32 research outputs found
Orbital parameters estimation for compact binary stars
Most stars in the Galaxy are found in multiple systems of two or more stars orbiting together. Two stars orbiting around their centre of mass are called binary stars. In close binary stars, the evolution of one star affects its companion and evolutionary expansion of one star allows for mass exchange between the components. In most cases, the material from the less massive star forms an accretion disc around the heavier companion that has evolved into a compact stellar remnant, the final state of stellar evolution. We call these systems compact binary stars (CBs). The study of CBs is key to the development of two fundamental phenomena: accretion and evolution of binary stars.
Statistical information on CBs can be deduced by extracting common properties and characteristic system parameter distributions from observed data. But, despite being fundamental for a wide range of astronomical phenomena, our comprehension of their formation and evolution is still poor, mainly because of the limited knowledge of crucial orbital parameters. This lack of reliable orbital parameters estimation is mainly due to observational handicaps, namely, the accretion disc outshines the system components. Astronomers have developed different techniques to overcome this, but are often very dependant of the signal to noise ratio of the data or are only able to obtain via target of opportunity programs (wait until the target is brighter).
The focus of this work is to test and develop techniques, based on indirect imaging methods, that can overcome the main observational handicaps to estimate orbital parameters of CBs. We combine these techniques with the exploitation of more “exotic” emission lines that trace the irradiated face of the donor star, namely Ca II NIR triplet and the Bowen blend. We made use of empirical properties of Doppler tomography to estimate the values of the phase zero Á0 and the velocity of the irradiated face of the secondary star (Kem). We then used synthetic models accounting for an irradiated secondary to fit our measured Kem and perform a K-correction to derive the radial velocity of the secondary K2. To derive K1, we used the centre of symmetry technique, testing its validity among several emission lines and the stability of the results depending on the selected area. Having strong constraints for K1 and K2, we find estimates for the mass ratio q. Furthermore, we developed a variation from the Doppler tomography secondary emission method to constrain the value of the systemic velocity ƴ. We derive meaningful uncertainties of these parameters with the bootstrap technique.
Using these techniques, we have successfully set dynamical constraints on the radial velocities of the binary components of CBs and derived fundamental orbital parameters, including the mass ratio, using basic properties of Doppler tomography
The chemical properties of the Milky Way's on-bar and off-bar regions: evidence for inhomogeneous star formation history in the bulge
Numerous studies of integrated starlight, stellar counts, and kinematics have
confirmed that the Milky Way is a barred galaxy. However, far fewer studies
have investigated the bar's stellar population properties, which carry valuable
independent information regarding the bar's formation history. Here we conduct
a detailed analysis of chemical abundance distributions ([Fe/H] and [Mg/Fe]) in
the on-bar and off-bar regions to study the azimuthal variation of star
formation history (SFH) in the inner Galaxy. We find that the on-bar and
off-bar stars at Galactocentric radii 3 5 kpc have remarkably
consistent [Fe/H] and [Mg/Fe] distribution functions and [Mg/Fe]--[Fe/H]
relation, suggesting a common SFH shared by the long bar and the disc. In
contrast, the bar and disc at smaller radii (2 3 kpc) show
noticeable differences, with relatively more very metal-rich ([Fe/H]~0.4) stars
but fewer solar abundance stars in the bar. Given the three-phase star
formation history proposed for the inner Galaxy in Lian et al. (2020b), these
differences could be explained by the off-bar disc having experienced either a
faster early quenching process or recent metal-poor gas accretion. Vertical
variations of the abundance distributions at small suggest a wider
vertical distribution of low- stars in the bar, which may serve as
chemical evidence for vertical heating through the bar buckling process. The
lack of such vertical variations outside the bulge may then suggest a lack of
vertical heating in the long bar.Comment: 10 pages, 5 figures. MNRAS in pres
Revealing the Chemical Structure of the Magellanic Clouds with APOGEE. III. Abundance Gradients of the Small Magellanic Cloud
We determine radial- and age-abundance gradients of the Small Magellanic
Cloud (SMC) using spectra of 2,062 red giant branch (RGB) field stars observed
by SDSS-IV / APOGEE-2S. With coverage out to 9 kpc in the SMC, these data
taken with the high resolution () APOGEE -band spectrograph
afford the opportunity to measure extensive radial gradients for as many as 24
abundance ratios. The SMC is found to have an overall metallicity gradient of
0.0546 0.0043 dex/kpc. Ages are calculated for every star to explore
the evolution of the different abundance gradients. As a function of age, many
of the gradients show a feature 3.66--5.58 Gyr ago, which is especially
prominent in the [X/H] gradients. Initially many gradients flatten until about
5.58 Gyr ago, but then steepen in more recent times. We previously
detected similar evolutionary patterns in the Large Magellanic Cloud (LMC)
which are attributed to a recent interaction between the LMC and SMC. It is
inferred that the feature in the SMC gradients was caused by the same
interaction. The age-[X/Fe] trends, which track average [X/Fe] over time, are
flat, demonstrating a slow enrichment history for the SMC. When comparing the
SMC gradients to the LMC and MW, normalized to disk scale length
(), the [X/Fe] and [X/Mg] gradients are similar, but there is a
dichotomy between the dwarfs and the Milky Way (MW) for the [X/H] gradients.
The median MW [X/H] gradient around 0.125 dex/ whilst the Clouds
have gradients of about 0.075 dex/.Comment: 27 pages, 22 figures, and 11 table
Revealing the Chemical Structure of the Magellanic Clouds with APOGEE. II. Abundance Gradients of the Large Magellanic Cloud
We present the abundance gradients of the Large Magellanic Cloud (LMC) for 25
elemental abundance ratios and their respective temporal evolution as well as
age-[X/Fe] trends using 6130 LMC field red giant branch (RGB) stars observed by
SDSS-IV / APOGEE-2S. APOGEE is a high resolution ( 22,500) -band
spectroscopic survey that gathered data on the LMC with broad radial and
azimuthal coverage out to 10\degr. The calculated overall metallicity
gradient of the LMC with no age binning is 0.0380 0.0022 dex/kpc. We
also find that many of the abundance gradients show a U-shaped trend as
functions of age. This trend is marked by a flattening of the gradient but then
a general steepening at more recent times. The extreme point at which all these
gradients (with the U-shaped trend) begin to steepen is 2 Gyr ago. In
addition, some of the age-[X/Fe] trends show an increase starting a few Gyr
before the extreme point in the gradient evolutions. A subset of the age-[X/Fe]
trends also show maxima concurrent with the gradients' extreme points, further
pinpointing a major event in the history of the LMC 2 Gyr ago. This time
frame is consistent with a previously proposed interaction between the
Magellanic Clouds suggesting that this is most likely the cause of the distinct
trend in the gradients and age-[X/Fe] trends.Comment: 25 pages, 19 figures, and 10 table
Close companions around young stars
Multiplicity is a fundamental property that is set early during stellar
lifetimes, and it is a stringent probe of the physics of star formation. The
distribution of close companions around young stars is still poorly constrained
by observations. We present an analysis of stellar multiplicity derived from
APOGEE-2 spectra obtained in targeted observations of nearby star-forming
regions. This is the largest homogeneously observed sample of high-resolution
spectra of young stars. We developed an autonomous method to identify double
lined spectroscopic binaries (SB2s). Out of 5007 sources spanning the mass
range of 0.05--1.5 \msun, we find 399 binaries, including both RV
variables and SB2s. The mass ratio distribution of SB2s is consistent with a
uniform for . The period
distribution is consistent with what has been observed in close binaries (
AU) in the evolved populations. Three systems are found to have 0.1,
with a companion located within the brown dwarf desert. There are not any
strong trends in the multiplicity fraction (MF) as a function of cluster age
from 1 to 100 Myr. There is a weak dependence on stellar density, with
companions being most numerous at stars/pc, and
decreasing in more diffuse regions. Finally, disk-bearing sources are deficient
in SB2s (but not RV variables) by a factor of 2; this deficit is
recovered by the systems without disks. This may indicate a quick dispersal of
disk material in short-period equal mass systems that is less effective in
binaries with lower .Comment: 25 pages, 20 figures. Accepted to A
Optical Monitoring of the Didymos–Dimorphos Asteroid System with the Danish Telescope around the DART Mission Impact
The NASA’s Double-Asteroid Redirection Test (DART) was a unique planetary defence and technology test mission, the first of its kind. The main spacecraft of the DART mission impacted the target asteroid Dimorphos, a small moon orbiting the asteroid Didymos (65803), on 2022 September 26. The impact brought up a mass of ejecta which, together with the direct momentum transfer from the collision, caused an orbital period change of 33 ± 1 minutes, as measured by ground-based observations. We report here the outcome of the optical monitoring campaign of the Didymos system from the Danish 1.54 m telescope at La Silla around the time of impact. The observations contributed to the determination of the changes in the orbital parameters of the Didymos–Dimorphos system, as reported by Thomas et al., but in this paper we focus on the ejecta produced by the DART impact. We present photometric measurements from which we remove the contribution from the Didymos–Dimorphos system using an H–G photometric model. Using two photometric apertures we determine the fading rate of the ejecta to be 0.115 ± 0.003 mag day−1 (in a 2″ aperture) and 0.086 ± 0.003 mag day−1 (5″) over the first week postimpact. After about 8 days postimpact we note the fading slows down to 0.057 ± 0.003 mag day−1 (2″ aperture) and 0.068 ± 0.002 mag day−1 (5″). We include deep-stacked images of the system to illustrate the ejecta evolution during the first 18 days, noting the emergence of dust tails formed from ejecta pushed in the antisolar direction, and measuring the extent of the particles ejected Sunward to be at least 4000 km
The EBLM Project VI. The mass and radius of five low-mass stars in F+M binaries discovered by the WASP survey
peer reviewedSome M-dwarfs around F-/G-type stars have been measured to be hotter and larger than predicted by stellar evolution models. Inconsistencies between observations and models need addressing with more mass, radius and luminosity measurements of low-mass stars to test and refine evolutionary models. Our aim is to measure the masses, radii and ages of the stars in five low-mass eclipsing binary systems discovered by the WASP survey. We use WASP photometry to establish eclipse-time ephemerides and to obtain initial estimates for the transit depth and width. Radial velocity measurements were simultaneously fitted with follow-up photometry to find the best-fitting orbital solution. This solution was combined with measurements of atmospheric parameters to interpolate evolutionary models and estimate the mass of the primary star, and the mass and radius of the M-dwarf companion. We assess how the best fitting orbital solution changes if an alternative limb- darkening law is used and quantify the systematic effects of unresolved companions. We also gauge how the best-fitting evolutionary model changes if different values are used for the mixing length parameter and helium enhancement. We report the mass and radius of five M-dwarfs and find little evidence of inflation with respect to evolutionary models. The primary stars in two systems are near the ``blue hook'' stage of their post sequence evolution, resulting in two possible solutions for mass and age. We find that choices in helium enhancement and mixing- length parameter can introduce an additional 3-5\,\% uncertainty in measured M-dwarf mass. Unresolved companions can introduce an additional 3-8\% uncertainty in the radius of an M-dwarf, while the choice of limb- darkening law can introduce up to an additional 2\% uncertainty
OGLE-2019-BLG-0825: Constraints on the Source System and Effect on Binary-lens Parameters arising from a Five Day Xallarap Effect in a Candidate Planetary Microlensing Event
We present an analysis of microlensing event OGLE-2019-BLG-0825. This event
was identified as a planetary candidate by preliminary modeling. We find that
significant residuals from the best-fit static binary-lens model exist and a
xallarap effect can fit the residuals very well and significantly improves
values. On the other hand, by including the xallarap effect in our
models, we find that binary-lens parameters like mass-ratio, , and
separation, , cannot be constrained well. However, we also find that the
parameters for the source system like the orbital period and semi major axis
are consistent between all the models we analyzed. We therefore constrain the
properties of the source system better than the properties of the lens system.
The source system comprises a G-type main-sequence star orbited by a brown
dwarf with a period of days. This analysis is the first to demonstrate
that the xallarap effect does affect binary-lens parameters in planetary
events. It would not be common for the presence or absence of the xallarap
effect to affect lens parameters in events with long orbital periods of the
source system or events with transits to caustics, but in other cases, such as
this event, the xallarap effect can affect binary-lens parameters.Comment: 19 pages, 7 figures, 6 tables. Accepted by A