ABSTRACT
We present the discovery of a white dwarf companion at ∼3.6 arcsec from GJ 3346, a nearby (π ∼ 42 mas) K star observed with SPHERE@VLT as part of an open time survey for faint companions to ...objects with significant proper motion discrepancies (Δμ) between Gaia DR1 and Tycho-2. Syrius-like systems like GJ 3346 AB, which include a main-sequence star and a white dwarf, can be difficult to detect because of the intrinsic faintness of the latter. They have, however, been found to be common contaminants for direct imaging (DI) searches. White dwarfs have in fact similar brightness to substellar companions in the infrared, while being much brighter in the visible bands like those used by Gaia. Combining our observations with Gaia DR2 and with several additional archival data sets, we were able to fully constrain the physical properties of GJ 3346 B, such as its effective temperature (11 × 103 ± 500 K) as well as the cooling age of the system (648 ± 58 Myr). This allowed us to better understand the system history and to partially explain the discrepancies previously noted in the age indicators for this object. Although further investigation is still needed, it seems that GJ 3346, which was previously classified as young, is in fact most likely to be older than 4 Gyr. Finally, given that the mass (0.58 ± 0.01 M⊙) and separation (85 au) of GJ 3346 B are compatible with the observed Δμ, this discovery represents a further confirmation of the potential of this kind of dynamical signatures as selection methods for DI surveys targeting faint, substellar companions.
Aims.
The goal of this paper is to increase the membership list of the Chamaeleon star-forming region and the
ϵ
Cha moving group, in particular for low-mass stars and substellar objects. We extended ...the search region significantly beyond the dark clouds.
Methods.
Our sample has been selected based on proper motions and colours obtained from
Gaia
and 2MASS. We present and discuss the optical spectroscopic follow-up of 18 low-mass stellar objects in Cha I and
ϵ
Cha. We characterize the properties of objects by deriving their physical parameters from spectroscopy and photometry.
Results.
We add three more low-mass members to the list of Cha I and increase the census of known
ϵ
Cha members by more than 40%, thereby spectroscopically confirming 13 new members and relying on X-ray emission as youth indicator for 2 more. In most cases the best-fitting spectral template is from objects in the TW Hya association, indicating that
ϵ
Cha has a similar age. The first estimate of the slope of the initial mass function in
ϵ
Cha down to the substellar regime is consistent with that of other young clusters. We estimate our initial mass function (IMF) to be complete down to ≈0.03
M
⊙
. The IMF can be represented by two power laws: for
M
< 0.5
M
⊙
α
= 0.42 ± 0.11 and for
M
> 0.5
M
⊙
α
= 1.44 ± 0.12.
Conclusions.
We find similarities between
ϵ
Cha and the southernmost part of Lower Centaurus Crux (LCC A0), which lie at similar distances and share the same proper motions. This suggests that
ϵ
Cha and LCC A0 may have been born during the same star formation event
Context.
The dominant formation channel of brown dwarfs (BDs) is not well constrained yet and a promising way to discriminate between scenarios is to test the environment-dependent efficiency in ...forming BDs. So far, the outcome of star formation, studied through the initial mass function, has been found to be very similar in all clusters that have been inspected.
Aims.
We aim to characterize the low-mass (sub)stellar population of the central portion (2.4 pc
2
) of the ∼2 Myr old cluster NGC 2244 using near-infrared spectroscopy. By studying this cluster, characterized by a low stellar density and numerous OB stars, we aim to explore the effect that OB stars may have on the production of BDs.
Methods.
We obtained near-infrared
HK
spectroscopy of 85 faint candidate members of NGC 2244. We derived the spectral type and extinction by comparison with spectral templates. We evaluated cluster membership using three gravity-sensitive spectral indices based on the shape of the
H
-band. Furthermore, we evaluated the infrared excess from
Spitzer
of all the candidate members of the cluster. Finally, we estimated the mass of all the candidate members of the cluster and derived the initial mass function, star-to-BD number ratio, and disk fraction.
Results.
The initial mass function is well represented by a power law (d
N
/d
M
∝
M
−
α
) below 0.4
M
⊙
, with a slope
α
= 0.7–1.1 depending on the fitted mass range. We calculated a star-to-BD number ratio of 2.2–2.8. We find the low-mass population of NGC 2244 to be consistent with nearby star-forming regions, although it is at the high end of BD production. We find BDs in NGC 2244 to be, on average, closer to OB stars than to low-mass stars, which could potentially be the first piece of evidence that OB stars affect the formation of BDs. We find a disk fraction of all the members with a spectral type later than K0 of 39 ± 9% which is lower than typical values found in nearby star-forming regions of similar ages.
Abstract
The last decade of direct imaging (DI) searches for sub-stellar companions has uncovered a widely diverse sample that challenges the current formation models, while highlighting the ...intrinsically low occurrence rate of wide companions, especially at the lower end of the mass distribution. These results clearly show how blind surveys, crucial to constrain the underlying planet and sub-stellar companion population, are not an efficient way to increase the sample of DI companions. It is therefore becoming clear that efficient target selection methods are essential to ensure a larger number of detections. We present the results of the COPAINS Survey conducted with SPHERE/VLT, searching for sub-stellar companions to stars showing significant proper motion differences (Δμ) between different astrometric catalogues. We observed twenty-five stars and detected ten companions, including four new brown dwarfs: HIP 21152 B, HIP 29724 B, HD 60584 B and HIP 63734 B. Our results clearly demonstrates how astrometric signatures, in the past only giving access to stellar companions, can now thanks to Gaia reveal companions well in the sub-stellar regime. We also introduce FORECAST (Finley Optimised REtrieval of Companions of Accelerating STars), a tool which allows to check the agreement between position and mass of the detected companions with the measured Δμ. Given the agreement between the values of the masses of the new sub-stellar companions from the photometry with the model-independent ones obtained with FORECAST, the results of COPAINS represent a significant increase of the number of potential benchmarks for brown dwarf and planet formation and evolution theories.
Several massive black holes exhibit flux variability on time-scales that correspond to source sizes of the order of few Schwarzschild radii. We survey the potential of near-infrared and X-ray ...polarimetry to constrain physical properties of such black hole systems, namely, their spin and inclination. We have focused on a model where an orbiting hotspot is embedded in an accretion disc. A new method of searching for the time-lag between orthogonal polarization channels is developed and applied to an ensemble of hotspot models that samples a wide range of parameter space. We found that the hotspot model predicts signatures in polarized light which are in the range to be measured directly in the near future. However, our estimations are predicted upon the assumption of a Keplerian velocity distribution inside the flow where the dominant part of the magnetic field is toroidal. We also found that if the right model of the accretion flow can be chosen for each source (e.g. on the basis of magnetohydrodynamic simulations), then the black hole spin and inclination can be constrained to a small two-dimensional area in the spin-inclination space. The results of the application of the method to the available near-infrared polarimetric data of Sagittarius A* (Sgr A*) are presented. It is shown that even with the currently available data, the spin and inclination of Sgr A* can be constrained. Next generations of near-infrared and X-ray polarimeters should be able to exploit this tool.
Context.
Studies of the low-mass population statistics in young clusters are the foundation for our understanding of the formation of low-mass stars and brown dwarfs. Robust low-mass populations can ...be obtained through near-infrared spectroscopy, which provides confirmation of the cool and young nature of member candidates. However, the spectroscopic analysis of these objects is often not performed in a uniform manner, and the assessment of youth generally relies on the visual inspection of youth features whose behavior is not well understood.
Aims.
We aim at building a method that efficiently identifies young low-mass stars and brown dwarfs from low-resolution near-infrared spectra, by studying gravity-sensitive features and their evolution with age.
Methods.
We have built a data set composed of all publicly available (∼2800) near-infrared spectra of dwarfs with spectral types between M0 and L3. First, we investigate methods for the derivation of the spectral type and extinction via comparison to spectral templates and various spectral indices. Then, we examine gravity-sensitive spectral indices and apply machine learning methods in order to efficiently separate young (≲10 Myr) objects from the field.
Results.
Using a set of six spectral indices for spectral typing, including two newly defined ones (TLI-
J
and TLI-
K
), we are able to achieve a precision below one spectral subtype across the entire spectral type range. We define a new gravity-sensitive spectral index (TLI-
g
) that consistently separates young objects from field objects; it shows a performance superior to other indices from the literature. Even better separation between the two classes can be achieved through machine learning methods that use the entire near-infrared spectra as an input. Moreover, we show that the
H
and
K
bands alone are sufficient for this purpose. Finally, we evaluate the relative importance of different spectral regions for gravity classification as returned by the machine learning models. We find that the
H
-band broadband shape is the most relevant feature, followed by the FeH absorption bands at 1.2 μm and 1.24 μm and the KI doublet at 1.24 μm.
Context. JWST and next-generation facilities are expected to uncover populations of free-floating objects below the deuterium-burning limit in a number of young clusters and star-forming regions. ...These young planetary-mass brown dwarfs have spectral types L and T; they are shaped by molecular absorption and modified by their low gravity, which makes them distinct from field objects. Aims. We aim to provide a detailed characterization of near-infrared spectra for young LT brown dwarfs, including robust spectral typing, calibrating spectral indices, identifying possible binaries, and selecting suitable spectral standards. Methods. We processed and analyzed archival spectra from VLT/X-shooter for a sample of 56 dwarfs with ages between 10 and 600 Myr and spectral types between late-M and mid-T. We redetermined their spectral types by comparing them with a set of literature templates. We assessed a large range of spectral indices, calibrated using a specifically designed literature sample. Results. We identify 15 spectral indices that are useful for spectral typing for specific spectral ranges discussed here and provide the scaling relations with spectral types. We also identify six spectral indices that can be used to separate young L dwarfs from the field ones. The equivalent-widths of the alkali lines show a correlation with age, increasing toward objects with higher surface gravities. From our sample, we confirm three that are likely to be binaries due to their anomalous spectra that appear to be better fitted by a combination of spectral types. We identify 12 objects as preliminary near-infrared spectral standards for young LT dwarfs. Conclusions. This study represents a significant step toward understanding the spectral sequence and properties of young L and T dwarfs. The relations and standards provided here will be useful for future spectroscopic work on young brown dwarfs and giant planets.
ABSTRACT
The last decade of direct imaging (DI) searches for sub-stellar companions has uncovered a widely diverse sample that challenges the current formation models, while highlighting the ...intrinsically low occurrence rate of wide companions, especially at the lower end of the mass distribution. These results clearly show how blind surveys, crucial to constrain the underlying planet and sub-stellar companion population, are not an efficient way to increase the sample of DI companions. It is therefore becoming clear that efficient target selection methods are essential to ensure a larger number of detections. We present the results of the COPAINS Survey conducted with SPHERE/VLT, searching for sub-stellar companions to stars showing significant proper motion differences (Δμ) between different astrometric catalogues. We observed 25 stars and detected ten companions, including four new BDs: HIP 21152 B, HIP 29724 B, HD 60584 B, and HIP 63734 B. Our results clearly demonstrate how astrometric signatures, in the past only giving access to stellar companions, can now thanks to Gaia reveal companions well in the sub-stellar regime. We also introduce FORECAST (Finely Optimised REtrieval of Companions of Accelerating STars), a tool which allows to check the agreement between position and mass of the detected companions with the measured Δμ. Given the agreement between the values of the masses of the new sub-stellar companions from the photometry with the model-independent ones obtained with FORECAST, the results of COPAINS represent a significant increase in the number of potential benchmarks for BD and planet formation and evolution theories.
Abstract
We present Atacama Large Millimeter/sub-millimeter Array Cycle-2 observations of the HBC 494 molecular outflow and envelope. HBC 494 is an FU Ori-like object embedded in the Orion A cloud ...and is associated with the reflection nebulae Re50 and Re50N. We use 12CO, 13CO and C18O spectral line data to independently describe the outflow and envelope structures associated with HBC 494. The moment-1 map of the 12CO emission shows the widest outflow cavities in a Class I object known to date (opening angle ∼150°). The morphology of the wide outflow is likely to be due to the interaction between winds originating in the inner disc and the surrounding envelope. The low-velocity blueshifted and redshifted 13CO and C18O emission trace the rotation and infall motion of the circumstellar envelope. Using molecular line data and adopting standard methods for correcting optical depth effects, we estimate its kinematic properties, including an outflow mass of the order of 10−1 M⊙. Considering the large estimated outflow mass for HBC 494, our results support recent theoretical work suggesting that wind-driven processes might dominate the evolution of protoplanetary discs via energetic outflows.
ABSTRACT
We present COPAINS (Code for Orbital Parametrization of Astrometrically Inferred New Systems), an innovative tool developed to identify previously undiscovered companions detectable via ...direct imaging, based on changes in stellar proper motions across multiple astrometric catalogues. This powerful procedure allows for dynamical predictions of the possible masses and separations of unseen companions compatible with observed astrometric trends, marginalized over unknown orbital elements. Validating our approach using well-constrained systems, we found that our tool provides a good indication of the region of the parameter space where undetected secondaries may be located. Comparing the output of the code to the measured or expected sensitivity from various imaging instruments, this in turn enables us to robustly select the most promising targets for direct imaging campaigns searching for low-mass companions. Such an informed selection method promises to reduce the null detection rates from current programmes and will significantly increase the current census of wide brown dwarfs and planetary companions to stars, which remain extremely rare in the surveys conducted so far.
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