, which we determine by comparing them to empirical templates and examining the strength of water absorption in the K-band. The medium resolution K-band spectra also allows us to derive effective temperatures for the individual components. All targets in the survey display several signs of youth, and some have kinematics similar to young moving groups, or low surface gravities which we determine from measuring equivalent widths of gravity sensitive alkali lines in the J-band. Resolved photometry from our targets is also compared with isochrones from theoretical evolutionary models, further implying young ages. Dynamical masses will be provided from ongoing monitoring of these systems, which can be seen as emblematic binary benchmarks that may be used to calibrate evolutionary models for low-mass stars in the future.</p
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