arXiv subjects

K. Kubiak

Publications and source records attributed to K. Kubiak.

6 recordsLinked to original sources

arXiv2026

CHARA/SPICA: The six-telescope visible combiner and near-infrared fringe tracker for the CHARA Array

The suite called Stellar Parameters and Images with a Cophased Array (SPICA) has two interferometric instruments installed at the focus of the CHARA Array located at Mount Wilson, CA. SPICA is made of SPICA-VIS, a fiber-fed six-beam visible spectrograph with three spectral resolutions, and SPICA-FT, a six-beam near-infrared fringe tracker for the fast stabilization of the fringes. SPICA is opening access to imaging in the visible domain with an unprecedented angular resolution down to 0.2 milliarcseconds. It has been designed around a large survey of fundamental parameters of stars over the Hertzsprung- Russell diagram, aiming at understanding the deviations from the standard empirical relations of stellar physics as a function of activity: limb darkening, multiplicity, rotation, winds, and environments. SPICA makes use of the advanced technologies in electron multiplying detectors in the visible and electron- avalanche photodiode arrays in the near-infrared. It benefits from the newly commissioned adaptive optics on the one-meter telescopes of the array. The modules of the visible instrument, SPICA-VIS, optimize the injection of light into single-mode fibers for spatial filtering before spectral dispersion in the image plane. The fringe tracker, SPICA-FT, performs group-delay and phase-delay tracking for six beams in the H band. SPICA-FT can use an all-in-one or ABCD encoding of the fringe signals. SPICA is operational on sky and is close to reaching the expected performance in low spectral resolution, in particular, for the Interferometric Survey of Stellar Parameters (ISSP). More work is still needed to achieve the ultimate performance in terms of sensitivity and to allow operations with higher spectral resolutions.

astro-ph.IM↗

arXiv2023

Spectroscopic substellar initial mass function of NGC 2244

We aim at characterizing the low-mass (sub)stellar population of the central portion (2.4 pc$^2$) of the $\sim$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 at exploring the effect that OB stars may have on the production of BDs. We obtain near infrared HK spectroscopy of 85 faint candidate members of NGC 2244. We derive the spectral type and extinction by comparison with spectral templates. We evaluate cluster membership using three gravity-sensitive spectral indices based on the shape of the $H$-band. Furthermore, we evaluate the infrared excess from Spitzer of all the candidate members of the cluster. Finally, we estimate the mass of all the candidate members of the cluster and derive the initial mass function, star-to-BD number ratio and disk fraction. The initial mass function is well represented by a power law ($dN/dM \propto M^{-\alpha}$) below 0.4 $M_\odot$, with a slope $\alpha$ = 0.7-1.1 depending on the fitted mass range. We calculate 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 evidence of OB stars affecting the formation of BDs. We find a disk fraction of all the members with spectral type later than K0 of 39$\pm$9% which is lower than typical values found in nearby star-forming regions of similar ages.

astro-ph.SR↗

arXiv2021

Youth analysis of near infrared spectra of young low-mass stars and brown dwarfs

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. We built a dataset composed of all publicly available ($\sim$2800) near-infrared spectra of dwarfs with spectral types between M0 and L3. First, we investigate methods for the derivation of the spectral type and extinction using comparison to spectral templates, and various spectral indices. Then, we examine gravity-sensitive spectral indices and apply machine learning methods, in order to efficiently separate young ($\lesssim$10 Myr) objects from the field. 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 1 spectral subtype across the entire spectral type range. We define a new gravity-sensitive spectral index (TLI-g) that consistently separates young from field objects, showing a performance superior to other indices from the literature. Even better separation between the two classes can be achieved through machine learning methods which use the entire NIR spectra as an input. Moreover, we show that the H- and K-bands alone are enough 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 broad-band 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 $μm$.

astro-ph.SR↗

arXiv2021

New low-mass members of Chamaeleon I and $ε$ Cha

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. 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, both from spectroscopy and photometry. 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%, confirming spectroscopically 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 sub-stellar regime is consistent with that of other young clusters. We estimate our IMF to be complete down to $\approx 0.03$M$_{\odot}$. The IMF can be represented by two power laws: for M $<$ 0.5 M$_{\odot}$ $α= 0.42 \pm 0.11$ and for M $>$ 0.5 M$_{\odot}$ $α= 1.44 \pm 0.12$. We find similarities between $ε$ Cha and the southernmost part of Lower Centaurus Crux (LCC A0), both lying at similar distances and sharing the same proper motions. This suggests that $ε$ Cha and LCC A0 may have been born during the same star formation event

astro-ph.SR↗

arXiv2016

Orion revisited III. The Orion Belt population

This paper continues our study of the foreground population to the Orion molecular clouds. The goal is to characterize the foreground population north of NGC 1981 and to investigate the star formation history in the large Orion star-forming region. We focus on a region covering about 25 square degrees, centered on the $ε$ Orionis supergiant (HD 37128, B0\,Ia) and covering the Orion Belt asterism. We used a combination of optical (SDSS) and near-infrared (2MASS) data, informed by X-ray (\textit{XMM-Newton}) and mid-infrared (WISE) data, to construct a suite of color-color and color-magnitude diagrams for all available sources. We then applied a new statistical multiband technique to isolate a previously unknown stellar population in this region. We identify a rich and well-defined stellar population in the surveyed region that has about 2\,000 objects that are mostly M stars. We infer the age for this new population to be at least 5\, Myr and likely $\sim10$\,Myr and estimate a total of about 2\,500 members, assuming a normal IMF. This new population, which we call the Orion Belt population, is essentially extinction-free, disk-free, and its spatial distribution is roughly centered near $ε$ Ori, although substructure is clearly present. The Orion Belt population is likely the low-mass counterpart to the Ori OB Ib subgroup. Although our results do not rule out Blaauw's sequential star formation scenario for Orion, we argue that the recently proposed blue streams scenario provides a better framework on which one can explain the Orion star formation region as a whole. We speculate that the Orion Belt population could represent the evolved counterpart of an Orion nebula-like cluster.

astro-ph.SR↗

arXiv2016

APOGEE strings: a fossil record of the gas kinematic structure

We compare APOGEE radial velocities (RVs) of young stars in the Orion A cloud with CO line gas emission and find a correlation between the two at large-scales, in agreement with previous studies. However, at smaller scales we find evidence for the presence of substructure in the stellar velocity field. Using a Friends-of-Friends approach we identify 37 stellar groups with almost identical RVs. These groups are not randomly distributed but form elongated chains or strings of stars with five or more members with low velocity dispersion, across lengths of 1-1.5~pc. The similarity between the kinematic properties of the APOGEE strings and the internal velocity field of the chains of dense cores and fibers recently identified in the dense ISM is striking and suggests that for most of the Orion A cloud, young stars keep memory of the parental gas substructure where they originated.

astro-ph.GA↗