arXiv subjects

Ya. V. Pavlenko

Publications and source records attributed to Ya. V. Pavlenko.

At least 19 recordsLinked to original sources

arXiv2026

Visual inspection of potential exocomet transits identified through machine learning and statistical methods

In this work, we explore several ways to detect possible exocomet transits in the TESS (The Transiting Exoplanet Survey Satellite) light curves. The first one has been presented in our previous work, a machine learning approach based on the Random Forest algorithm. It was trained on asymmetric transit profiles calculated as a result of the modelling of a comet transit, and then applied to real star light curves from Sector 1 of TESS. This allowed us to detect 32 candidates with weak and non-periodic brightness dips that may correspond to comet-like events. The aim of this work is to analyse the events identified by the visual inspection to make sure that the features detected were not caused by instrumental effects. The second approach to detect possible exocomet transits, which is proposed, is an independent statistical method to test the results of the machine learning algorithm and to look for asymmetric minima directly in the light curves. This approach was applied to \b{eta} Pictoris light curves using TESS data from Sectors 5, 6, 32, and 33. The algorithm reproduced nearly all previously known events deeper than 0.03 % of the star flux, showing that it is efficient to detect shallow and irregular flux changes in the different sectors of the TESS data and at the different levels of noise. The combination of machine learning, visual inspection, and statistical analysis facilitates the identification of faint and short-lived asymmetric transits in photometric data. Although the number of confirmed exocomet transits is still small, the growing amount of observations points to their likely presence in many young planetary systems.

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arXiv2024

Primeval very low-mass stars and brown dwarfs -- VIII. The first age benchmark L subdwarf, a wide companion to a halo white dwarf

We report the discovery of five white dwarf + ultracool dwarf systems identified as common proper motion wide binaries in the Gaia Catalogue of Nearby Stars. The discoveries include a white dwarf + L subdwarf binary, VVV 1256-62AB, a gravitationally bound system located 75.6(+1.9/-1.8) pc away with a projected separation of 1375(+35/-33) au. The primary is a cool DC white dwarf with a hydrogen dominated atmosphere, and has a total age of 10.5(+3.3/-2.1) Gyr, based on white dwarf model fitting. The secondary is an L subdwarf with a metallicity of [M/H] = -0.72(+0.08/-0.10) (i.e. [Fe/H] = -0.81+/-0.10) and Teff = 2298(+45/-43) K based on atmospheric model fitting of its optical to near infrared spectrum, and likely has a mass just above the stellar/substellar boundary. The sub-solar metallicity of the L subdwarf and the system's total space velocity of 406 km/s indicates membership in the Galactic halo, and it has a flat eccentric Galactic orbit passing within 1~kpc of the centre of the Milky Way every ~0.4Gyr and extending to 15-31 kpc at apogal. VVV 1256-62B is the first L subdwarf to have a well-constrained age, making it an ideal benchmark of metal-poor ultracool dwarf atmospheres and evolution.

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arXiv2024

Hunting for exocomet transits in the TESS database using the Random Forest method

This study introduces an approach to detecting exocomet transits in the dataset of the Transiting Exoplanet Survey Satellite (TESS), specifically within its Sector 1. Given the limited number of exocomet transits detected in the observed light curves, creating a sufficient training sample for the machine learning method was challenging. We developed a unique training sample by encapsulating simulated asymmetric transit profiles into observed light curves, thereby creating realistic data for the model training. To analyze these light curves, we employed the TSFresh software, which was a tool for extracting key features that were then used to refine our Random Forest model training. Considering that cometary transits typically exhibit a small depth, less than 1% of the star's brightness, we chose to limit our sample to the CDPP parameter. Our study focused on two target samples: light curves with a CDPP of less than 40 ppm and light curves with a CDPP of up to 150 ppm. Each sample was accompanied by a corresponding training set. This methodology achieved an accuracy of approximately 96%, with both precision and recall rates exceeding 95% and a balanced F1-score of around 96%. This level of accuracy was effective in distinguishing between 'exocomet candidate' and 'non-candidate' classifications for light curves with a CDPP of less than 40 ppm, and our model identified 12 potential exocomet candidates. However, when applying machine learning to less accurate light curves (CDPP up to 150 ppm), we noticed a significant increase in curves that could not be confidently classified, but even in this case, our model identified 20 potential exocomet candidates. These promising results within Sector 1 motivate us to extend our analysis across all TESS sectors to detect and study comet-like activity in the extrasolar planetary systems.

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arXiv2022

Rise and fall of silicate dust in RS Ophiuchi following the 2006 eruption

We present an analysis of archival Spitzer InfraRed Spectrograph (IRS) observations of the recurrent nova RS Ophiuchi obtained on several occasions, beginning about 7 months after the outburst in 2006. These data show atomic emission lines, absorption bands due to photospheric SiO, and the well known silicate dust features at $9.7\,μ$m and $18\,μ$m. The dust emission, arising in the wind of the secondary star, is fitted by Dusty models for mass-loss rates in the range $1.0-1.7\times10^{-7}$M$_{\odot}$yr$^{-1}$. The silicate features are similar in profile to those seen in circumstellar environments of isolated late-type stars and some dusty symbiotic binaries, although the longer wavelength feature peaks at $17\,μ$m,, instead of the usual $18\,μ$m, indicating peculiar grain properties. The dust features are variable, appearing stronger in 2006-2007 during outburst than in 2008-2009 when the system was in the quiescent state. This variability is attributed to changes in the ultraviolet output and the reformation of the accretion disk, although a decline in the mass-loss rate of the red giant secondary star could also play a role. Further observations, in the aftermath of the 2021 eruption, could provide a definitive conclusion.

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arXiv2022

The recurrent nova V3890~Sgr: a near-infrared and optical study of the red giant component and its environment

We present an analysis of the red giant component of the recurrent nova V3890 Sgr, using data obtained before and after its 2019 eruption. Its effective temperature is $T_{\rm eff}=3050\pm$200 K for $\log{g}=0.7$, although there are modest changes in $T_{\rm eff}$. There is an overabundance of both carbon ($0.20\pm0.05$~dex) and sodium ($1.0\pm0.3$~dex) relative to their solar values, possibly the result of ejecta from the 1990 nova eruption being entrained into the red giant photosphere. We find $^{12}$C/$^{13}$C $=25\pm2$, a value similar to that found in red giants in other recurrent novae. The interpretation of the quiescent spectrum in the 5--38$\,mu$m region requires the presence of photospheric SiO absorption and cool ($\sim400$~K) dust in the red giant environment. The spectrum in the region of the Na{\sc i} D lines is complex, and includes at least six interstellar components, together with likely evidence for interaction between ejecta from the 2019 eruption and material accumulated in the plane of the binary. Three recurrent novae with giant secondaries have been shown to have environments with different dust content, but photospheres with similar $^{12}$C/$^{13}$C ratios. The SiO fundamental bands most likely have a photospheric origin in the all three stars.

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arXiv2020

Isotopic ratios in the red giant component of the recurrent nova T Coronae Borealis

We report the determination of abundances and isotopic ratios for C, O and Si in the photosphere of the red giant component of the recurrent nova T Coronae Borealis from new 2.284--2.402 $μ$m and 3.985--4.155 $μ$m spectroscopy. Abundances and isotopic ratios in the photosphere may be affected by (i) processes in the red giant interior which are brought to the surface during dredge-up, (ii) contamination of the red giant, either during the common envelope phase of the binary evolution or by material synthesised in recurrent nova eruptions, or a combination of the two. We find that the abundances of C, O and Si are reasonably consistent with the expected composition of a red giant after first dredge-up, as is the $^{16}$O/$^{17}$O ratio. The $^{28}$Si/$^{29}$Si ratio is found to be $8.6\pm3.0$, and that for $^{28}$Si/$^{30}$Si is $21.5\pm3.0$. The $^{12}$C/$^{13}$C ratio ($10\pm2$) is somewhat lower than expected for first dredge-up. The $^{16}$O/$^{18}$O ratio ($41\pm3$) is highly inconsistent with that expected either from red giant evolution ($\sim550$) or from contamination of the red giant by the products of a nova thermonuclear runaway. In particular the C and O isotopic ratios taken in combination are a puzzle. We urge confirmation of our results using spectroscopy at high resolution. We also encourage a thorough theoretical study of the effects on the secondary star in a recurrent nova system of contamination by ejecta having anomalous abundances and isotopic ratios.

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arXiv2020

Analysis of the TiO isotopologues in stellar optical spectra

This study is based on the use of the new ExoMol TiO rovibronic line lists to identify and model TiO isotopologue features in spectra of M-dwarfs. We aim to investigate problems involving the computation of electronic bands for different isotopologues of TiO by modelling optical spectra of late-type stars and to determine their Ti isotopic abundances, to compare the TiO isotopologues spectra computed using line lists by different authors. We fit theoretical synthetic spectra to the observed stellar molecular bands of TiO. We model spectra of two M-dwarfs: GJ 15A (M1V) and GJ 15B (M3 V) to determine Ti isotopic ratios in their atmospheres. We demonstrate the accuracy of the ExoMol {\sc Toto} line list for differentisotopologues of TiO and the possibility of determining accurate Ti isotope abundances in a numberof spectral ranges. The 7580--7594 Å spectral range seems particularly useful, with two atomic linesof Fe I and molecular band heads of $^{50}Ti$O, $^{49}Ti$O, $^{48}Ti$O, $^{47}Ti$O clearly observable in our two M-dwarf spectra. We determine non-solar Ti isotopic ratios of $^{46}Ti$ : $^{47}Ti$ : $^{48}Ti$ : $^{49}Ti$ : $^{50}Ti$ = 7.9 : 5.2 : 72.8 : 7.9 :6.2 for GJ 15A and 7.4 : 4.2 : 76.6 : 5.8 : 6.0 for GJ 15B with accuracy of order $\pm$ 0.2. [Ti] = 0.040 and 0.199 within accuracy $\pm$ 0.10 were also determined for GJ 15A and GJ 15B, respectively. We find that the ExoMol {\sc Toto} TiO line list: a) describes the fine details in line position and intensities of the M-dwarf spectra better than other available TiO line lists; b) correctly reproduces the positions and intensities of the TiO isotopologue band heads observed in M-dwarf spectra; c) can be used to determine Ti isotope abundances in atmospheres of M-stars.

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arXiv2019

Analysis of first overtone bands of isotopologues of CO and SiO in stellar spectra

The first overtone ($Δv$ = 2) bands of the monosubstituted isotopologues of CO at 2.3 $μ$m in the spectrum of Arcturus (K2 III), and of the monosubstituted isotopologues of SiO at 4 $μ$m in the spectrum of the red giant HD196610 (M6 III) are modelled. To investigate problems involving the computation of the first overtone bands of isotopologues of CO and SiO in spectra of late-type stars and to determine isotopic abundances. We use fits of theoretical synthetic spectra to the observed stellar molecular bands of CO and SiO to determine abundances of isotopes of C, O and Si. Fits of synthetic spectra of the \CO first overtone bands at 2.3 $μ$m computed with three available line lists (Goorvitch, HITEMP2010 and Li et al.) to the observed spectrum of Arcturus provide the same carbon abundance [C]=$-0.6$ and isotopic ratio of carbon $^{12}$C/$^{13}$C=10 $\pm$ 2. However, the quality of fits to the observed spectrum differ for three line lists used. Furthermore, the derived oxygen isotopic ratio $^{16}$O/$^{18}$O = 2000 $\pm$ 500 is larger than that known in the solar system where $^{16}$O/$^{18}$O = 500. The silicon isotopic ratio in the atmosphere of the red giant HD196610 is revised. Using the ExoMol SiO line list with appropriate statistical weights for the SiO isotopologues the `non-solar' ratio $^{28}$Si:$^{29}$Si:$^{30}$Si = 0.86$\pm$0.03:0.12$\pm$0.02:0.02$\pm$0.01 is obtained. We found that a) the computed isotopic carbon and silicon ratios determined by the fits to the observed spectrum depend on the adopted abundance of C and Si, respectively; b) Correct treatment of the nuclear spin degeneracies parameter is of crucial importance for the use of nowadays HITRAN/ExoMol line lists in the astrophysical computations.

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arXiv2019

Temporal changes of the flare activity of Proxima Cen

We study temporal variations of the emission lines of Halpha, Hepsilon, H and K Ca II, D1 and D2 Na I, 4026 and 5876 A He I in the HARPS spectra of Proxima Centauri across an extended time of 13.2 years, from May 27, 2004, to September 30, 2017. Aims. We analyse the common behaviour and differences in the intensities and profiles of different emission lines in flare and quiet modes of Proxima activity. Methods. We compare the pseudo-equivalent widths (pEW) and profiles of the emission lines in the HARPS high-resolution (R ~ 115,000) spectra observed at the same epochs. Results. All emission lines show variability with a timescale of at least 10 min. The strength of all lines except He I 4026 A correlate with \Halpha. During strong flares the `red asymmetry' appears in the Halpha emission line indicating the infall of hot condensed matter into the chromosphere with velocities greater than 100 km/s disturbing chromospheric layers. As a result, the strength of the Ca II lines anti-correlates with Halpha during strong flares. The He I lines at 4026 and 5876 A appear in the strong flares. The cores of D1 and D2 Na I lines are also seen in emission. During the minimum activity of Proxima Centauri, Ca II lines and Hepsilon almost disappear while the blue part of the Na I emission lines is affected by the absorption in the extending and condensing flows. Conclusions. We see different behaviour of emission lines formed in the flare regions and chromosphere. Chromosphere layers of Proxima Cen are likely heated by the flare events; these layers are cooled in the `non-flare' mode. The self-absorption structures in cores of our emission lines vary with time due to the presence of a complicated system of inward and outward matter flows in the absorbing layers.

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arXiv2019

Gas phase SiO in the circumstellar environment of the recurrent nova T Coronae Borealis

We report the discovery of the diatomic molecule SiO in the gas phase in the environment of the recurrent nova T Coronae Borealis. While some of the SiO is photospheric, a substantial portion must arise in the wind from the red giant component of T CrB. A simple fit to the SiO feature, assuming local thermodynamic equilibrium, suggests a SiO column density of 2.8x10^17 /cm2 and temperature ~1000K; the SiO column density is similar to that present in the winds of field red giants. A search for SiO maser emission is encouraged both before and after the next anticipated eruption. We find that the 12C/13C ratio in the red giant is <9, with a best fit value of ~5, a factor ~18 times lower than the solar value of 89. We find no convincing evidence for the presence of dust in the environment of T CrB, which we attribute to the destructive effects on nucleation sites of hard X-ray emission. When the next eruption of T CrB occurs, the ejected material will shock the wind, producing X-ray and coronal line emission, as is the case for the recurrent nova RS Oph. T CrB is also a good candidate for very high energy gamma-ray emission, as first observed during the 2010 outburst of V407 Cyg. We include in the paper a wide variety of infrared spectroscopic and photometric data.

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arXiv2018

Infrared spectroscopy of the merger candidate KIC 9832227

Context: It has been predicted that the object KIC 9832227 - a contact binary star - will undergo a merger in 2022.2+/-0.7. We describe the near infrared spectrum of this object as an impetus to obtain pre-merger data. Aims: We aim to characterise (i)~the nature of the individual components of the binary and (ii)~the likely circumbinary environment, so that the merger - if and when it occurs - can be interpreted in an informed manner. Methods: We use infrared spectroscopy in the wavelength range 0.7micron - 2.5microns, to which we fit model atmospheres to represent the individual stars. We use the binary ephemeris to determine the orbital phase at the time of observation. Results: We find that the infrared spectrum is best fitted by a single component having effective temperature 5920K, log[g]=4.1 and solar metallicity, consistent with the fact that the system was observed at conjunction. Conclusions: The strength of the infrared H lines is consistent with a high value of log[g], and the strength of the CaII triplet indicates the presence of a chromosphere, as might be expected from rapid stellar rotation. The HeI absorption we observe likely arises in He excited by coronal activity in a circumstellar envelope, suggesting that the weakness of the CaII triplet is also likely chromospheric in origin.

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arXiv2017

A detailed study of Lithium in 107 CHEPS dwarf stars

We report results from lithium abundance determinations using high resolution spectral analysis of the 107 metal-rich stars from the Calan-Hertfordshire Extrasolar Planet Search programme. We set out to understand the lithium distribution of the population of stars taken from this survey. The lithium abundance with account NLTE effects was determined from the fits to the Li I 6708 Å~resonance doublet profiles in the observed spectra. We find that a) fast rotators tend to have higher lithium abundances, b) $\log$ N(Li) is higher in more massive/hot stars, c) $\log$ N(Li) is higher in less evolved stars, i.e. stars of lower \logg, d) stars with the metallicities $>$0.25~dex do not show the presence of lithium lines in their spectra, e) most of our planet hosts rotate slower, f) our estimate of a lower limit of lithium isotopic ratio is \Li $>$10 in the atmospheres of two SWP and two non-SWP stars. Measurable lithium abundances were found in the atmospheres of 45 stars located at distances of 20-170 pc from the Sun, for the other 62 stars the upper limits of log N(Li) were computed. We found well defined dependences of lithium abundances on \Teff, \vsini, and less pronounced for the \logg. In case of \vsini we see two sequences of stars: with measurable lithium and with the upper limit of log N(Li). About 10\% of our targets are known to host planets. Only two SWP have notable lithium abundances, so we found a lower proportion of stars with detectable Li among known planet hosts than among stars without planets. However, given the small sample size of our planet-host sample, our analysis does not show any statistically significant differences in the lithium abundance between SWP and stars without known planets.

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arXiv2017

Flare activity and photospheric analysis of Proxima Centauri

We present the analysis of emission lines in high-resolution optical spectra of the planet-host star Proxima Centauri (Proxima) classified as a M5.5V\@. We carry out the detailed analysis of observed spectra to get a better understanding of the physical conditions of the atmosphere of this star. We identify the emission lines in a serie series of 147 high-resolution optical spectra of the star at different levels of activity and compare them with the synthetic spectra computed over a wide spectral range. Our synthetic spectra computed with the PHOENIX 2900/5.0/0.0 model atmosphere fits pretty well the observed optical-to-near-infrared spectral energy distribution. However, modelling strong atomic lines in the blue spectrum (3900--4200Å) requires implementing additional opacity. We show that high temperature layers in Proxima Centauri consist in at least three emitting parts: a) a stellar chromosphere where numerous emission lines form. We suggest that some emission cores of strong absorption lines of metals form there; b) flare regions above the chromosphere, where hydrogen Balmer lines up to high transition levels (10--2) form; c) a stellar wind component with V${r}$\,=\,$-$30 \kmps{} seen in some Balmer lines as blue shifted emission lines. We believe that the observed He line at 4026Å in emission can be formed in that very hot region. We show, that real structure of the atmosphere of Proxima is rather complicated. The photosphere of the star is best fit by a normal M5 dwarf spectrum. On the other hand emission lines form in the chromosphere, flare regions and extended hot envelope.

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arXiv2017

Surprisingly different starspot distributions on the near equal-mass equal-rotation-rate stars in the M dwarf binary GJ 65 AB

We aim to understand how stellar parameters such as mass and rotation impact the distribution of starspots on the stellar surface. To this purpose, we have used Doppler imaging to reconstruct the surface brightness distributions of three fully convective M dwarfs with similar rotation rates. We secured high cadence spectral time series observations of the 5.5 AU separation binary GJ 65, comprising GJ 65A (M5.5V, Prot = 0.24 d) and GJ 65B (M6V, Prot = 0.23 d). We also present new observations of GJ 791.2A (M4.5V, Prot = 0.31 d). Observations of each star were made on two nights with UVES, covering a wavelength range from 0.64 - 1.03 microns. The time series spectra reveal multiple line distortions, which we interpret as cool starspots and which are persistent on both nights suggesting stability on the timescale of 3 days. Spots are recovered with resolutions down to 8.3 degs at the equator. The global spot distributions for GJ 791.2A are similar to observations made a year earlier. Similar high latitude and circumpolar spot structure is seen on GJ791.2A and GJ 65A. However, they are surprisingly absent on GJ 65B, which instead reveals more extensive, larger, spots concentrated at intermediate latitudes. All three stars show small amplitude latitude-dependent rotation that is consistent with solid body rotation. We compare our measurements of differential rotation with previous Doppler imaging studies and discuss the results in the wider context of other observational estimates and recent theoretical predictions.

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arXiv2017

Primeval very low-mass stars and brown dwarfs - II. The most metal-poor substellar object

SDSS J010448.46+153501.8 has previously been classified as an sdM9.5 subdwarf. However, its very blue $J-K$ colour ($-0.15 \pm 0.17$) suggests a much lower metallicity compared to normal sdM9.5 subdwarfs. Here, we re-classify this object as a usdL1.5 subdwarf based on a new optical and near-infrared spectrum obtained with X-shooter on the Very Large Telescope. Spectral fitting with BT-Settl models leads to $T_{\rm eff}$ = 2450 $\pm$ 150 K, [Fe/H] = $-$2.4 $\pm$ 0.2 and log $g$ = 5.5 $\pm$ 0.25. We estimate a mass for SDSS J010448.46+153501.8 of 0.086 $\pm$ 0.0015 M$_{\odot}$ which is just below the hydrogen-burning minimum mass at [Fe/H] = $-$2.4 ($\sim$0.088 M$_{\odot}$) according to evolutionary models. Our analysis thus shows SDSS J0104+15 to be the most metal-poor and highest mass substellar object known to-date. We found that SDSS J010448.46+153501.8 is joined by another five known L subdwarfs (2MASS J05325346+8246465, 2MASS J06164006$-$6407194, SDSS J125637.16$-$022452.2, ULAS J151913.03$-$000030.0 and 2MASS J16262034+3925190) in a 'halo brown dwarf transition zone' in the $T_{\rm eff}-$[Fe/H] plane, which represents a narrow mass range in which unsteady nuclear fusion occurs. This halo brown dwarf transition zone forms a 'substellar subdwarf gap' for mid L to early T types.

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arXiv2016

The Cool Giant HD 77361 - A Super Li-Rich Star

Super Li-rich stars form a very small and enigmatic group whose existence cannot be explained in terms of the standard stellar evolution theory. The goal of our study is to check the reality of this group of cool giants based on an independent technique. We have carried out such a check using the K giant HD 77361 (HR 3597), which has previously been assigned to this rare type, as an example. We have redetermined the effective temperature Teff and surface gravity log g for this star. We have applied two different methods, photometric and spectroscopic, to estimate Teff (the accuracy of the Li-abundance determination depends significantly on this parameter). The value of log g has been found from the highly accurate parallax of this nearby star. To apply the photometric method of determining Teff, we have performed UBV observations of the star, which yielded V = 6.18 +/- 0.03, B - V = 1.13 +/- 0.01, and U - B = 1.18+/-0.05. The following parameters of the star have been found: effective temperature Teff = 4370+/- 100 K, surface gravity log g = 2.30 +/- 0.10, iron abundance log e(Fe) = 7.49 +/- 0.14, microturbulence Vt = 1.1 +/- 0.2 km/s, rotational velocity V sin i = 4.5 km/s, and mass M = 1.3 +/- 0.2 Msun. The lithium abundance has been determined from a non-LTE analysis of three Li I lines: the resonance line at 6707.8 ?A and the subordinate lines at 6103.6 and 8126.4 ?A (the latter in a blend with a CN molecular line). We have found a high lithium abundance, log e(Li) = 3.75 +/- 0.11, which exceeds considerably the initial abundance log e(Li) = 3.2 +/- 0.1 for young stars in the solar neighborhood. Thus, we have confirmed that the K giant HD 77361 actually belongs to the type of super Li-rich stars.

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arXiv2015

Modelling the spectral energy distribution of the red giant in RS Ophiuchi: evidence for irradiation

We present an analysis of optical and infrared spectra of the recurrent nova RS Oph obtained during between 2006 and 2009. The best fit to the optical spectrum for 2006 September 28 gives effective temperature Tef = 3900~K for log g = 2.0, while for log g = 0.0 we find Tef = 4700~K, and a comparison with template stellar spectra provides Tef $\sim$ 4500 K. The observed spectral energy distribution (SED), and the intensities of the emission lines, vary on short ($\sim 1$~day) time-scales, due to disc variability. We invoke a simple one-component model for the accretion disc, and a model with a hot boundary layer, with high ($\sim 3.9 \times 10^{-6}$ \Mdot) and low ($\sim 2 \times 10^{-8}$ \Mdot) accretion rates, respectively. Fits to the accretion disc-extracted infrared spectrum (2008 July 15) yield effective temperatures for the red giant of Tef = 3800 +/- 100~K (log g = 2.0) and Tef = 3700 +/- 100~K (log g = 0.0). Furthermore, using a more sophisticated approach, we reproduced the optical and infrared SEDs of the red giant in the RS Oph system with a two-component model atmosphere, in which 90\% of the surface has Tef = 3600 K and 10\% has Tef = 5000~K. Such structure could be due to irradiation of the red giant by the white dwarf.

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arXiv2015

Starspot distributions on fully convective M dwarfs: implications for radial velocity planet searches

Since M4.5 - M9 dwarfs exhibit equatorial rotation velocities of order 10 km/s on average, radial velocity surveys targeting this stellar population will likely need to find methods to effectively remove starspot jitter. We present the first high resolution Doppler images of the M4.5 dwarf, GJ 791.2A, and the M9 dwarf, LP 944-20. The time series spectra of both objects reveal numerous line profile distortions over the rotation period of each star which we interpret as starspots. The transient distortions are modelled with spot/photosphere contrast ratios that correspond to model atmosphere temperature differences of Tphot-Tspot = 200 and 300 K. GJ 791.2A is a fully convective star with vsini = 35.1 km/s. Although we find more starspot structure at high latitudes, we reconstruct spots at a range of phases and latitudes with a mean spot filling of ~3%. LP 944-20 is one of the brightest known late-M dwarfs, with spectral type M9V and vsini = 30.8 km/s. Its spectral time series exhibits two dominant transient line distortions that are reconstructed as high latitude spots, while a mean spot filling factor of only 1.5% is found. The occurrence of low-contrast spots at predominantly high latitudes, which we see in both targets here, is in general likely to be responsible for the low amplitude photometric variability seen in late-M dwarfs. For GJ 791.2A, the radial velocities induced by the starspot features yield an rms velocity variability of 138 m/s, which can be reduced by a factor of 1.9 using our reconstructed surface brightness distributions.

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