High-speed Stars Research Articles

High speed stars

Context. The Gaia satellite has provided the community with three releases containing astrometrical and photometric data as well as by products, such as stellar parameters and variability indicators. Aims. By selecting in the Gaia database, one can select stars with the requested characteristics, such as high speed. At present any selection is based on available Gaia releases including a subset of the observations. This, for some stars, can show some limitations, for example there is still not a sufficient number of observations to detect binarity. Methods. We investigated a star selected in Gaia EDR3 for its high speed that appears unbound to the Galaxy. We requested high-quality spectra to derive more information on the star. Results. From the spectroscopic investigation we confirm the low metallicity content of the star, and we derive a detailed chemical composition. The star is poor in carbon and very rich in oxygen: [(C+N+O)/Fe] = +0.65. From the two spectra observed we conclude that the star is in a binary system and from the investigation of the ionisation balance we derive that the star is closer than implied by the Gaia DR3 parallax, and thus has a a lower intrinsic luminosity. Conclusions. The star is probably still unbound, but there is the possibility that it is bound to the Galaxy. Its low carbon abundance suggests that the star was formed in a dwarf galaxy.

High-speed stars. II. An unbound star, young stars, bulge metal-poor stars, and Aurora candidates

The data from the Gaia satellite led us to revise our conception of the Galaxy structure and history. Hitherto unknown components have been discovered and a deep re-thinking of what the Galactic halo is is in progress. We selected from the Gaia catalogue stars with extreme transverse velocities with respect to the Sun ($|V_T| > 500 $ and observed them with FORS2 at the ESO VLT, to classify them using both their chemical and dynamical properties. Two apparently young stars, identified in paper\,I, were observed with UVES. We derived abundances for Na, Mg, Ca, Ti, Mn, and Fe, analysing the spectra with while for Ba we used line profile fitting. We computed actions from parallaxes and kinematical data. The stars span the metallicity range $ Fe/H -0.5$ with $ Fe/H = -1.6$. Star GHS143 has a total speed of about 1440 which is almost three times faster than the local escape velocity of 522\ strongly implying this star is unbound to the Galaxy. Remarkably, this star is not escaping from the Galaxy, but it is falling into it. Ten stars are apparently young with masses in excess of 1.3 $M Their interpretation as evolved blue stragglers is doubtful. The existence of a young metal-poor population is possible. The two stars observed with UVES show no lithium, suggesting they are blue stragglers. We detected a metal-poor population, confined to the bulge, that we call SpiteF, and argue that it is the result of a recent accretion event. We detect 102 candidates of the Aurora population that should have formed prior to the formation of the disc. Our sample is non-homogeneous and mainly retrograde. The stars are metal poor, and 23<!PCT!> have Fe/H -2.0$. Our selection is efficient at finding very metal-poor stars, but it selects peculiar populations.

High-speed stars: Galactic hitchhikers

Context.The search for stars born in the very early stages of the Milky Way star formation history is of paramount importance in the study of the early Universe since their chemistry carries irreplaceable information on the conditions in which early star formation and galaxy buildup took place. The search for these objects has generally taken the form of expensive surveys for faint extremely metal-poor stars, the most obvious but not the only candidates to a very early formation.Aims.Thanks toGaiaDR2 radial velocities and proper motions, we identified 72 bright cool stars displaying heliocentric transverse velocities in excess of 500 km s−1. These objects are most likely members of extreme outer-halo populations, either formed in the early Milky Way build-up or accreted from since-destroyed self-gravitating stellar systems.Methods.We analysed low-resolution FORS spectra of the 72 stars in the sample and derived the abundances of a few elements. Despite the large uncertainties on the radial velocity determination, we derived reliable orbital parameters for these objects.Results.The stars analysed are mainly slightly metal poor, with a few very metal-poor stars. Their chemical composition is much more homogeneous than expected. All the stars have very eccentric halo orbits, some extending well beyond the expected dimension of the Milky Way.Conclusions.These stars can be the result of a disrupted small galaxy or they could have been globular cluster members. Age estimates suggest that some of them are evolved blue stragglers, now on the subgiant or asymptotic giant branches.

Design and Simulation of a High-Speed Star Tracker for Direct Optical Feedback Control in ADCS.

Star Trackers are often the most accurate instrument in an Attitude Determination and Control Systems, but often present a slow update rate, requiring additional sensor and sensor fusion algorithms to provide a smoother and faster output. However, the available rate gyros are either noisy, or expensive and heavy. The proposed work investigates the feasibility of high-speed star trackers with modern optics, sensors, and computing systems. Firstly, we investigate the sensitivity of an optoelectrical acquisition system stimulated by dim stars, secondly, we propose and evaluate an algorithm designed to operate at high speed and to be compatible with an Field-Programmable Gate Array implementation, before evaluating the performance of the implementation on FPGA. Finally, we debate the usability of such a system, both in terms of compatibility with a mission and CubeSat ecosystems, and in terms of performance. As a result, aside from removing the need for a rate gyro, Attitude Determination and Control Systems overall pointing performances can be increased. The proposed attitude determination system achieved a 0.001° accuracy, with a 99.1% sky coverage and an ability to reject false-positive while performing a single-frame lost-in-space star identification at a 50 Hz update rate with a total delay of 19 ms, including 13 ms.

Nearby High-speed Stars in Gaia DR2

Abstract We investigate the nature of nearby (10–15 kpc) high-speed stars in the Gaia DR2 archive identified on the basis of parallax, proper motion and radial velocity. Together with a consideration of their kinematic, orbital, and photometric properties, we develop a novel strategy for evaluating whether high-speed stars are statistical outliers of the bound population or unbound stars capable of escaping the Galaxy. Out of roughly 1.5 million stars with radial velocities, proper motions, and 5σ parallaxes, we identify just over 100 high-speed stars. Of these, only two have a nearly 100% chance of being unbound, with an indication that they are not just bound outliers; both are likely hyper-runaway stars. The rest of the high-speed stars are likely statistical outliers. We use the sample of high-speed stars to demonstrate that radial velocity alone provides a poor discriminant of nearby, unbound stars. However, these stars are efficiently identified from the tangential velocity, using just parallax and proper motion. Within the full Gaia DR2 archive of stars with 5σ parallax and proper motion but no radial velocity, we identify a sample of 19 with speeds significantly larger than the local escape speed of the Milky Way based on tangential motion alone.

The Nearest Extreme-velocity Stars among Gaia DR2 High-proper-motion Stars

Among about 1.8 million Gaia DR2 high proper motion stars with $\mu>60$ mas/yr and moderately significant parallaxes ($\varpi>3\sigma_{\varpi}$), we have selected 109 high-speed star candidates with Galactic rest frame tangential velocities (corrected for solar motion) $v_{t,g}$$>$700 km/s, well above the local Galactic escape velocity. After applying various Gaia DR2 astrometric quality criteria, only 39 candidates remained, including all three hypervelocity runaway white dwarfs (HVR WDs) found by Shen et al. (2018) but no additional such candidates (falling in the same region of the colour-magnitude diagram). Considering only candidates with highly significant parallaxes ($\varpi>5\sigma_{\varpi}$) as reliable extreme tangential velocity candidates (cf. Bromley et al. 2018), our final list consists of six objects with parallaxes of 0.17 mas $\lesssim \varpi \lesssim$ 1.05 mas and tangential velocities of 730 km/s $\lesssim v_{t,g} \lesssim$ 1600 km/s, including two candidates of Bromley et al. (2018). Only one of the three HVR WDs of Shen et al. (2018), D6-2 (= NLTT 51732), falls in that list but can be considered a doubtful high-speed candidate because of its relatively large astrometric quality parameters. One of the three new extreme velocity candidates in our list, Gaia DR2 6097052289696317952, is relatively bright ($G\approx$13.5 mag) and blue $BP$$-$$RP$$\approx$0.6 mag and has the smallest parallax and highest tangential velocity. Radial velocity measurements are needed to clarify whether these high-speed candidates are unbound to the Galaxy.

SHORT-DURATION LENSING EVENTS. I. WIDE-ORBIT PLANETS? FREE-FLOATING LOW-MASS OBJECTS? OR HIGH-VELOCITY STARS?

Short duration lensing events tend to be generated by low-mass lenses or by lenses with high transverse velocities. Furthermore, for any given lens mass and speed, events of short duration are preferentially caused by nearby lenses (mesolenses) that can be studied in detail, or else by lenses so close to the source star that finite-source-size effects may be detected, yielding information about both the Einstein ring radius and the surface of the lensed star. Planets causing short-duration events may be in orbits with any orientation, and may have semimajor axes smaller than an AU, or they may reach the outer limits of their planetary systems, in the region corresponding to the Solar System's Oort Cloud. They can have masses larger than Jupiter's or smaller than Pluto's. Lensing therefore has a unique potential to expand our understanding of planetary systems. A particular advantage of lensing is that it can provide precision measurements of system parameters, including the masses of and projected separation between star and planet. We demonstrate how the parameters can be extracted and show that a great deal can be learned. For example, it is remarkable that the gravitational mass of nearby free-floating planet-mass lenses can be measured by complementing observations of a photometric event with deep images that detect the planet itself. A fraction of short events may be caused by high-velocity stars located within a kpc. Many high-velocity lenses are likely to be neutron stars that received large natal kicks. Other high-speed stars may be members of the halo population. Still others may be hypervelocity stars that have been ejected from the Galactic Center, or runaway stars escaped from close binaries, possibly including the progenitor binaries of Type Ia supernovae.

Analysis of Static Load Sharing in Star Gearing

In order to analyze the sharing mechanism of marine high-speed star gearing system, a static mechanical model is presented in the paper, in which manufacturing errors, assembly errors, bearing errors and tooth thickness error are all considered. In consideration of equivalent mesh error and static balance of floating components, the load sharing coefficient of the system in different kinds of errors was obtained. And the sensitivities of the load sharing coefficient on different types of errors are analyzed. The result shows that the effects of the eccentric error of sun and tooth thickness error of star gear upon load sharing coefficient are larger than those of the other errors. The influences on load sharing coefficient of each error have cumulative effects. The research has provided a useful reference for improving the load sharing capability of marine star gearing system.

Membership of Globular Cluster NGC 6121

We have developed a method for obtaining membership of globular cluster NGC 6121 by radius-velocity profiles of a self- gravitating system. Previous works usually identified the membership of clusters by spatial distribution (radius), velocity distribution (proper motion), and photometry (isochrone in colour-magnitude diagram). However, field star contamination might be severe by method of spatial distribution and photometric method, and many high speed stars near the central region might be missed by velocity distribution method. More complete members could be identified by this method.

The Basic Escape Rate of Stars from Clusters.

In the escape of stars from an isolated cluster the basic mechanism is the relaxation of a velocity distribution as a result of stellar encounters. All estimates of the rate of escape of stars therefore contain a multiplicative factor that expresses this relaxation rate. Short of solving the entire problem of the dynamics of an isolated cluster, the best method of estimating this basic escape rate is to solve an idealized problem that represents the relaxation process without any further dynamical complications. This idealized problem is to find a velocity distribution in which encounters maintain a steady state, subject to a cutoff at the escape velocity. As stars disappear at the cutoff, the total number of stars decreases while the shape of the velocity distribution remains unchanged. The problem has been solved previously under the assumption that the stars under consideration encounter another group of stars having a Gaussian velocity distribution. The present paper removes this restriction and treats the case of a group of stars encountering each other, so that the unknown velocity distribution in the Fokker-Planck equation appears not only in the usual derivatives but also in the encounter integrals. The usual separation-of-variables technique can be used, but a scale change in the velocities must be included, in order to conserve total energy as high-speed stars disappear across the cutoff. The resulting solution gives an escape rate about 50% higher than that recently found by Spitzer and Ha~rm. The escape rate depends somewhat on the way in which energy is fed into the velocity distribution by processes such as contraction of the cluster. % higher than that recently found by Spitzer and Ha~rm. The escape rate depends somewhat on the way in which energy is fed into the velocity distribution by processes such as contraction of the cluster. % higher than that recently found by Spitzer and Ha~rm. The escape rate depends somewhat on the way in which energy is fed into the velocity distribution by processes such as contraction of the cluster. % higher than that recently found by Spitzer and Ha~rm. The escape rate depends somewhat on the way in which energy is fed into the velocity distribution by processes such as contraction of the cluster.

The Radial Velocities of Long-Period Variable Stars. Second Paper.

view Abstract Citations (34) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Radial Velocities of Long-Period Variable Stars. Second Paper. Merrill, Paul W. Abstract Measurements of radial velocity-These were taken from 6i8 Mount Wilson spectrograms of 206 long- period variables of spectral classes Me and Se and are recorded in Table i. For 152 of these objects no previous measurements have been reported. The velocities of 305 variables in Table 3 summarize all known measurements. It is recommended that velocities derived from the absorption lines (which differ appreciably from those derived from the emission lines) be used in studies of galactic motions. For 72 stars of Table 3, direct measurements of the absorption lines are available; for the other stars the meas- ured emission-line displacements were corrected in the manner described in Mt. W. Contr., No. 6~. A brief statistical study is given of the types, periods, and galactic latitudes of the stars in Table ~. Discussion of galactic motions-The residual radial velocities computed by applying the corrections for ordinary solar motion are decidedly high, the arithmetic mean being 36 km/sec. The higher velocities are associated with the earlier spectral types and the shorter periods (Tables 6 and 7 and Figs. 3 and 4). The relationship with period is partiularly well marked. Among 152 stars with periods shorter than 300 days, i6 have residual velocities greater than 100 km/sec, io have velocities between 80 and 100 km/sec; while among 153 stars with periods of 300 days or longer not one has a velocity exceeding 100 km/sec, and oniy 2 have velocities between 8o and 100 km/sec. The average velocity decreases steadily from 8o km/sec for a group of 27 stars with periods from i~o to 199 days to 17 km/sec for 35 stars with periods greater than 399 days. The evolutionary significance of these results is important but obscure. After correction for solar motion, the 305 variables have a group motion, V0, of 31 km/sec toward the apex a 3 i6°, ~ = + 5Q0 (see Table 8). When the stars are divided according to the absolute values of the residual velocity, V0 is only 4 km/sec for the group with velocities less than 25 km/sec, while for the remaining stars Vo is 6o km/sec. A somewhat similar result is obtained when the stars are divided ac- cording to period of light-variation. On the other hand, when the stars are divided according to distance from the sun, V0 is nearly the same for both groups. The well-marked relationship of group motion to random stellar velocity (Fig. 5) is an excellent example of the asymmetry of high stellar velocities. The apex for the high-speed stars is near that point of the sky toward which stars near the sun are moving in their huge galactic orbits. This fact confirms the view that the velocity asymmetry is an effect exhibited by stars having unusually small circular velocities in their galactic orbits. The radial components may be exceptionally large Publication: The Astrophysical Journal Pub Date: September 1941 DOI: 10.1086/144325 Bibcode: 1941ApJ....94..171M full text sources ADS | Related Materials (1) Reprint: 1941CMWCI.649....1M