Context.
The detection of Earth-mass exoplanets in the habitable zone around solar-mass stars using the radial velocity technique requires extremely high precision, on the order of 10 cm s
−1
. This ...puts the required noise floor below the intrinsic variability of even relatively inactive stars, such as the Sun. One such variable is convective blueshift varying temporally, spatially, and between spectral lines.
Aims.
We develop a new approach for measuring convective blueshift and determine the strength of convective blueshift for 810 stars observed by the HARPS spectrograph, spanning spectral types late-F, G, K, and early-M. We derive a model for infering blueshift velocity for lines of any depth in later-type stars of any effective temperature.
Methods.
Using a custom list of spectral lines, covering a wide range of absorption depths, we create a model for the line-core shift as a function of line depth, commonly known as the third signature of granulation. For this we utilize an extremely-high-resolution solar spectrum (
R
~ 1 000 000) to empirically account for the nonlinear nature of the third signature. The solar third signature is then scaled to all 810 stars. Through this we obtain a measure of the convective blueshift relative to the Sun as a function of stellar effective temperature.
Results.
We confirm the general correlation of increasing convective blueshift with effective temperature and establish a tight, cubic relation between the two that strongly increases for stars above ~5800 K. For stars between ~4100 and ~4700 K we show, for the first time, a plateau in convective shift and a possible onset of a plateau for stars above 6000 K. Stars below ~4000 K show neither blueshift nor redshift. We provide a table that lists expected blueshift velocities for each spectral subtype in the data set to quickly access the intrinsic noise floor through convective blueshift for the radial velocity technique.
Flat-relative optimal extraction Zechmeister, M; Anglada-Escude, G; Reiners, A
Astronomy and astrophysics (Berlin),
1/2014, Letnik:
561
Journal Article
Recenzirano
Odprti dostop
Context. Optimal extraction is a key step in processing the raw images of spectra as registered by two-dimensional detector arrays to a one-dimensional format. Previously reported algorithms ...reconstruct models for a mean one-dimensional spatial profile to assist a properly weighted extraction. Aims. We outline a simple optimal extraction algorithm (including error propagation), which is very suitable for stabilised, fibre-fed spectrographs and does not model the spatial profile shape. Methods. A high signal-to-noise ratio, master-flat image serves as reference image and is directly used as an extraction profile mask. Each extracted spectral value is the scaling factor relative to the cross-section of the unnormalised master flat that contains all information about the spatial profile, as well as pixel-to-pixel variations, fringing, and blaze. The extracted spectrum is measured relative to the flat spectrum. Results. Using echelle spectra of the HARPS spectrograph we demonstrate a competitive extraction performance in terms of a signal-to-noise ratio and show that extracted spectra can be used for high precision radial velocity measurement. Conclusions. Pre- or post-flat-fielding of the data is not necessary, since all spectrograph inefficiencies inherent to the extraction mask are automatically accounted for. Also the reconstruction of the mean spatial profile by models is not needed, thereby reducing the number of operations to extract spectra. Flat-relative optimal extraction is a simple, efficient, and robust method that can be applied easily to stabilised, fibre-fed spectrographs.
In a volume-limited sample of 63 ultracool dwarfs of spectral type M7-M9.5, we have obtained high-resolution spectroscopy with UVES at the Very Large Telescope and HIRES at Keck Observatory. In this ...first paper we introduce our volume-complete sample from DENIS and 2MASS targets, and we derive radial velocities and space motion. Kinematics of our sample are consistent with the stars being predominantly members of the young disk. The kinematic age of the sample is 3.1 Gyr. We find that six of our targets show strong Li lines implying that they are brown dwarfs younger than several hundred million years. Five of the young brown dwarfs were unrecognized before. Comparing the fraction of Li detections to later spectral types, we see a hint of an unexpected local maximum of this fraction at spectral type M9. It is not yet clear whether this maximum is due to insufficient statistics, or to a combination of physical effects including spectral appearance of young brown dwarfs, Li line formation, and the star formation rate at low masses.
We compare the amount of magnetic flux measured in Stokes V and Stokes I in a sample of early- and mid-M stars around the boundary to full convection (~M 3.5). Early-M stars possess a radiative core, ...mid-M stars are fully convective. While Stokes V is sensitive to the net polarity of magnetic flux arising mainly from large-scale configurations, Stokes I measurements can see the total mean flux. We find that in early-M dwarfs, only ~6% of the total magnetic flux is detected in Stokes V. This ratio is more than twice as large, ~14%, in fully convective mid-M dwarfs. The bulk of the magnetic flux on M-dwarfs is not seen in Stokes V. This is presumably because magnetic flux is mainly stored in small scale components. There is also more to learn about the effect of the weak-field approximation on the accuracy of strong field detections. In our limited sample, we see evidence for a change in magnetic topology at the boundary to full convection. Fully convective stars store a 2–3 times higher fraction of their flux in fields visible to Stokes V. We estimate the total magnetic energy detected in Stokes I and compare it to results from Stokes V. We find that in early-M dwarfs only ~0.5% of the total magnetic energy is detected in Stokes V while this fraction is ~2.5% in mid-M dwarfs.
Context.The rotation-activity connection explains stellar activity in terms of rotation and convective overturn time. It is well established in stars of spectral types F–K as well as in M-type stars ...of young clusters, in which rotation is still very rapid even among M-dwarfs. The rotation-activity connection is not established in field M-dwarfs, because they rotate very slowly, and detecting rotation periods or rotational line broadening is a challenge. In field M-dwarfs, saturation sets in below vrot = 5 km s-1, hence they are expected to populate the non-saturated part of the rotation-activity connection. Aims.This work for the first time shows intrinsically resolved spectral lines of slowly rotating M-dwarfs and makes a first comparison to estimates of convective velocities. By measuring rotation velocities in a sample of mostly inactive M-dwarfs, the unsaturated part of the rotation-activity connection is followed into the regime of very low activity. Methods.Spectra of ten M-dwarfs are taken at a resolving power of $R = 200000$ at the CES in the near infrared region where molecular FeH has strong absorption bands. The intrinsically very narrow lines are compared to model calculations of convective flows, and rotational broadening is measured. Results.In one star, an upper limit of $v \sin i$ = 1 km s-1 was found, significant rotation was detected in the other nine objects. All inactive stars show rotation below or equal to 2 km s-1. In the two active stars AD Leo and YZ CMi, rotation velocities are found to be 40–50% below the results from earlier studies. Conclusions. The rotation activity connection holds in field early-M stars, too. Activity and rotation velocities of the sample stars are well in agreement with the relation found in earlier and younger stars. The intrinsic absorption profiles of molecular FeH lines are consistent with calculations from atomic Fe lines. Investigation of FeH line profiles is a very promising tool to measure convection patterns at the surfaces of M-stars.
Context. Co-rotating spots at different latitudes on the stellar surface generate periodic photometric variability and can be useful proxies for detecting differential rotation (DR). This is a major ...ingredient of the solar dynamo, but observations of stellar DR are very sparse. Because the Kepler space telescope steadily collects more data, we are interested in detecting DR using photometric information of a star. Aims. The main goal of this paper is to develop a fast method for determining stellar DR from photometric data. Methods. We ran an extensive Monte Carlo simulation of differentially rotating spotted stars with very different properties to investigate the detectability of DR. For different noise levels the resulting light curves were prewhitened using Lomb-Scargle periodograms to derive parameters for a global sine fit to detect periodicities. Results. We show under which conditions DR can successfully be detected from photometric data, and in which cases the light curve provides insufficient or even misleading information on the stellar rotation law. In our simulations, the most significant period P1out is on average 2.4% shorter than the actual spot rotation-rate. This period was detected in 96.2% of all light curves. The signature of DR is a second period close to P1out in our model. For the noise-free case, we found such a period in 64.2% of all stars. Calculating the measured latitudinal shear of two distinct spots αout, and comparing this with the known original spot rotation-rates shows that the real value is on average 3.2% lower. Comparing the total equator-to-pole shear α to αout, we find that α is underestimated by 8.8%, especially the detection of DR for stars with α < 6% is challenging. Finally, we applied our method to four differentially rotating Kepler stars and found close agreement with results from detailed modeling. Conclusions. The method we developed is capable of measuring stellar rotation periods and detecting DR with relatively high accuracy and is suitable for large data sets. We will apply our analysis to more Kepler data in a forthcoming paper.
Context.
Solar surface magneto-convection appears as granulation pattern that impacts spectral lines in terms of both shape and wavelength. Such induced effects also tend to vary over the observed ...solar disc because of the changing observation angle and, thus, the changing observation height as well. Centre-to-limb observations of the resolved Sun offer an insight into the variable spectral behaviour across different heliocentric observing positions, providing crucial information about limb darkening, convective velocities, and line profile variability relevant to radial velocity (RV) calculations. Thus, RV measurements and exoplanet transit spectroscopy depend on precise reference templates.
Aims.
We want to provide a spectroscopic centre-to-limb solar atlas at high spectral resolution and high-frequency accuracy. The atlas shall help improve the understanding of the solar atmosphere and convection processes.
Methods.
We performed high-resolution observations of the resolved quiet Sun with a Fourier transform spectrograph at the Institut für Astrophysik und Geophysik in Göttingen. Our dataset contains a wavelength range from 4200 Å to 8000 Å. We obtained 165 spectra in total, with a spectral resolution of Δ
ν
= 0.024 cm
−1
, corresponding to a resolving power
R
of 700 000 at ∼6000 Å.
Results.
We present a centre-to-limb solar atlas containing 14 heliocentric positions. To check for consistency, we investigated the Fe
I
6175 Å absorption line and compared our line profiles with previous centre-to-limb observations and also with simulations. The line profile and also the bisector profiles are generally consistent with previous observations, but we have identified differences to model line profiles, especially close to the solar limb.
We present a new radio survey of about 100 late-M and L dwarfs undertaken with the Very Large Array. The sample was chosen to explore the role of rotation in the radio activity of ultracool dwarfs. ...As part of the survey we discovered radio emission from three new objects, 2MASS J 0518113 - 310153 (M6.5), 2MASS J 0952219 - 192431 (M7), and 2MASS J 1314203 + 132001 (M7), and made an additional detection of LP 349-25 (M8). Combining the new sample with results from our previous studies and from the literature, we compile the largest sample to date of ultracool dwarfs with radio observations and measured rotation velocities (167 objects). In the spectral type range M0-M6 we find a radio activity-rotation relation, with saturation at L sub(rad)/L sub(bol) approx = 10 super(-7.5) above vsin i approx = 5 km s super(-1), similar to the relation in H alpha and X-rays. However, at spectral types > ~M7 the ratio of radio to bolometric luminosity increases significantly regardless of rotation velocity, and the scatter in radio luminosity increases. In particular, while the most rapid rotators (vsin i > ~20 km s super(-1)) exhibit "super-saturation" in X-rays and H alpha , this effect is not seen in the radio. We also find that ultracool dwarfs with vsin i > ~20 km s super(-1) have a higher radio detection fraction by about a factor of three compared to objects with vsin i < ~10 km s super(-1). When measured in terms of the Rossby number (Ro), the radio activity-rotation relation follows a single trend and with no apparent saturation from G to L dwarfs and down to Ro ~ 10 super(-3); in X-rays and H alpha there is clear saturation at Ro < ~0.1, with super-saturation beyond M7. A similar trend is observed for the radio surface flux (L sub(rad)/R super(2)low *) as a function of Ro. The continued role of rotation in the overall level of radio activity and in the fraction of active sources, and the single trend of L sub(rad)/L sub(bol) and L sub(rad)/R super(2)low * as a function of Ro from G to L dwarfs, indicates that rotation effects are important in regulating the topology or strength of magnetic fields in at least some fully convective dwarfs. The fact that not all rapid rotators are detected in the radio provides additional support to the idea of dual dynamo states proposed from spectropolarimetric observations.
We present magnetic flux measurements in seven rapidly rotating M dwarfs. Our sample stars have X-ray and H alpha emission indicative of saturated emission, i.e., emission at a high level, ...independent of rotation rate. Our measurements are made using near-infrared FeH molecular spectra observed with the High Resolution Echelle Spectrometer at Keck. Because of their large convective overturn times, the rotation velocity of M stars with small Rossby numbers is relatively slow and does not hamper the measurement of Zeeman splitting. The Rossby numbers of our sample stars are as small as 0.01. All our sample stars exhibit magnetic flux of kG strength. We find that the magnetic flux saturates in the same regime as saturation of coronal and chromospheric emission, at a critical Rossby number of around 0.1. The filling factors of both field and emission are near unity by then. We conclude that the strength of surface magnetic fields remains independent of rotation rate below that; making the Rossby number yet smaller by a factor of 10 has little effect. These saturated M-star dynamos generate an integrated magnetic flux of roughly 3 kG, with a scatter of about 1 kG. The relation between emission and flux also has substantial scatter.
M-dwarf stars are the most numerous stars in the Universe; they span a wide range in mass and are in the focus of ongoing and planned exoplanet surveys. To investigate and understand their physical ...nature, detailed spectral information and accurate stellar models are needed. We use a new synthetic atmosphere model generation and compare model spectra to observations. To test the model accuracy, we compared the models to four benchmark stars with atmospheric parameters for which independent information from interferometric radius measurements is available. We used chi super(2)-based methods to determine parameters from high-resolution spectroscopic observations. Our synthetic spectra are based on the new PHOENIX grid that uses the ACES description for the equation of state. This is a model generation expected to be especially suitable for the low-temperature atmospheres. We identified suitable spectral tracers of atmospheric parameters and determined the uncertainties in T sub(eff), logg, and Fe/H resulting from degeneracies between parameters and from shortcomings of the model atmospheres. The inherent uncertainties we find are sigmaT sub(eff)= 35K, sigma sub(log)g= 0.14, and sigma sub(Fe/H)= 0.11. The new model spectra achieve a reliable match to our observed data; our results for T sub(eff) and logg are consistent with literature values to within 1sigma. However, metallicities reported from earlier photometric and spectroscopic calibrations in some cases disagree with our results by more than 3sigma. A possible explanation are systematic errors in earlier metallicity determinations that were based on insufficient descriptions of the cool atmospheres. At this point, however, we cannot definitely identify the reason for this discrepancy, but our analysis indicates that there is a large uncertainty in the accuracy of M-dwarf parameter estimates.
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