- ID:
- ivo://CDS.VizieR/J/ApJ/807/82
- Title:
- Rotational velocities of APOKASC red giants
- Short Name:
- J/ApJ/807/82
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We investigate the occurrence rate of rapidly rotating (vsini>10km/s), low-mass giant stars in the Apache Point Observatory Galaxy Evolution Experiment-Kepler (APOKASC) fields with asteroseismic mass and surface gravity measurements. Such stars are likely merger products and their frequency places interesting constraints on stellar population models. We also identify anomalous rotators, i.e., stars with 5km/s<vsini<10km/s that are rotating significantly faster than both angular momentum evolution predictions and the measured rates of similar stars. Our data set contains fewer rapid rotators than one would expect given measurements of the Galactic field star population, which likely indicates that asteroseismic detections are less common in rapidly rotating red giants. The number of low-mass moderate (5-10km/s) rotators in our sample gives a lower limit of 7% for the rate at which low-mass stars interact on the upper red giant branch because single stars in this mass range are expected to rotate slowly. Finally, we classify the likely origin of the rapid or anomalous rotation where possible. KIC 10293335 is identified as a merger product and KIC 6501237 is a possible binary system of two oscillating red giants.
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Search Results
- ID:
- ivo://CDS.VizieR/J/ApJ/732/39
- Title:
- Rotational velocities of K giant stars
- Short Name:
- J/ApJ/732/39
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the results of a search for unusually rapidly rotating giant stars in a large sample of K giants (~1300 stars) that had been spectroscopically monitored as potential targets for the Space Interferometry Mission's Astrometric Grid. The stars in this catalog are much fainter and typically more metal-poor than those of other catalogs of red giant star rotational velocities, but the spectra generally only have signal-to-noise ratio (S/N) of ~20-60, making the measurement of the widths of individual lines difficult. To compensate for this, we have developed a cross-correlation method to derive rotational velocities in moderate S/N echelle spectra to efficiently probe this sample for rapid rotator candidates. We have discovered 28 new red giant rapid rotators as well as one extreme rapid rotator with a vsin i of 86.4km/s. Rapid rotators comprise 2.2% of our sample, which is consistent with other surveys of brighter, more metal-rich K giant stars. Although we find that the temperature distribution of rapid rotators is similar to that of the slow rotators, this may not be the case with the distributions of surface gravity and metallicity. The rapid rotators show a slight overabundance of low-gravity stars and as a group are significantly more metal-poor than the slow rotators, which may indicate that the rotators are tidally locked binaries.
- ID:
- ivo://CDS.VizieR/J/other/NatAs/4.03
- Title:
- Rotation and activity across HR diagram
- Short Name:
- J/other/NatAs/4.
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present mean chromospheric CaII H and K activity levels and rotation periods, derived from the Mount Wilson Observatory HK Project time series. These are combined with theoretically calculated convective turnover times to derive Rossby numbers and connect the rotational scaling of chromospheric activity of main sequence and post-main sequence stars with each other.
- ID:
- ivo://CDS.VizieR/J/A+AS/127/277
- Title:
- Rotation and binary rate among giant F stars
- Short Name:
- J/A+AS/127/277
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have tested the hypothesis of Berthet (1991A&A...251..171B) which foresees that Am stars become giant metallic A and F stars (defined by an enhanced value of the blanketing parameter {delta}m2 of the Geneva photometry) when they evolved. If this hypothesis is right, Am and metallic A-FIII stars need to have the same rate of binaries and a similar distribution of vsini. From our new spectroscopic data and from vsini and radial velocities in the literature, we show that it is not the case. The metallic giant stars are often fast rotators with vsini larger than 100 km/s, while the maximum rotational velocity for Am stars is about 100 km/s. The rate of tight binaries with periods less than 1000 days is less than 30 % among metallic giants, which is incompatible with the value of 75 % for Am stars (Abt & Levy, 1985ApJS...59..229A). Therefore, the simplest way to explain the existence of giant metallic F stars is to suggest that all normal A and early F stars might go through a short "metallic" phase when they are finishing their life on the main sequence. Besides, it is shown that only giant stars with spectral type comprised between F0 and F6 may have a really enhanced {delta}m2 value, while all A-type giants seem to be normal.
- ID:
- ivo://CDS.VizieR/J/A+A/616/A24
- Title:
- Rotation in RGBs from Kepler asteroseismology
- Short Name:
- J/A+A/616/A24
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Asteroseismology allows us to probe stellar interiors. In the case of red giant stars, conditions in the stellar interior are such as to allow for the existence of mixed modes, consisting in a coupling between gravity waves in the radiative interior and pressure waves in the convective envelope. Mixed modes can thus be used to probe the physical conditions in red giant cores. However, we still need to identify the physical mechanisms that transport angular momentum inside red giants, leading to the slow-down observed for red giant core rotation. Thus large-scale measurements of red giant core rotation are of prime importance to obtain tighter constraints on the efficiency of the internal angular momentum transport, and to study how this efficiency changes with stellar parameters. This work aims at identifying the components of the rotational multiplets for dipole mixed modes in a large number of red giant oscillation spectra observed by Kepler. Such identification provides us with a direct measurement of the red giant mean core rotation. We compute stretched spectra that mimic the regular pattern of pure dipole gravity modes. Mixed modes with the same azimuthal order are expected to be almost equally spaced in stretched period, with a spacing equal to the pure dipole gravity mode period spacing. The departure from this regular pattern allows us to disentangle the various rotational components and therefore to determine the mean core rotation rates of red giants. We automatically identify the rotational multiplet components of 1183 stars on the red giant branch with a success rate of 69 per cent with respect to our initial sample. As no information on the internal rotation can be deduced for stars seen pole-on, we obtain mean core rotation measurements for 875 red giant branch stars. This large sample includes stars with a mass as large as 2.5 solar masses, allowing us to test the dependence of the core slow-down rate on the stellar mass. Disentangling rotational splittings from mixed modes is now possible in an automated way for stars on the red giant branch, even for the most complicated cases, where the rotational splittings exceed half the mixed-mode spacing. This work on a large sample allows us to refine previous measurements of the evolution of the mean core rotation on the red giant branch. Rather than a slight slow-down, our results suggest rotation is constant along the red giant branch, with values independent of the mass.
- ID:
- ivo://CDS.VizieR/J/AJ/156/64
- Title:
- RV measurements for 6 K giants in the SENS program
- Short Name:
- J/AJ/156/64
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We report the discovery of long-period radial velocity (RV) variations in six intermediate-mass K-giant stars using precise RV measurements. These discoveries are part of the Search for Exoplanets around Northern Circumpolar Stars (SENS) survey being conducted at the Bohyunsan Optical Astronomy Observatory. The nature of the RV variations was investigated by looking for photometric and line shape variations. We can find no variability with the RV period in these quantities and conclude that RV variations are most likely due to unseen sub-stellar companions. Orbital solutions for the six stars yield orbital periods in the range 418-1065 days and minimum masses in the range 1.9-8.5 M_J_. These properties are typical on planets around intermediate-mass stars. Our SENS survey so far has about an 8% confirmed planet occurrence rate, and it will provide better statistics on planets around giant stars when the survey is completed.
- ID:
- ivo://CDS.VizieR/J/AJ/157/136
- Title:
- RVs of RGs with suspected massive companions
- Short Name:
- J/AJ/157/136
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Motivated by the existence of binary systems where a stellar-mass black hole is bound to a normal star, we selected four red giants with large radial velocity (RV) variation from the survey of Space Interferometry Mission (SIM) grid stars and monitored their RVs for several months. None turned out to contain a massive companion above 2.5 solar masses. The red giant TYC 9299-1080-1 with a large RV and a large proper motion is a single-lined spectroscopic binary with a period of 81 days. It is an extreme halo object moving at 350 km/s almost directly toward the Galactic center. HD 206092 is a double-lined binary with a short period of 4.37 days. It belongs to the rare class of active RS CVn-type binaries with evolved primary components, apparently undergoing mass transfer. The X-ray luminosity of HD 206092 is about twice as high as the most luminous coronal X-ray emitters observed by ROSAT, including II Peg and the prototype star RS CVn. HD 318347 has a variable double-peaked emission-line spectrum (not a giant), while HD 324668 has a constant RV. Despite the overall good quality of the SIM survey data confirmed by a comparison with Gaia Data Release 2 (DR2, Cat. I/345) mean RVs, the few large RV variations are explained, mostly, by erroneous data. We discuss the significance of the non-detection of massive companions in the SIM grid sample and the associated work.
- ID:
- ivo://CDS.VizieR/J/AJ/155/120
- Title:
- RV variability of the K-giant {gamma} Draconis
- Short Name:
- J/AJ/155/120
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present precise stellar radial velocity (RV) measurements of {gamma} Dra taken from 2003 to 2017. The data from 2003 to 2011 show coherent, long-lived variations with a period of 702 days. These variations are consistent with the presence of a planetary companion having m sin i=10.7 M_Jup_ whose orbital properties are typical for giant planets found around evolved stars. An analysis of the Hipparcos photometry, Ca II S-index measurements, and measurements of the spectral line shapes during this time show no variations with the RV of the planet, which seems to "confirm" the presence of the planet. However, RV measurements taken from 2011-2017 seem to refute this. From 2011-2013, the RV variations virtually disappear, only to return in 2014 but with a noticeable phase shift. The total RV variations are consistent either with amplitude variations on timescales of ~10.6 year, or the beating effect between two periods of 666 and 801 days. It seems unlikely that both these signals stem from a two-planet system. A simple dynamical analysis indicates that there is only a 1%-2% chance that the two-planet system is stable. Rather, we suggest that this multi-periodic behavior may represent a new form of stellar variability, possibly related to oscillatory convective modes. If such intrinsic stellar variability is common around K giant stars and is attributed to planetary companions, then the planet occurrence rate among these stars may be significantly lower than thought.
- ID:
- ivo://CDS.VizieR/J/A+A/604/A128
- Title:
- S abundances for 1301 stars from GES
- Short Name:
- J/A+A/604/A128
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Due to their volatile nature, when sulphur and zinc are observed in external galaxies, their determined abundances represent the gas-phase abundances in the interstellar medium. This implies that they can be used as tracers of the chemical enrichment of matter in the Universe at high redshift. Comparable observations in stars are more difficult and, until recently, plagued by small number statistics. We wish to exploit the Gaia ESO Survey (GES) data to study the behaviour of sulphur and zinc abundances of a large number of Galactic stars, in a homogeneous way. By using the UVES spectra of the GES sample, we are able to assemble a sample of 1301 Galactic stars, including stars in open and globular clusters in which both sulphur and zinc were measured. We confirm the results from the literature that sulphur behaves as an alpha-element. We find a large scatter in [Zn/Fe] ratios among giant stars around solar metallicity. The lower ratios are observed in giant stars at Galactocentric distances less than 7.5kpc. No such effect is observed among dwarf stars, since they do not extend to that radius. Given the sample selection, giants and dwarfs are observed at different Galactic locations, and it is plausible, and compatible with simple calculations, that Zn-poor giants trace a younger population more polluted by SN Ia yields. It is necessary to extend observations in order to observe both giants and dwarfs at the same Galactic location. Further theoretical work on the evolution of zinc is also necessary.
- ID:
- ivo://CDS.VizieR/J/ApJ/787/110
- Title:
- SAGA: Stromgren survey of seismic KIC stars
- Short Name:
- J/ApJ/787/110
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Asteroseismology has the capability of precisely determining stellar properties that would otherwise be inaccessible, such as radii, masses, and thus ages of stars. When coupling this information with classical determinations of stellar parameters, such as metallicities, effective temperatures, and angular diameters, powerful new diagnostics for Galactic studies can be obtained. The ongoing Stromgren survey for Asteroseismology and Galactic Archaeology (SAGA) has the goal of transforming the Kepler field into a new benchmark for Galactic studies, similar to the solar neighborhood. Here we present the first results from a stripe centered at a Galactic longitude of 74{deg} and covering latitude from about 8{deg} to 20{deg}, which includes almost 1000 K giants with seismic information and the benchmark open cluster NGC 6819. We describe the coupling of classical and seismic parameters, the accuracy as well as the caveats of the derived effective temperatures, metallicities, distances, surface gravities, masses, and radii. Confidence in the achieved precision is corroborated by the detection of the first and secondary clumps in a population of field stars with a ratio of 2 to 1 and by the negligible scatter in the seismic distances among NGC 6819 member stars. An assessment of the reliability of stellar parameters in the Kepler Input Catalog is also performed, and the impact of our results for population studies in the Milky Way is discussed, along with the importance of an all-sky Stromgren survey.
