Orbital capability is a decisive step forward for nanosatellites in general and CubeSats in particular. Although trajectory maneuvers and their implementation have been thoroughly studied for ...classical satellites, the high level of constraints on CubeSats in terms of mass, volume and power, makes the transition delicate. Orbit, attitude and power control subsystems available for this format limit too optimistic performance available in literature. To verify this hypothesis, we simulate trajectory maneuvers in Earth orbit with representative CubeSat hardware and software. A low-thrust trajectory solver based on classical orbital elements from the literature is adapted to our context. A home-made attitude control simulation tool is coupled to include both control and perturbative dynamics. Increases in time and propellant consumption of more than 100% are caused by thrust direction errors such as misalignments and attitude control limitations, sometimes leading to mission loss. These results highlight an important increase in complexity for the CubeSat format that is not covered by the usual approach. Such limitations should be considered from the very start of the design of a nanosatellite mission with trajectory modification requirements.
•Usual approach considers GNC and ADCS independently on CubeSats.•Thrust disturbance torques are unavoidable and saturate reaction wheels (RW).•RW desaturation either extend maneuver duration or increase propellant consumption.•Mutual impacts between GNC and ADCS can lead to CubeSat loss.•Design of CubeSat with propulsion must focus on ADCS before trajectory optimization.
Oscillating red-giant stars have provided a wealth of asteroseismic information regarding their interiors and evolutionary states, which enables detailed studies of the Milky Way. The objective of ...this work is to determine what fraction of red-giant stars shows photometric rotational modulation, and understand its origin. One of the underlying questions is the role of close binarity in this population, which relies on the fact that red giants in short-period binary systems (less than 150 days or so) have been observed to display strong rotational modulation. We selected a sample of about 4500 relatively bright red giants observed by
Kepler
, and show that about 370 of them (∼8%) display rotational modulation. Almost all have oscillation amplitudes below the median of the sample, while 30 of them are not oscillating at all. Of the 85 of these red giants with rotational modulation chosen for follow-up radial-velocity observation and analysis, 34 show clear evidence of spectroscopic binarity. Surprisingly, 26 of the 30 nonoscillators are in this group of binaries. On the contrary, about 85% of the active red giants with detectable oscillations are not part of close binaries. With the help of the stellar masses and evolutionary states computed from the oscillation properties, we shed light on the origin of their activity. It appears that low-mass red-giant branch stars tend to be magnetically inactive, while intermediate-mass ones tend to be highly active. The opposite trends are true for helium-core burning (red clump) stars, whereby the lower-mass clump stars are comparatively more active and the higher-mass ones are less active. In other words, we find that low-mass red-giant branch stars gain angular momentum as they evolve to clump stars, while higher-mass ones lose angular momentum. The trend observed with low-mass stars leads to possible scenarios of planet engulfment or other merging events during the shell-burning phase. Regarding intermediate-mass stars, the rotation periods that we measured are long with respect to theoretical expectations reported in the literature, which reinforces the existence of an unidentified sink of angular momentum after the main sequence. This article establishes strong links between rotational modulation, tidal interactions, (surface) magnetic fields, and oscillation suppression. There is a wealth of physics to be studied in these targets that is not available in the Sun.
We present the first APOKASC catalog of spectroscopic and asteroseismic data for dwarfs and subgiants. Asteroseismic data for our sample of 415 objects have been obtained by the Kepler mission in ...short (58.5 s) cadence, and light curves span from 30 up to more than 1000 days. The spectroscopic parameters are based on spectra taken as part of the Apache Point Observatory Galactic Evolution Experiment and correspond to Data Release 13 of the Sloan Digital Sky Survey. We analyze our data using two independent scales, the spectroscopic values from DR13 and those derived from SDSS griz photometry. We use the differences in our results arising from these choices as a test of systematic temperature uncertainties and find that they can lead to significant differences in the derived stellar properties. Determinations of surface gravity ( ), mean density ( ), radius (R), mass (M), and age (τ) for the whole sample have been carried out by means of (stellar) grid-based modeling. We have thoroughly assessed random and systematic error sources in the spectroscopic and asteroseismic data, as well as in the grid-based modeling determination of the stellar quantities provided in the catalog. We provide stellar properties determined for each of the two scales. The median combined (random and systematic) uncertainties are 2% (0.01 dex; ), 3.4% ( ), 2.6% (R), 5.1% (M), and 19% (τ) for the photometric scale and 2% ( ), 3.5% ( ), 2.7% (R), 6.3% (M), and 23% (τ) for the spectroscopic scale. We present comparisons with stellar quantities in the asteroseismic catalog by Chaplin et al. that highlight the importance of having metallicity measurements for determining stellar parameters accurately. Finally, we compare our results with those coming from a variety of sources, including stellar radii determined from TGAS parallaxes and asteroseismic analyses based on individual frequencies. We find a very good agreement for all inferred quantities. The latter comparison, in particular, gives strong support to the determination of stellar quantities based on global seismology, a relevant result for future missions such as TESS and PLATO.
When the core hydrogen is exhausted during stellar evolution, the central region of a star contracts and the outer envelope expands and cools, giving rise to a red giant. Convection takes place over ...much of the star's radius. Conservation of angular momentum requires that the cores of these stars rotate faster than their envelopes; indirect evidence supports this. Information about the angular-momentum distribution is inaccessible to direct observations, but it can be extracted from the effect of rotation on oscillation modes that probe the stellar interior. Here we report an increasing rotation rate from the surface of the star to the stellar core in the interiors of red giants, obtained using the rotational frequency splitting of recently detected 'mixed modes'. By comparison with theoretical stellar models, we conclude that the core must rotate at least ten times faster than the surface. This observational result confirms the theoretical prediction of a steep gradient in the rotation profile towards the deep stellar interior.
Abstract
We present a catalog of fundamental stellar properties for ∼7500 evolved stars, including stellar radii and masses, determined from the combination of spectroscopic observations from the ...Apache Point Observatory Galactic Evolution Experiment, part of the Sloan Digital Sky Survey IV, and asteroseismology from K2. The resulting APO-K2 catalog provides spectroscopically derived temperatures and metallicities, asteroseismic global parameters, evolutionary states, and asteroseismically derived masses and radii. Additionally, we include kinematic information from Gaia. We investigate the multidimensional space of abundance, stellar mass, and velocity with an eye toward applications in Galactic archaeology. The APO-K2 sample has a large population of low-metallicity stars (∼288 with M/H ≤ −1), and their asteroseismic masses are larger than astrophysical estimates. We argue that this may reflect offsets in the adopted fundamental temperature scale for metal-poor stars rather than metallicity-dependent issues with interpreting asteroseismic data. We characterize the kinematic properties of the population as a function of
α
enhancement and position in the disk and identify those stars in the sample that are candidate components of the Gaia-Enceladus merger. Importantly, we characterize the selection function for the APO-K2 sample as a function of metallicity, radius, mass,
ν
max
, color, and magnitude referencing Galactic simulations and target selection criteria to enable robust statistical inferences with the catalog.
The detection of mixed modes in subgiants and red giants allows us to monitor stellar evolution from the main sequence to the asymptotic giant branch and draw seismic evolutionary tracks. Quantified ...asteroseismic definitions that characterize the change in the evolutionary stages have been defined. This seismic information can now be used for stellar modelling, especially for studying the energy transport in the helium burning core or for specifying the inner properties of stars all along their evolution. Modelling will also allow us to study stars identified in the helium subflash stage, high-mass stars either arriving or quitting the secondary clump, or stars that could be in the blue-loop stage.
We report the observations of the clear-sky fraction at the Concordia station during winter 2006 and derive from it the duty cycle for the astronomical observations. The duty cycle and observation ...duration at Dome C allow for efficient asteroseismic observations. This performance is analyzed and compared to network observations. For network observations, simulations were run using the helioseismic Global Oscillation Network Group as a reference. Observations with one site in Antarctica provide performance similar to or better than that with a six-site network, since the duty cycle limited by meteorology is as high as 92%. On bright targets, a 100 day long time series with a duty cycle of about 87% can be observed, which is not possible for a network observation. Based on observations made by E. Aristidi during the 2006 second overwinter at the French-Italian Concordia Station, Dome C, Antarctica.
The Kepler space mission has observed many solar-like pulsators, and helped to decipher their fundamental parameters (e.g: mass, radius, rotation). Most of the achievements recently obtained in that ...domain result from the analysis of the mode frequencies. However, unique information on non-adiabatic physics derives from the height and width of the modes. In this study, we aim at measuring the mode widths of the pressure modes in thousands of Kepler red giants and to analyze their variations in function of stellar parameters. To achieve that, we used a peakbagging technique on the star radial modes. The results show a relation between the radial mode linewidth and the effective temperature of the star as theoretically predicted. We also unveil a clear dependence with mass and stellar evolution for the radial mode width. This means that the mode damping depends on the evolutionary status of the stars.
Rapidly rotating red giants Gehan, Charlotte; Mosser, Benoît; Michel, Eric
EPJ Web of Conferences,
01/2017, Letnik:
160
Journal Article, Conference Proceeding
Recenzirano
Odprti dostop
Stellar oscillations give seismic information on the internal properties of stars. Red giants are targets of interest since they present mixed modes, wich behave as pressure modes in the convective ...envelope and as gravity modes in the radiative core. Mixed modes thus directly probe red giant cores, and allow in particular the study of their mean core rotation. The high–quality data obtained by CoRoT and Kepler satellites represent an unprecedented perspective to obtain thousands of measurements of red giant core rotation, in order to improve our understanding of stellar physics in deep stellar interiors. We developed an automated method to obtain such core rotation measurements and validated it for stars on the red giant branch. In this work, we particularly focus on the specific application of this method to red giants having a rapid core rotation. They show complex spectra where it is tricky to disentangle rotational splittings from mixed-mode period spacings. We demonstrate that the method based on the identification of mode crossings is precise and efficient. The determination of the mean core rotation directly derives from the precise measurement of the asymptotic period spacing ΔΠ1 and of the frequency at which the crossing of the rotational components is observed.
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