Context. The current paradigm to explain the presence of Jupiter-like planets with small orbital periods (P < 10 days; hot Jupiters), which involves their formation beyond the snow line following ...inward migration, has been challenged by recent works that explore the possibility of in situ formation. Aims. We aim to test whether stars harbouring hot Jupiters and stars with more distant gas-giant planets show any chemical peculiarity that could be related to different formation processes. Methods. Our methodology is based on the analysis of high-resolution échelle spectra. Stellar parameters and abundances of C, O, Na, Mg, Al, Si, S, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, and Zn for a sample of 88 planet hosts are derived. The sample is divided into stars hosting hot (a < 0.1 au) and cool (a > 0.1 au) Jupiter-like planets. The metallicity and abundance trends of the two sub-samples are compared and set in the context of current models of planet formation and migration. Results. Our results show that stars with hot Jupiters have higher metallicities than stars with cool distant gas-giant planets in the metallicity range +0.00/+0.20 dex. The data also shows a tendency of stars with cool Jupiters to show larger abundances of α elements. No abundance differences between stars with cool and hot Jupiters are found when considering iron peak, volatile elements or the C/O, and Mg/Si ratios. The corresponding p-values from the statistical tests comparing the cumulative distributions of cool and hot planet hosts are 0.20, <0.01, 0.81, and 0.16 for metallicity, α, iron-peak, and volatile elements, respectively. We confirm previous works suggesting that more distant planets show higher planetary masses as well as larger eccentricities. We note differences in age and spectral type between the hot and cool planet host samples that might affect the abundance comparison. Conclusions. The differences in the distribution of planetary mass, period, eccentricity, and stellar host metallicity suggest a different formation mechanism for hot and cool Jupiters. The slightly larger α abundances found in stars harbouring cool Jupiters might compensate their lower metallicities allowing the formation of gas-giant planets.
Context. Most of our current understanding of the planet formation mechanism is based on the planet metallicity correlation derived mostly from solar-type stars harbouring gas-giant planets. Aims. To ...achieve a more extensive grasp on the substellar formation process, we aim to analyse in terms of their metallicity a diverse sample of stars (in terms of mass and spectral type) covering the whole range of possible outcomes of the planet formation process (from planetesimals to brown dwarfs and low-mass binaries). Methods. Our methodology is based on the use of high-precision stellar parameters derived by our own group in previous works from high-resolution spectra by using the iron ionisation and equilibrium conditions. All values were derived in an homogeneous way, except for the M dwarfs where a methodology based on the use of pseudo equivalent widths of spectral features was used. Results. Our results show that as the mass of the substellar companion increases the metallicity of the host star tends to lower values. The same trend is maintained when analysing stars with low-mass stellar companions and a tendency towards a wide range of host star’s metallicity is found for systems with low-mass planets. We also confirm that more massive planets tend to orbit around more massive stars. Conclusions. The core-accretion formation mechanism for planet formation achieves its maximum efficiency for planets with masses in the range 0.2–2 MJup. Substellar objects with higher masses have higher probabilities of being formed as stars. Low-mass planets and planetesimals might be formed by core-accretion even around low-metallicity stars.
Context. Currently, the core accretion model has its strongest observational evidence on the chemical signature of mostly main sequence stars with planets. Aims. We aim to test whether the ...well-established correlation between the metallicity of the star and the presence of giant planets found for main sequence stars still holds for the evolved and generally more massive giant and subgiant stars. Although several attempts have been made so far, the results are not conclusive since they are based on small or inhomogeneous samples. Methods. We determine in a homogeneous way the metallicity and individual abundances of a large sample of evolved stars, with and without known planetary companions, and discuss their metallicity distribution and trends. Our methodology is based on the analysis of high-resolution échelle spectra (R ≥ 67 000) from 2−3 m class telescopes. It includes the calculation of the fundamental stellar parameters (Teff, log g, microturbulent velocity, and metallicity) by applying iron ionisation and excitation equilibrium conditions to several isolated Fe i and Fe ii lines, as well as, calculating individual abundances of different elements such as Na, Mg, Si, Ca, Ti, Cr, Co, or Ni. Results. The metallicity distributions show that giant stars hosting planets are not preferentially metal-rich because they have similar abundance patterns to giant stars without known planetary companions. We have found, however, a very strong relation between the metallicity distribution and the stellar mass within this sample. We show that while the less massive giant stars with planets (M⋆ ≤ 1.5 M⊙) are not metal rich, the metallicity of the sample of massive (M⋆ > 1.5 M⊙), young (age < 2 Gyr) giant stars with planets is higher than that of a similar sample of stars without planets. Regarding other chemical elements, giant stars with and without planets in the mass domain M⋆ ≤ 1.5 M⊙ show similar abundance patterns. However, planet and non-planet hosts with masses M⋆ > 1.5 M⊙ show differences in the abundances of some elements, specially Na, Co, and Ni. In addition, we find the sample of subgiant stars with planets to be metal rich, showing similar metallicities to main-sequence planet hosts. Conclusions. While the metallicity distribution of planet-hosting subgiant stars and giant stars with stellar masses M⋆ > 1.5 M⊙ fits well in the predictions of current core-accretion models, the fact that giant planet hosts in the mass domain M⋆ ≤ 1.5 M⊙ do not show metal enrichment is difficult to explain. Given that these stars have similar stellar parameters to subgiants and main-sequence planet hosts, the lack of the metal-rich signature in low-mass giants could be explained by a pollution scenario in the main sequence that gets erased as the star becomes fully convective. However, there is no physical reason why it should play a role for giants with masses M⋆ ≤ 1.5 M⊙ yet not be observed for giants with M⋆ > 1.5 M⊙.
Context. Tentative correlations between the presence of dusty circumstellar debris discs and low-mass planets have recently been presented. In parallel, detailed chemical abundance studies have ...reported different trends between samples of planet and non-planet hosts. Whether these chemical differences are indeed related to the presence of planets is still strongly debated. Aims. We aim to test whether solar-type stars with debris discs show any chemical peculiarity that could be related to the planet formation process. Methods. We determine in a homogeneous way the metallicity, Fe/H, and abundances of individual elements of a sample of 251 stars including stars with known debris discs, stars harbouring simultaneously debris discs and planets, stars hosting exclusively planets, and a comparison sample of stars without known discs or planets. High-resolution échelle spectra (R ~ 57 000) from 2−3 m class telescopes are used. Our methodology includes the calculation of the fundamental stellar parameters (Teff, log g, microturbulent velocity, and metallicity) by applying the iron ionisation and equilibrium conditions to several isolated Fe i and Fe ii lines, as well as individual abundances of C, O, Na, Mg, Al, Si, S, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, and Zn. Results. No significant differences have been found in metallicity, individual abundances or abundance-condensation temperature trends between stars with debris discs and stars with neither debris nor planets. Stars with debris discs and planets have the same metallicity behaviour as stars hosting planets, and they also show a similar ⟨ X/Fe ⟩ − TC trend. Different behaviour in the ⟨ X/Fe ⟩ − TC trends is found between the samples of stars without planets and the samples of planet hosts. In particular, when considering only refractory elements, negative slopes are shown in cool giant planet hosts, whilst positive ones are shown in stars hosting low-mass planets. The statistical significance of the derived slopes is low, however, probably because of the wide range of stellar parameters of our samples. Stars hosting exclusively close-in giant planets behave in a different way, showing higher metallicities and positive ⟨ X/Fe ⟩ − TC slope. A search for correlations between the ⟨ X/Fe ⟩ − TC slopes and the stellar properties reveals a moderate but significant correlation with the stellar radius and a weak correlation with the stellar age, which remain even if Galactic chemical evolution effects are considered. No correlation between the ⟨ X/Fe ⟩ − TC slopes and the disc/planet properties are found. Conclusions. The fact that stars with debris discs and stars with low-mass planets do not show either metal enhancement or a different ⟨ X/Fe ⟩ − TC trend might indicate a correlation between the presence of debris discs and the presence of low-mass planets. We extend results from previous works based mainly on solar analogues with reported differences in the ⟨ X/Fe ⟩ − TC trends between planet hosts and non-hosts to a wider range of parameters. However, these differences tend to be present only when the star hosts a cool distant planet and not in stars hosting exclusively low-mass planets. The interpretation of these differences as a signature of planetary formation should be considered with caution since moderate correlations between the TC-slopes with the stellar radius and the stellar age are found, suggesting that an evolutionary effect might be at work.
Context. Debris discs are a consequence of the planet formation process and constitute the fingerprints of planetesimal systems. Their solar system counterparts are the asteroid and Edgeworth-Kuiper ...belts. Aims. The DUNES survey aims at detecting extra-solar analogues to the Edgeworth-Kuiper belt around solar-type stars, putting in this way the solar system into context. The survey allows us to address some questions related to the prevalence and properties of planetesimal systems. Methods. We used Herschel/PACS to observe a sample of nearby FGK stars. Data at 100 and 160 μm were obtained, complemented in some cases with observations at 70 μm, and at 250, 350 and 500 μm using SPIRE. The observing strategy was to integrate as deep as possible at 100 μm to detect the stellar photosphere. Results. Debris discs have been detected at a fractional luminosity level down to several times that of the Edgeworth-Kuiper belt. The incidence rate of discs around the DUNES stars is increased from a rate of ~12.1% ± 5% before Herschel to ~20.2% ± 2%. A significant fraction (~52%) of the discs are resolved, which represents an enormous step ahead from the previously known resolved discs. Some stars are associated with faint far-IR excesses attributed to a new class of cold discs. Although it cannot be excluded that these excesses are produced by coincidental alignment of background galaxies, statistical arguments suggest that at least some of them are true debris discs. Some discs display peculiar SEDs with spectral indexes in the 70–160 μm range steeper than the Rayleigh-Jeans one. An analysis of the debris disc parameters suggests that a decrease might exist of the mean black body radius from the F-type to the K-type stars. In addition, a weak trend is suggested for a correlation of disc sizes and an anticorrelation of disc temperatures with the stellar age.
Abstract
HST UV spectra were obtained to study whether the
φ
Leo system has one or more stellar companions, as shown for the A-star dwarf, HR10, stellar pulsation, or transiting exocomets. During ...five partial HST observation sets spread over 13 months, redshifted gas with radial velocities up to +245 km s
−1
was observed on one occasion, with lower velocity infalling material seen twice. Blueshifted gas features with velocities as high as −150 km s
−1
with up to three features in two spectra were also seen. Low-mass stellar companions to late A stars are coronal sources. Assuming an age of 450 Myr, G-K companions to
φ
Leo, and multiple M-star companions would have been detected in the ROSAT All-Sky Survey data, but are not seen. Further, the star shows only 1 major pulsation component, suggesting it is single, excluding an A- or early F-star companion. Low covering factor absorption features with
∣
v
∣
≤ 200 km s
−1
have been reported in optical spectra with variation on timescales consistent with nonradial pulsation. However, the highest velocity infall feature has a broader, continuous absorption profile extending from the stellar velocity to +245 km s
−1
, with a covering factor of ∼25% which is more consistent with a transit by a body on a star-grazing orbit.
Exocomets: A spectroscopic survey Rebollido, I.; Eiroa, C.; Montesinos, B. ...
Astronomy & astrophysics,
07/2020, Volume:
639
Journal Article, Web Resource
Peer reviewed
Open access
Context.
While exoplanets are now routinely detected, the detection of small bodies in extrasolar systems remains challenging. Since the discovery of sporadic events, which are interpreted to be ...exocomets (falling evaporating bodies) around
β
Pic in the early 1980s, only ∼20 stars have been reported to host exocomet-like events.
Aims.
We aim to expand the sample of known exocomet-host stars, as well as to monitor the hot-gas environment around stars with previously known exocometary activity.
Methods.
We have obtained high-resolution optical spectra of a heterogeneous sample of 117 main-sequence stars in the spectral type range from B8 to G8. The data were collected in 14 observing campaigns over the course of two years from both hemispheres. We analysed the Ca
II
K&H and Na
I
D lines in order to search for non-photospheric absorptions that originated in the circumstellar environment and for variable events that could be caused by the outgassing of exocomet-like bodies.
Results.
We detected non-photospheric absorptions towards 50% of the sample, thus attributing a circumstellar origin to half of the detections (i.e. 26% of the sample). Hot circumstellar gas was detected in the metallic lines inspected via narrow stable absorptions and/or variable blue- and red-shifted absorption events. Such variable events were found in 18 stars in the Ca
II
and/or Na
I
lines; six of them are reported in the context of this work for the first time. In some cases, the variations we report in the Ca
II
K line are similar to those observed in
β
Pic. While we do not find a significant trend in the age or location of the stars, we do find that the probability of finding CS gas in stars with larger
v
sin
i
is higher. We also find a weak trend with the presence of near-infrared excess and with anomalous (
λ
Boo-like) abundances, but this would require confirmation by expanding the sample.
Context. Around 16% of the solar-like stars in our neighbourhood show IR-excesses due to dusty debris discs and a fraction of them are known to host planets. Determining whether these stars follow ...any special trend in their properties is important to understand debris disc and planet formation. Aims. We aim to determine in a homogeneous way the metallicity of a sample of stars with known debris discs and planets. We attempt to identify trends related to debris discs and planets around solar-type stars. Methods. Our analysis includes the calculation of the fundamental stellar parameters Teff, log g, microturbulent velocity, and metallicity by applying the iron ionisation equilibrium conditions to several isolated Fe i and Fe ii lines. High-resolution échelle spectra (R ~ 57 000) from 2, 3 m class telescopes are used. Our derived metallicities are compared with other results in the literature, which finally allows us to extend the stellar samples in a consistent way. Results. The metallicity distributions of the different stellar samples suggest that there is a transition toward higher metallicities from stars with neither debris discs nor planets to stars hosting giant planets. Stars with debris discs and stars with neither debris nor planets follow a similar metallicity distribution, although the distribution of the first ones might be shifted towards higher metallicities. Stars with debris discs and planets have the same metallicity behaviour as stars hosting planets, irrespective of whether the planets are low-mass or gas giants. In the case of debris discs and giant planets, the planets are usually cool, – semimajor axis larger than 0.1 AU (20 out of 22 planets), even ≈65% have semimajor axis larger than 0.5 AU. The data also suggest that stars with debris discs and cool giant planets tend to have a low dust luminosity, and are among the less luminous debris discs known. We also find evidence of an anticorrelation between the luminosity of the dust and the planet eccentricity. Conclusions. Our data show that the presence of planets, not the debris disc, correlates with the stellar metallicity. The results confirm that core-accretion models represent suitable scenarios for debris disc and planet formation. These conclusions are based on a number of stars with discs and planets considerably larger than in previous works, in particular stars hosting low-mass planets and debris discs. Dynamical instabilities produced by eccentric giant planets could explain the suggested dust luminosity trends observed for stars with debris discs and planets.
Context. High-resolution imaging reveals a large morphological variety of protoplanetary disks. To date, no constraints on their global evolution have been found from this census. An evolutionary ...classification of disks was proposed based on their IR spectral energy distribution, with the Group I sources showing a prominent cold component ascribed to an earlier stage of evolution than Group II. Aims. Disk evolution can be constrained from the comparison of disks with different properties. A first attempt at disk taxonomy is now possible thanks to the increasing number of high-resolution images of Herbig Ae/Be stars becoming available. Methods. Near-IR images of six Group II disks in scattered light were obtained with VLT/NACO in polarimetric differential imaging, which is the most efficient technique for imaging the light scattered by the disk material close to the stars. We compare the stellar/disk properties of this sample with those of well-studied Group I sources available from the literature. Results. Three Group II disks are detected. The brightness distribution in the disk of HD 163296 indicates the presence of a persistent ring-like structure with a possible connection with the CO snowline. A rather compact (<100 AU) disk is detected around HD 142666 and AK Sco. A taxonomic analysis of 17 Herbig Ae/Be sources reveals that the difference between Group I and Group II is due to the presence or absence of a large disk cavity (≳5 AU). There is no evidence supporting the evolution from Group I to Group II. Conclusions. Group II disks are not evolved versions of the Group I disks. Within the Group II disks, very different geometries exist (both self-shadowed and compact). HD 163296 could be the primordial version of a typical Group I disk. Other Group II disks, like AK Sco and HD 142666, could be smaller counterparts of Group I unable to open cavities as large as those of Group I.
Context. The scarcity of accretion rate estimates and accretion tracers available for Herbig Ae/Be (HAeBe) stars contrasts with the extensive studies for lower mass objects. Aims. This work aims to ...derive accretion rates from the UV Balmer excess for a sample of 38 HAeBe stars. We look for possible empirical correlations with the strength of the Hα, O i6300, and Brγ emission lines. Methods. Shock modelling within the context of magnetospheric accretion (MA) was applied to each star. We obtained the accretion rates from the excess in the Balmer discontinuity, derived from mean values of multi-epoch Johnson’s UB photometry. The accretion rates were related to both mean Hα luminosities, Hα 10% widths, and O i6300 luminosities from simultaneous spectra, and to Brγ luminosities from the literature. Results. The typical -median- mass accretion rate is 2 × 10-7 M⊙ yr-1 in our sample, 36% of the stars showing values ≤10-7 M⊙ yr-1, 35% between 10-7 and 10-6, and 29% > 10-6 M⊙ yr-1. The model fails to reproduce the large Balmer excesses shown by the four hottest stars (T∗ > 12 000 K). When accretion is related to the stellar masses and luminosities (1 ≤ M∗/M⊙ ≤ 6; 2 ≤ L∗/L⊙ ≤ 103), we derive Ṁacc ∝ M∗5 and Lacc ∝ L∗1.2, with scatter. Empirical calibrations relating the accretion and the Hα, O i6300, and Brγ luminosities are provided. The slopes in our expressions are slightly shallower than those for lower mass stars, but the difference is within the uncertainties, except for the O i6300 line. The Hα 10% width is uncorrelated with Ṁacc, unlike for the lower mass regime. The mean Hα width shows higher values as the projected rotational velocities of HAe stars increase, which agrees with MA. The accretion rate variations in the sample are typically lower than 0.5 dex on timescales of days to months. Our data suggest that the changes in the Balmer excess are uncorrelated to the simultaneous changes of the line luminosities. Conclusions. The Balmer excesses and Hα line widths of HAe stars can be interpreted within the context of MA, which is not the case for several HBes. The steep trend relating Ṁacc and M∗ can be explained from the mass-age distribution characterizing HAeBe stars. The line luminosities used for low-mass objects are also valid to estimate typical accretion rates for the intermediate-mass regime under similar empirical expressions. However, we suggest that several of these calibrations are driven by the stellar luminosity.
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