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A multi-method approach to radial-velocity measurement for single-object spectra
摘要： Context. The derivation of radial velocities from large numbers of spectra that typically result from survey work, requires automation. However, except for the classical cases of slowly rotating late-type spectra, existing methods of measuring Doppler shifts require fine-tuning to avoid a loss of accuracy due to the idiosyncrasies of individual spectra. The radial velocity spectrometer (RVS) on the Gaia mission, which will start operating very soon, prompted a new attempt at creating a measurement pipeline to handle a wide variety of spectral types. Aims. The present paper describes the theoretical background on which this software is based. However, apart from the assumption that only synthetic templates are used, we do not rely on any of the characteristics of this instrument, so our results should be relevant for most telescope-detector combinations. Methods. We propose an approach based on the simultaneous use of several alternative measurement methods, each having its own merits and drawbacks, and conveying the spectral information in a different way, leading to different values for the measurement. A comparison or a combination of the various results either leads to a 'best estimate' or indicates to the user that the observed spectrum is problematic and should be analysed manually. Results. We selected three methods and analysed the relationships and differences between them from a unified point of view; with each method an appropriate estimator for the individual random error is chosen. We also develop a procedure for tackling the problem of template mismatch in a systematic way. Furthermore, we propose several tests for studying and comparing the performance of the various methods as a function of the atmospheric parameters of the observed objects. Finally, we describe a procedure for obtaining a knowledge-based combination of the various Doppler-shift measurements.
关键词： techniques: radial velocities,surveys,stars: kinematics and dynamics,methods: data analysis,techniques: spectroscopic
更新于2025-09-23 15:23:52
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Thermal structure of a hot non-flaring corona from Hinode/EIS
摘要： Aims. In previous studies, a very hot plasma component has been diagnosed in solar active regions through the images in three different narrow-band channels of Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO). This diagnostic from extreme ultraviolet (EUV) imaging data has also been supported by the matching morphology of emission in the hot Ca XVII line, as observed with Extreme-Ultraviolet Imaging Spectrometer (EIS) on board Hinode. This evidence is debated because of the unknown distribution of the emission measure along the line of sight. Here we investigate in detail the thermal distribution of one such region using EUV spectroscopic data. Methods. In an active region observed with SDO/AIA, Hinode/EIS, and X-ray telescope (XRT), we select a sub-region with a very hot plasma component and another cooler sub-region for comparison. The average spectrum is extracted for both, and 14 intense lines are selected for analysis that probe the 5.5 < log T < 7 temperature range uniformly. From these lines, the emission measure distributions are reconstructed with the Markov-chain Monte Carlo method. Results are cross-checked in comparison with the two sub-regions, with a different inversion method, with the morphology of the images, and with the addition of fluxes measured with narrow, and broadband imagers. Results. We find that, whereas the cool region has a flat and featureless distribution that drops at temperature log T ≥ 6.3, the distribution of the hot region shows a well-defined peak at log T = 6.6 and gradually decreasing trends on both sides, thus supporting the very hot nature of the hot component diagnosed with imagers. The other cross-checks are consistent with this result. Conclusions. This study provides a completion of the analysis of active region components, and the resulting scenario supports the presence of a minor very hot plasma component in the core, with temperatures log T > 6.6.
关键词： techniques: imaging spectroscopy,Sun: UV radiation,Sun: corona,techniques: spectroscopic,Sun: X-rays, gamma rays
更新于2025-09-23 15:23:52
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Using the multi-object adaptive optics demonstrator RAVEN to observe metal-poor stars in and towards the Galactic Centre
摘要： The chemical abundances for five metal-poor stars in and towards the Galactic bulge have been determined from the H-band infrared spectroscopy taken with the RAVEN multi-object adaptive optics science demonstrator and the Infrared Camera and Spectrograph at the Subaru 8.2-m telescope. Three of these stars are in the Galactic bulge and have metallicities between ?2.1 < [Fe/H] < ?1.5, and high [α/Fe] ～ +0.3, typical of Galactic disc and bulge stars in this metallicity range; [Al/Fe] and [N/Fe] are also high, whereas [C/Fe] < +0.3. An examination of their orbits suggests that two of these stars may be confined to the Galactic bulge and one is a halo trespasser, though proper motion values used to calculate orbits are quite uncertain. An additional two stars in the globular cluster M22 show [Fe/H] values consistent to within 1σ, although one of these two stars has [Fe/H] = ?2.01 ± 0.09, which is on the low end for this cluster. The [α/Fe] and [Ni/Fe] values differ by 2σ, with the most metal-poor star showing significantly higher values for these elements. M22 is known to show element abundance variations, consistent with a multipopulation scenario though our results cannot discriminate this clearly given our abundance uncertainties. This is the first science demonstration of multi-object adaptive optics with high-resolution infrared spectroscopy, and we also discuss the feasibility of this technique for use in the upcoming era of 30-m class telescope facilities.
关键词： stars: abundances,Galaxy: centre,instrumentation: adaptive optics,globular clusters: individual: M22,techniques: spectroscopic
更新于2025-09-23 15:23:52
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A library of near-infrared integral field spectra of young M–L dwarfs
摘要： Context. At young ages, low surface gravity affects the atmospheric properties of ultracool dwarfs. The impact on medium-resolution near-infrared (NIR) spectra has only been slightly investigated at the M–L transition so far. Aims. We present a library of NIR (1.1–2.45 μm) medium-resolution (R ～ 1500–2000) integral field spectra of 15 young M6?L0 dwarfs. We aim at deriving updated NIR spectral type, luminosity, and physical parameters (Teff, log g, M, L/L☉) for each source. This work also aims at testing the latest generation of BT-SETTL atmospheric models. Methods. We estimated spectral types using spectral indices and the spectra of young objects classified in the optical. We used the 2010 and 2012 releases of the BT-SETTL synthetic spectral grid and cross-checked the results with the DRIFT-PHOENIX models to derive the atmospheric properties of the sources. Results. We do not find significant differences between the spectra of young companions and those of young isolated brown dwarfs in the same spectral type range. We derive infrared spectral types L0 ± 1, L0 ± 1, M9.5 ± 0.5, M9.5 ± 0.5, M9.25 ± 0.25, M8+0.5?0.75, and M8.5 ± 0.5 for AB Pic b, Cha J110913-773444, USco CTIO 108B, GSC 08047-00232 B, DH Tau B, CT Cha b, and HR7329B, respectively. The BT-SETTL and DRIFT-PHOENIX models yield close Teff and log g estimates for each source. The models seem to show a 600+600?300 K drop in the effective temperature at the M–L transition. Assuming the former temperatures are correct, we then derive new mass estimates that confirm that DH Tau B, USco CTIO 108B, AB Pic b, KPNO Tau 4, OTS 44, and Cha1109 lie inside or at the boundary of the planetary mass range. We combine the empirical luminosities of the M9.5–L0 sources to the Teff to derive semi-empirical radii estimates that do not match 'hot-start' evolutionary models predictions at 1–3 Myr. We use complementary data to demonstrate that atmospheric models are able to reproduce the combined optical and infrared spectral energy distribution, together with the NIR spectra of these sources simultaneously. But the models still fail to represent the dominant features in the optical. This issue raises doubts on the ability of these models to predict effective temperatures from NIR spectra alone. Conclusions. The library provides templates for characterizing other young and late type objects. The study advocates the use of photometric and spectroscopic information over a broad range of wavelengths to study the properties of very low-mass young companions to be detected with the planet imagers (Subaru/SCExAO, LBT/LMIRCam, Gemini/GPI, VLT/SPHERE).
关键词： stars: low-mass,techniques: spectroscopic,planetary systems,brown dwarfs
更新于2025-09-23 15:23:52
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Rotationally resolved spectroscopy of (20000) Varuna in the near-infrared
摘要： Context. Models of the escape and retention of volatiles by minor icy objects exclude any presence of volatile ices on the surface of trans-Neptunian objects (TNOs) smaller than ～1000 km in diameter at the typical temperature in this region of the solar system, whereas the same models show that water ice is stable on the surface of objects over a wide range of diameters. Collisions and cometary activity have been used to explain the process of surface refreshing of TNOs and Centaurs. These processes can produce surface heterogeneity that can be studied by collecting information at di?erent rotational phases. Aims. The aims of this work are to study the surface composition of (20000) Varuna, a TNO with a diameter 668+154 ?86 km and to search for indications of rotational variability. Methods. We observed (20000) Varuna during two consecutive nights in January 2011 with the near-infrared camera and spectrometer NICS at the Telescopio Nazionale Galileo, La Palma, Spain. We used the low resolution mode with the AMICI prism to obtain a set of spectra covering the whole rotation period of the Varuna (Pr = 6.34 h). We ?t the resulting relative re?ectance with radiative transfer models of the surface of atmosphereless bodies. Results. After studying the spectra corresponding to di?erent rotational phases of Varuna, we did not ?nd any indication of surface variability at 2σ level. In all the spectra, we detect an absorption at 2.0 μm, suggesting the presence of water ice on the surface. We do not detect any other volatiles on the surface, although the signal-to-noise ratio is not high enough to discard their presence in small quantities. Based on scattering models, we present two possible compositions compatible with our set of data and discuss their implications in the framework of the collisional history of the trans-Neptunian belt. Conclusions. We ?nd that the most probable composition for the surface of Varuna is a mixture of amorphous silicates, complex organics, and water ice. This composition is compatible with all the materials being primordial, so no replenishment mechanism is needed in the equation. However, our data can also be ?tted by models containing up to a 10% of methane ice. For an object with the characteristics of Varuna, this volatile could not be primordial, so an event, such as an energetic impact, would be needed to explain its presence on the surface.
关键词： Kuiper belt objects: individual: (2000) Varuna,planets and satellites: composition,methods: numerical,techniques: spectroscopic,methods: observational
更新于2025-09-23 15:23:52
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Detection of He?I λ10830 ? absorption on HD 189733 b with CARMENES high-resolution transmission spectroscopy
摘要： We present three transit observations of HD 189733 b obtained with the high-resolution spectrograph CARMENES at Calar Alto. A strong absorption signal is detected in the near-infrared He I triplet at 10830 ? in all three transits. During mid-transit, the mean absorption level is 0.88 ± 0.04% measured in a 10 km s?1 range at a net blueshift of ?3.5 ± 0.4 km s?1. The absorption signal exhibits radial velocities of +6.5 ± 1.0 km s?1 and ?12.6 ± 3.1 km s?1 during ingress and egress, respectively; all radial velocities are measured in the planetary rest frame. We show that stellar activity related pseudo-signals interfere with the planetary atmospheric absorption signal. They could contribute as much as 80% of the observed signal and might also affect the observed radial velocity signature, but pseudo-signals are very unlikely to explain the entire signal. The observed line ratio between the two unresolved and the third line of the He I triplet is 2.8 ± 0.2, which strongly deviates from the value expected for an optically thin atmospheres. When interpreted in terms of absorption in the planetary atmosphere, this favors a compact helium atmosphere with an extent of only 0.2 planetary radii and a substantial column density on the order of 4 × 1012 cm?2. The observed radial velocities can be understood either in terms of atmospheric circulation with equatorial superrotation or as a sign of an asymmetric atmospheric component of evaporating material. We detect no clear signature of ongoing evaporation, like pre- or post-transit absorption, which could indicate material beyond the planetary Roche lobe, or radial velocities in excess of the escape velocity. These findings do not contradict planetary evaporation, but only show that the detected helium absorption in HD 189733 b does not trace the atmospheric layers that show pronounced escape signatures.
关键词： stars: activity,planets and satellites: individual: HD 189773 b,infrared: planetary systems,planets and satellites: atmospheres,planet-star interactions,techniques: spectroscopic
更新于2025-09-23 15:23:52
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Measuring precise radial velocities on individual spectral lines
摘要： Context. Stellar activity is the main limitation to the detection of an Earth-twin using the radial-velocity (RV) technique. Despite many efforts in trying to mitigate the effect of stellar activity using empirical and statistical techniques, it seems that we are facing an obstacle that will be extremely difficult to overcome using current techniques. Aims. In this paper, we investigate a novel approach to derive precise RVs considering the wealth of information present in high-resolution spectra. Methods. This new method consists of building a master spectrum from all available observations and measure the RVs of each individual spectral line in a spectrum relative to this master. When analysing several spectra, the final product of this approach is the RVs of each individual line as a function of time. Results. We demonstrate on three stars intensively observed with HARPS that our new method gives RVs that are extremely similar to the one derived from the HARPS data reduction software. Our new approach to derive RVs demonstrates that the non-stability of daily HARPS wavelength solution induces night-to-night RV offsets with an standard deviation of 0.4 m s?1, and we propose a solution to correct for this systematic. Finally, and this is probably the most astrophysically relevant result of this paper, we demonstrate that some spectral lines are strongly affected by stellar activity while others are not. By measuring the RVs on two carefully selected subsample of spectral lines, we demonstrate that we can boost by a factor of two or mitigate by a factor of 1.6 the red noise induced by stellar activity in the 2010 RV measurements of α Cen B. Conclusions. By measuring the RVs of each spectral line, we are able to reach the same RV precision as other approved techniques. In addition, this new approach allows us to demonstrate that each spectral line is differently affected by stellar activity. Preliminary results show that studying in details the behaviour of each spectral line is probably the key to overcome the obstacle of stellar activity.
关键词： stars: individual: HD 128621,stars: activity,stars: individual: HD 10700,stars: individual: HD 10180,techniques: radial velocities,techniques: spectroscopic
更新于2025-09-23 15:22:29
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LOW-RESOLUTION NEAR-INFRARED STELLAR SPECTRA OBSERVED BY THE COSMIC INFRARED BACKGROUND EXPERIMENT (CIBER)
摘要： We present near-infrared (0.8–1.8 μm) spectra of 105 bright (mJ < 10) stars observed with the low-resolution spectrometer on the rocket-borne Cosmic Infrared Background Experiment. As our observations are performed above the Earth?s atmosphere, our spectra are free from telluric contamination, which makes them a unique resource for near-infrared spectral calibration. Two-Micron All-Sky Survey photometry information is used to identify cross-matched stars after reduction and extraction of the spectra. We identify the spectral types of the observed stars by comparing them with spectral templates from the Infrared Telescope Facility library. All the observed spectra are consistent with late F to M stellar spectral types, and we identify various infrared absorption lines.
关键词： stars: general,catalogs,infrared: stars,techniques: spectroscopic
更新于2025-09-23 15:22:29
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An Automated Galaxy Spectra Recognition Method Basing on Spectral Lines Information
摘要： For the vast amounts of spectra produced by LAMOST, the pipeline basing on PCAZ method is limited by the bad ?ux calibration and low S/N data. This work focuses on the study of the e?cient recognition methods of galaxy spectra of LAMOST basing on spectral lines information. The new method searches spectral lines and extracts the information of spectral lines (position, height, and width et al.) automatically. Using the spectral lines information which are less in?uenced by the quality of ?ux calibration and the S/N ratio, galaxy spectra are recognized with the redshift measured through spectral lines matching method. The experiment veri?ed it is feasible for the LAMOST galaxy spectra: the correct recognition rate > 80% for the data with SN R g > 5, and > 90% for the data with SN R r > 5. Compared with the redshift of SDSS, the systematic error of our method is 0, and the standard deviation of the error is 0.0002.
关键词： galaxies: fundamental parameters (classi?cation,methods: data analysis,telescopes:LAMOST,luminosities,colors,radii,techniques: spectroscopic,etc.),masses
更新于2025-09-23 15:22:29
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SEPIA – a new single pixel receiver at the APEX telescope ( <i>Corrigendum</i> )
摘要： A mistake occurred in the first paragraph of Sect. 2.1 during the production process. One should read the following sentences. One of the major challenges of bringing the SEPIA receiver with installed ALMA cartridges to the APEX telescope is the necessity to implement tertiary optics, which should not only provide the required and frequency independent illumination of the secondary but also be compatible with the existing optical layout of the APEX Cabin A where all single-pixel heterodyne receivers are installed. Another serious constraint is a clearance of the APEX Cabin A Nasmyth tube, whose rim is limited by the elevation encoder down to 150 mm in diameter requiring precision alignment possibilities to avoid Band 5 receiver beam truncation. ALMA receiver cartridges have built-in cold optics optimized for their respective position inside the ALMA Front-End (FE) receiver cryostat place at the antenna focal plane. In particular, depending on the cartridge position offset from the FE center, the beam tilt offset compensating is different for, for example, ALMA Band 5 – 2.38 degree and for ALMA Band 9 cartridges – 0.93 degree. SEPIA tertiary optics shall accommodate all these constrains and differences with minimum number of reflecting surfaces (thus minimizing the reflecting loss) and fit a very confined volume within the APEX Cabin A. Specific for SEPIA and in contrast to ALMA FE, we use a rotating cartridge-selection mirror. Such an optical switch addresses limitations of the Nasmyth layout when one receiver channel has access to the sky at a time (Fig. 1). The cartridge-selection mirror (NMF3, Fig. 1) with its precision computer-controlled rotating mechanism facilitates the accommodation of different ALMA cartridges having specific differences in the incoming beam positioning as outlined above.
关键词： errata,techniques: spectroscopic,addenda,instrumentation: detectors
更新于2025-09-23 15:22:29