arXiv subjects

B. Montesinos

Publications and source records attributed to B. Montesinos.

At least 73 records · Page 4Linked to original sources

arXiv2012

$α$ Centauri A in the far infrared

Chromospheres and coronae are common phenomena on solar-type stars. Understanding the energy transfer to these heated atmospheric layers requires direct access to the relevant empirical data. Study of these structures has, by and large, been limited to the Sun thus far. The region of the temperature reversal can be directly observed only in the far infrared and submm. We aim at the determination of the characteristics of the atmosphere in the region of the temperature minimum of the solar sister star alpha Cen A. For the nearby binary system alpha Centauri, stellar parameters are known with high accuracy from measurements. For the basic model parameters Teff, log g and [Fe/H], we interpolate in the grid of GAIA/PHOENIX stellar model atmospheres and compute the corresponding model for the G2 V star alpha Cen A. Comparison with photometric measurements shows excellent agreement between observed photospheric data in the optical and infrared. For longer wavelengths, the modelled spectral energy distribution is compared to MIPS, PACS, SPIRE and LABOCA photometry. A specifically tailored Uppsala model based on the MARCS code and extending further in wavelength is used to gauge the emission characteristics of alpha Cen A in the FIR. Similar to the Sun, the FIR emission of alpha Cen A originates in the minimum temperature region above the stellar photosphere in the visible. However, in comparison with the solar case, the FIR photosphere of alpha Cen A appears marginally cooler, Tmin=T160mu=3920+/-375 K. Beyond the minimum near 160mu, the brightness temperatures increase and this radiation likely originates in warmer regions of the chromosphere of alpha Cen A. To the best of our knowledge this is the first time a temperature minimum has been directly measured on a main-sequence star other than the Sun.

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arXiv2012

HD 172555: detection of 63 microns [OI] emission in a debris disc

Context. HD 172555 is a young A7 star belonging to the Beta Pictoris Moving Group that harbours a debris disc. The Spitzer IRS spectrum of the source showed mid-IR features such as silicates and glassy silica species, indicating the presence of a warm dust component with small grains, which places HD 172555 among the small group of debris discs with such properties. The IRS spectrum also shows a possible emission of SiO gas. Aims. We aim to study the dust distribution in the circumstellar disc of HD 172555 and to asses the presence of gas in the debris disc. Methods. As part of the GASPS Open Time Key Programme, we obtained Herschel-PACS photometric and spectroscopic observations of the source. We analysed PACS observations of HD 172555 and modelled the Spectral Energy Distribution (SED) with a modified blackbody and the gas emission with a two-level population model with no collisional de-excitation. Results. We report for the first time the detection of OI atomic gas emission at 63.18 microns in the HD 172555 circumstellar disc. We detect excesses due to circumstellar dust toward HD 172555 in the three photometric bands of PACS (70, 100, and 160 microns). We derive a large dust particle mass of 4.8e-4 Earth masses and an atomic oxygen mass of 2.5e-2*R^2 Earth masses, where R in AU is the separation between the star and the inner disc. Thus, most of the detected mass of the disc is in the gaseous phase.

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arXiv2012

Comparative Modelling of the Spectra of Cool Giants

Our ability to extract information from the spectra of stars depends on reliable models of stellar atmospheres and appropriate techniques for spectral synthesis. Various model codes and strategies for the analysis of stellar spectra are available today. We aim to compare the results of deriving stellar parameters using different atmosphere models and different analysis strategies. The focus is set on high-resolution spectroscopy of cool giant stars. Spectra representing four cool giant stars were made available to various groups and individuals working in the area of spectral synthesis, asking them to derive stellar parameters from the data provided. The results were discussed at a workshop in Vienna in 2010. Most of the major codes currently used in the astronomical community for analyses of stellar spectra were included in this experiment. We present the results from the different groups, as well as an additional experiment comparing the synthetic spectra produced by various codes for a given set of stellar parameters. Similarities and differences of the results are discussed. Several valid approaches to analyze a given spectrum of a star result in quite a wide range of solutions. The main causes for the differences in parameters derived by different groups seem to lie in the physical input data and in the details of the analysis method. This clearly shows how far from a definitive abundance analysis we still are.

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arXiv2012

GASPS observations of Herbig Ae/Be stars with PACS/Herschel. The atomic and molecular content of their protoplanetary discs

We observed a sample of 20 representative Herbig Ae/Be stars and five A-type debris discs with PACS onboard of Herschel. The observations were done in spectroscopic mode, and cover far-IR lines of [OI], [CII], CO, CH+, H2O and OH. We have a [OI]63 micron detection rate of 100% for the Herbig Ae/Be and 0% for the debris discs. [OI]145 micron is only detected in 25%, CO J=18-17 in 45% (and less for higher J transitions) of the Herbig Ae/Be stars and for [CII] 157 micron, we often found spatially variable background contamination. We show the first detection of water in a Herbig Ae disc, HD 163296, which has a settled disc. Hydroxyl is detected as well in this disc. CH+, first seen in HD 100546, is now detected for the second time in a Herbig Ae star, HD 97048. We report fluxes for each line and use the observations as line diagnostics of the gas properties. Furthermore, we look for correlations between the strength of the emission lines and stellar or disc parameters, such as stellar luminosity, UV and X-ray flux, accretion rate, PAH band strength, and flaring. We find that the stellar UV flux is the dominant excitation mechanism of [OI]63 micron, with the highest line fluxes found in those objects with a large amount of flaring and greatest PAH strength. Neither the amount of accretion nor the X-ray luminosity has an influence on the line strength. We find correlations between the line flux of [OI]63 micron and [OI]145 micron, CO J = 18-17 and [OI]6300 Å, and between the continuum flux at 63 micron and at 1.3 mm, while we find weak correlations between the line flux of [OI]63 micron and the PAH luminosity, the line flux of CO J = 3-2, the continuum flux at 63 micron, the stellar effective temperature and the Brgamma luminosity. (Abbreviated version)

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arXiv2012

Accretion-related properties of Herbig Ae/Be stars. Comparison with T Tauris

We look for trends relating the mass accretion rate (Macc) and the stellar ages (t), spectral energy distributions (SEDs), and disk masses (Mdisk) for a sample of 38 HAeBe stars, comparing them to analogous correlations found for classical T Tauri stars. Our goal is to shed light on the timescale and physical processes that drive evolution of intermediate-mass pre-main sequence objects. Macc shows a dissipation timescale τ= 1.3^{+1.0}_{-0.5} Myr from an exponential law fit, while a power law yields Macc(t) \propto t^{-η}, with η= 1.8^{+1.4}_{-0.7}. This result is based on our whole HAeBe sample (1-6 Msun), but the accretion rate decline most probably depends on smaller stellar mass bins. The near-IR excess is higher and starts at shorter wavelengths (J and H bands) for the strongest accretors. Active and passive disks are roughly divided by 2 x 10^{-7} Msun/yr. The mid-IR excess and the SED shape from the Meeus et al. classification are not correlated with Macc. We find Macc \propto Mdisk^{1.1 +- 0.3}. Most stars in our sample with signs of inner dust dissipation typically show accretion rates ten times lower and disk masses three times smaller than the remaining objects. The trends relating Macc with the near-IR excess and Mdisk extend those for T Tauri stars, and are consistent with viscous disk models. The differences in the inner gas dissipation timescale, and the relative position of the stars with signs of inner dust clearing in the Macc-Mdisk plane, could be suggesting a slightly faster evolution, and that a different process - such as photoevaporation - plays a more relevant role in dissipating disks in the HAeBe regime compared to T Tauri stars. Our conclusions must consider the mismatch between the disk mass estimates from mm fluxes and the disk mass estimates from accretion, which we also find in HAeBe stars.

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arXiv2012

A peculiar class of debris disks from Herschel/DUNES - A steep fall off in the far infrared

Aims. We present photometric data of debris disks around HIP 103389 (HD 199260), HIP 107350 (HN Peg, HD206860), and HIP 114948 (HD 219482), obtained in the context of our Herschel Open Time Key Program DUNES (DUst around NEarby Stars). Methods. We used Herschel/PACS to detect the thermal emission of the three debris disks with a 3 sigma sensitivity of a few mJy at 100 um and 160 um. In addition, we obtained Herschel/PACS photometric data at 70 um for HIP 103389. Two different approaches are applied to reduce the Herschel data to investigate the impact of data reduction on the photometry. We fit analytical models to the available spectral energy distribution (SED) data. Results. The SEDs of the three disks potentially exhibit an unusually steep decrease at wavelengths > 70 um. We investigate the significance of the peculiar shape of these SEDs and the impact on models of the disks provided it is real. Our modeling reveals that such a steep decrease of the SEDs in the long wavelength regime is inconsistent with a power-law exponent of the grain size distribution -3.5 expected from a standard equilibrium collisional cascade. In contrast, a very distinct range of grain sizes is implied to dominate the thermal emission of such disks. However, we demonstrate that the understanding of the data of faint sources obtained with Herschel is still incomplete and that the significance of our results depends on the version of the data reduction pipeline used. Conclusions. A new mechanism to produce the dust in the presented debris disks, deviations from the conditions required for a standard equilibrium collisional cascade (grain size exponent of -3.5), and/or significantly different dust properties would be necessary to explain the potentially steep SED shape of the three debris disks presented. (abridged)

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arXiv2012

Metallicity of solar-type stars with debris discs and planets

Around 16% of the solar-like stars in our neighbourhood show IR-excesses due to debris discs and a fraction of them are known to host planets. We aim to determine in a homogeneous way the metallicity of a sample of stars with known debris discs and planets. Our analysis includes the calculation of the fundamental stellar parameters by applying the iron ionisation equilibrium conditions to several isolated Fe I and Fe II lines. 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. 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. 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. Dynamical instabilities produced by eccentric giant planets could explain the suggested dust luminosity trends observed for stars with debris discs and planets.

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arXiv2011

Detection of warm water vapour in Taurus protoplanetary discs by Herschel

Line spectra of 68 Taurus T Tauri stars were obtained with the Herschel-PACS (Photodetector Array Camera & Spectrometer) instrument as part of the GASPS (Gas Evolution in Protoplanetary Systems) survey of protoplanetary discs. A careful examination of the line scans centred on the [OI] 63.18 microns fine-structure line unveiled a line at 63.32 micron in some of these spectra. We identify this line with a transition of ortho-water. It is detected confidently (i.e., >3 sigma) in eight sources, i.e., 24% of the sub-sample with gas-rich discs. Several statistical tests were used to search for correlations with other disc and stellar parameters such as line fluxes of [OI] 6300 Armstrong and 63.18 microns; X-ray luminosity and continuum levels at 63 microns and 850 microns. Correlations are found between the water line fluxes and the [OI] 63.18 microns line luminosity, the dust continuum, and possibly with the stellar X-ray luminosity. This is the first time that this line of warm water vapour has been detected in protoplanetary discs. We discuss its origins, in particular whether it comes from the inner disc and/or disc surface or from shocks in outflows and jets. Our analysis favours a disc origin, with the observed water vapour line produced within 2-3AU from the central stars, where the gas temperature is of the order of 500-600 K.

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arXiv2011

Gas modelling in the disc of HD 163296

We present detailed model fits to observations of the disc around the Herbig Ae star HD 163296. This well-studied object has an age of ~ 4 Myr, with evidence of a circumstellar disc extending out to ~ 540AU. We use the radiation thermo-chemical disc code ProDiMo to model the gas and dust in the circumstellar disc of HD 163296, and attempt to determine the disc properties by fitting to observational line and continuum data. These include new Herschel/PACS observations obtained as part of the open-time key program GASPS (Gas in Protoplanetary Systems), consisting of a detection of the [OI]63mic line and upper limits for several other far infrared lines. We complement this with continuum data and ground-based observations of the 12CO 3-2, 2-1 and 13CO J=1-0 line transitions, as well as the H2 S(1) transition. We explore the effects of stellar ultraviolet variability and dust settling on the line emission, and on the derived disc properties. Our fitting efforts lead to derived gas/dust ratios in the range 9-100, depending on the assumptions made. We note that the line fluxes are sensitive in general to the degree of dust settling in the disc, with an increase in line flux for settled models. This is most pronounced in lines which are formed in the warm gas in the inner disc, but the low excitation molecular lines are also affected. This has serious implications for attempts to derive the disc gas mass from line observations. We derive fractional PAH abundances between 0.007 and 0.04 relative to ISM levels. Using a stellar and UV excess input spectrum based on a detailed analysis of observations, we find that the all observations are consistent with the previously assumed disc geometry.

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arXiv2011

Herschel discovery of a new class of cold, faint debris discs

We present Herschel PACS 100 and 160 micron observations of the solar-type stars alpha Men, HD 88230 and HD 210277, which form part of the FGK stars sample of the Herschel Open Time Key Programme (OTKP) DUNES (DUst around NEarby S tars). Our observations show small infrared excesses at 160 micron for all three stars. HD 210277 also shows a small excess at 100 micron, while the 100 micron fluxes of alpha Men and HD 88230 agree with the stellar photospheric predictions. We attribute these infrared excesses to a new class of cold, faint debris discs. alpha Men and HD 88230 are spatially resolved in the PACS 160 micron images, while HD 210277 is point-like at that wavelength. The projected linear sizes of the extended emission lie in the range from ~ 115 to ~ 250 AU. The estimated black body temperatures from the 100 and 160 micron fluxes are $\lesssim$ 22 K, while the fractional luminosity of the cold dust is Ldust/Lstar ~ 10E-6, close to the luminosity of the Solar-System's Kuiper belt. These debris discs are the coldest and faintest discs discovered so far around mature stars and cannot easily be explained by invoking "classical" debris disc models.

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arXiv2011

Accretion rates and accretion tracers of Herbig Ae/Be stars

This work aims to derive accretion rates for a sample of 38 HAeBe stars. We apply magnetospheric accretion (MA) shock modelling to reproduce the observed Balmer excesses. We look for possible correlations with the strength of the Halpha, [OI]6300, and Brgamma emission lines. The median mass accretion rate is 2 x 10^-7 Msun yr^-1 in our sample. The model fails to reproduce the large Balmer excesses shown by the four hottest stars (T* > 12000 K). We derive Macc propto M*^5 and Lacc propto L*^1.2 for our sample, with scatter. Empirical calibrations relating the accretion and the Halpha, [OI]6300, and Brgamma 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 [OI]6300 line. The Halpha 10% width is uncorrelated with Macc, unlike for the lower mass regime. The mean Halpha 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. The Balmer excesses and Halpha 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 Macc 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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arXiv2011

A Herschel resolved far-infrared dust ring around HD 207129

Dusty debris discs around main sequence stars are thought to be the result of continuous collisional grinding of planetesimals in the system. The majority of these systems are unresolved and analysis of the dust properties is limited by the lack of information regarding the dust location.vThe Herschel DUNES key program is observing 133 nearby, Sun-like stars (<20 pc, FGK spectral type) in a volume limited survey to constrain the absolute incidence of cold dust around these stars by detection of far infrared excess emission at flux levels comparable to the Edgeworth-Kuiper belt (EKB). We have observed the Sun-like star HD 207129 with Herschel PACS and SPIRE. In all three PACS bands we resolve a ring-like structure consistent with scattered light observations. Using α Boötis as a reference point spread function (PSF), we deconvolved the images, clearly resolving the inner gap in the disc at both 70 and 100 μm. We have resolved the dust-producing planetesimal belt of a debris disc at 100 μm for the first time. We measure the radial profile and fractional luminosity of the disc, and compare the values to those of discs around stars of similar age and/or spectral type, placing this disc in context of other resolved discs observed by Herschel/DUNES.

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arXiv2011

Optical spectroscopic variability of Herbig Ae/Be stars

We analysed 337 multi-epoch optical spectra of 38 Herbig Ae/Be (HAeBe) stars to gain insights into the variability behaviour of the circumstellar (CS) atomic gas. Equivalent widths (EWs) and line fluxes of the Halpha, [OI]6300, HeI5876 and NaID lines were obtained for each spectrum; the Halpha line width at 10% of peak intensity (W10) and profile shapes were also measured and classified. The mean line strengths and relative variabilities were quantified for each star. Simultaneous optical photometry was used to estimate the line fluxes. We present a homogeneous spectroscopic database of HAeBe stars. The lines are variable in practically all stars and timescales, although 30 % of the objects show a constant EW in [OI]6300, which is also the only line that shows no variability on timescales of hours. The HeI5876 and NaID EW relative variabilities are typically the largest, followed by those in [OI]6300 and Halpha. The EW changes can be larger than one order of magnitude for the HeI5876 line, and up to a factor 4 for Halpha. The [OI]6300 and Halpha EW relative variabilities are correlated for most stars in the sample. The Halpha mean EW and W10 are uncorrelated, as are their relative variabilities. The Halpha profile changes in 70 % of the objects. The massive stars in the sample usually show more stable Halpha profiles with blueshifted self-absorptions and less variable 10% widths. Our data suggest multiple causes for the different line variations, but the [OI]6300 and Halpha variability must share a similar origin in many objects. The physical mechanism responsible for the Halpha line broadening does not depend on the amount of emission; unlike in lower-mass stars, physical properties based on the Halpha luminosity and W10 would significantly differ. Our results provide additional support to previous works that reported different physical mechanisms in Herbig Ae and Herbig Be stars.

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arXiv2010

Spectroscopic properties of nearby late-type stars, members of stellar kinematic groups

Nearby late-type stars are excellent targets to look for young objects in stellar associations and moving groups. The study of these groups goes back more than one century ago however, their origin is still misunderstood. Although their existence have been confirmed by statistical studies of large sample of stars, the identification of a group of stars as member of moving groups, is not an easy task, list of members often change with time and most members have been identified by means of kinematics criteria which is not sufficient since many old stars can share the same spatial motion of those stars in moving groups. In this contribution we attempt to identify unambiguous moving groups members, among a sample of nearby-late type stars. High resolution echelle spectra is used to i) derive accurate radial velocities which allow us to study the stars' kinematics and make a first selection of moving groups members; and ii) analyze several age-related properties for young late-type stars (i.e., lithium LiI 6707.8 Amstrongs, R'HK index). The different age-estimators are compared and the moving group membership of the kinematically selected candidates are discussed.

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arXiv2010

A spectroscopy study of nearby late-type stars, possible members of stellar kinematic groups

Nearby late-type stars are excellent targets for seeking young objects in stellar associations and moving groups. The origin of these structures is still misunderstood, and lists of moving group members often change with time and also from author to author. Most members of these groups have been identified by means of kinematic criteria, leading to an important contamination of previous lists by old field stars. We attempt to identify unambiguous moving group members among a sample of nearby-late type stars by studying their kinematics, lithium abundance, chromospheric activity, and other age-related properties. High-resolution echelle spectra ($R \sim 57000$) of a sample of nearby late-type stars are used to derive accurate radial velocities that are combined with the precise Hipparcos parallaxes and proper motions to compute galactic-spatial velocity components. Stars are classified as possible members of the classical moving groups according to their kinematics. The spectra are also used to study several age-related properties for young late-type stars, i.e., the equivalent width of the lithium Li~{\sc i} \space 6707.8 \space Å\space line or the $R'_{\rm HK}$ index. Additional information like X-ray fluxes from the ROSAT All-Sky Survey or the presence of debris discs is also taken into account. The different age estimators are compared and the moving group membership of the kinematically selected candidates are discussed. From a total list of 405 nearby stars, 102 have been classified as moving group candidates according to their kinematics. i.e., only $\sim$ 25.2 \% of the sample. The number reduces when age estimates are considered, and only 26 moving group candidates (25.5\% of the 102 candidates) have ages in agreement with the star having the same age as an MG member

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arXiv2010

Gas in Protoplanetary Discs (GASPS) 1. First results

Context - Circumstellar discs are ubiquitous around young stars, but rapidly dissipate their gas and dust on timescales of a few Myr. The Herschel space observatory allows for the study of the warm disc atmosphere, using far-infrared spectroscopy to measure gas content and excitation conditions, and far-IR photometry to constrain the dust distribution. Aims - We aim to detect and characterize the gas content of circumstellar discs in four targets as part of the Herschel science demonstration phase. Methods - We carried out sensitive medium resolution spectroscopy and high sensitivity photometry at lambda ~60-190 micron using the Photodetector Array Camera and Spectrometer instrument on the Herschel space observatory. Results - We detect [OI] 63 micron emission from the young stars HD 169142, TW Hydrae, and RECX 15, but not HD 181327. No other lines, including [CII] 158 and [OI] 145, are significantly detected. All four stars are detected in photometry at 70 and 160 micron. Extensive models are presented in associated papers.

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arXiv2010

Gas in the protoplanetary disc of HD 169142: Herschel's view

In an effort to simultaneously study the gas and dust components of the disc surrounding the young Herbig Ae star HD 169142, we present far-IR observations obtained with the PACS instrument onboard the Herschel Space Observatory. This work is part of the Open Time Key Project GASPS, which is aimed at studying the evolution of protoplanetary discs. To constrain the gas properties in the outer disc, we observed the star at several key gas-lines, including [OI] 63.2 and 145.5 micron, [CII] 157.7 micron, CO 72.8 and 90.2 micron, and o-H2O 78.7 and 179.5 micron. We only detect the [OI] 63.2 micron line in our spectra, and derive upper limits for the other lines. We complement our data set with PACS photometry and 12/13CO data obtained with the Submillimeter Array. Furthermore, we derive accurate stellar parameters from optical spectra and UV to mm photometry. We model the dust continuum with the 3D radiative transfer code MCFOST and use this model as an input to analyse the gas lines with the thermo-chemical code ProDiMo. Our dataset is consistent with a simple model in which the gas and dust are well-mixed in a disc with a continuous structure between 20 and 200 AU, but this is not a unique solution. Our modelling effort allows us to constrain the gas-to-dust mass ratio as well as the relative abundance of the PAHs in the disc by simultaneously fitting the lines of several species that originate in different regions. Our results are inconsistent with a gas-poor disc with a large UV excess; a gas mass of 5.0 +/- 2.0 times 10^(-3) Msun is still present in this disc, in agreement with earlier CO observations.

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arXiv2010

Herschel-PACS observation of the 10 Myr old T Tauri disk TW Hya: Constraining the disk gas mass

Planets are formed in disks around young stars. With an age of ~10 Myr, TW Hya is one of the nearest T Tauri stars that is still surrounded by a relatively massive disk. In addition a large number of molecules has been found in the TW Hya disk, making TW Hya the perfect test case in a large survey of disks with Herschel-PACS to directly study their gaseous component. We aim to constrain the gas and dust mass of the circumstellar disk around TW Hya. We observed the fine-structure lines of [OI] and [CII] as part of the Open-time large program GASPS. We complement this with continuum data and ground-based 12CO 3-2 and 13CO 3-2 observations. We simultaneously model the continuum and the line fluxes with the 3D Monte-Carlo code MCFOST and the thermo-chemical code ProDiMo to derive the gas and dust masses. We detect the [OI] line at 63 micron. The other lines that were observed, [OI] at 145 micron and [CII] at 157 micron, are not detected. No extended emission has been found. Preliminary modeling of the photometric and line data assuming [12CO]/[13CO]=69 suggests a dust mass for grains with radius < 1 mm of ~1.9 times 10^-4 Msun (total solid mass of 3 times 10^-3 Msun) and a gas mass of (0.5--5) times 10^-3 Msun. The gas-to-dust mass may be lower than the standard interstellar value of 100.

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