Exocomets: A spectroscopic survey Rebollido, I.; Eiroa, C.; Montesinos, B. ...
Astronomy and astrophysics (Berlin),
07/2020, Letnik:
639
Journal Article, Web Resource
Recenzirano
Odprti dostop
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. Infrared-dark clouds (IRDCs) are the precursors to massive stars and stellar clusters. G011.11–0.12 is a well-studied filamentary IRDC, though, to date, the absence of far-infrared data with ...sufficient spatial resolution has limited the understanding of the structure and star-formation activity. Aims. We use Herschel to study the embedded population of young pre- and protostellar cores in this IRDC. Methods. We examine the cloud structure, which appears in absorption at short wavelength and in emission at longer wavelength. We derive the properties of the massive cores from the spectral energy distributions of bright far-infrared point sources detected with the PACS instrument aboard Herschel. Results. We report on the detection and characterization of pre- and protostellar cores in a massive filamentary infrared-dark cloud G011.11–0.12 using PACS. We characterize 18 cores directly associated with the filament, two of which have masses over 50 $M_\odot$, making them the best candidates to become massive stars in G011.11–0.12. These cores are likely at various stages of protostar formation, showing elevated temperature ($\langle$ T $\rangle$ ~ 22 K) with respect to the ambient gas reservoir. The core masses ($\langle$ M $\rangle$ ~ 24 $M_\odot$) are small compared to that in the cold filament. The mean core separation is 0.9 pc, well in excess of the Jeans length in the filament. Conclusions. We confirm that star formation in IRDCs is underway and diverse, and IRDCs have the capability of forming massive stars and clusters.
The nuclear bulge of the Galaxy Launhardt, R.; Zylka, R.; Mezger, P. G.
Astronomy and astrophysics (Berlin),
03/2002, Letnik:
384, Številka:
1
Journal Article
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.
Abridged: We detail and benchmark two sophisticated chemical models developed by the Heidelberg and Bordeaux astrochemistry groups. The main goal of this study is to elaborate on a few well-described ...tests for state-of-the-art astrochemical codes covering a range of physical conditions and chemical processes, in particular those aimed at constraining current and future interferometric observations of protoplanetary disks. We consider three physical models: a cold molecular cloud core, a hot core, and an outer region of a T Tauri disk. Our chemical network (for both models) is based on the original gas-phase osu_03_2008 ratefile and includes gas-grain interactions and a set of surface reactions for the H-, O-, C-, S-, and N-bearing molecules. The benchmarking is performed with the increasing complexity of the considered processes: (1) the pure gas-phase chemistry, (2) the gas-phase chemistry with accretion and desorption, and (3) the full gas-grain model with surface reactions. Using atomic initial abundances with heavily depleted metals and hydrogen in its molecular form, the chemical evolution is modeled within 10^9 years. The time-dependent abundances calculated with the two chemical models are essentially the same for all considered physical cases and for all species, including the most complex polyatomic ions and organic molecules. This result however required a lot of efforts to make all necessary details consistent through the model runs, e.g. definition of the gas particle density, density of grain surface sites, the strength and shape of the UV radiation field, etc. The reference models and the benchmark setup, along with the two chemical codes and resulting time-dependent abundances are made publicly available in the Internet: www.mpia.de/homes/semenov/Chemistry_benchmark/home.html
Context. Stars are born deeply embedded in molecular clouds. In the earliest embedded phases, protostars emit the bulk of their radiation in the far-infrared wavelength range, where $Herschel$ is ...perfectly suited to probe at high angular resolution and dynamic range. In the high-mass regime, the birthplaces of protostars are thought to be in the high-density structures known as infrared-dark clouds (IRDCs). While massive IRDCs are believed to have the right conditions to give rise to massive stars and clusters, the evolutionary sequence of this process is not well-characterized.Aims: As part of the Earliest Phases of Star formation (EPoS) $Herschel$ guaranteed time key program, we isolate the embedded structures within IRDCs and other cold, massive molecular clouds. We present the full sample of 45 high-mass regions which were mapped at PACS 70, 100, and 160 μm and SPIRE 250, 350, and 500 μm. In the present paper, we characterize a population of cores which appear in the PACS bands and place them into context with their host molecular cloud and investigate their evolutionary stage.Methods: We construct spectral energy distributions (SEDs) of 496 cores which appear in all PACS bands, 34% of which lack counterparts at 24 μm. From single-temperature modified blackbody fits of the SEDs, we derive the temperature, luminosity, and mass of each core. These properties predominantly reflect the conditions in the cold, outer regions. Taking into account optical depth effects and performing simple radiative transfer models, we explore the origin of emission at PACS wavelengths. Results: The core population has a median temperature of 20 K and has masses and luminosities that span four to five orders of magnitude. Cores with a counterpart at 24 μm are warmer and bluer on average than cores without a 24 μm counterpart. We conclude that cores bright at 24 μm are on average more advanced in their evolution, where a central protostar(s) have heated the outer bulk of the core, than 24 μm-dark cores. The 24 μm emission itself can arise in instances where our line of sight aligns with an exposed part of the warm inner core. About 10% of the total cloud mass is found in a given cloud's core population. We uncover over 300 further candidate cores which are dark until 100 μm. These are possibly starless objects, and further observations will help us determine the nature of these very cold cores.
Context. Stellar rotation is a crucial parameter driving stellar magnetism, activity and mixing of chemical elements. Measuring rotational velocities of young stars can give additional insight in the ...initial conditions of the star formation process. Furthermore, the evolution of stellar rotation is coupled to the evolution of circumstellar disks. Disk-braking mechanisms are believed to be responsible for rotational deceleration during the accretion phase, and rotational spin-up during the contraction phase after decoupling from the disk for fast rotators arriving at the ZAMS. On the ZAMS, stars get rotationally braked by solar-type winds. Aims. We investigate the projected rotational velocities v $\sin$ i of a sample of young stars with respect to the stellar mass and disk evolutionary state to search for possible indications of disk-braking mechanisms. Furthermore, we search for signs of rotational spin-up of stars that have already decoupled from their circumstellar disks. Methods. We analyse the stellar spectra of 220 nearby (mostly <100 pc) young (2–600 Myr) stars for their v $\sin$ i, stellar age, Hα emission, and accretion rates. The stars have been observed with FEROS at the 2.2 m MPG/ESO telescope and HARPS at the 3.6 m telescope in La Silla, Chile. The spectra have been cross-correlated with appropriate theoretical templates. We build a new calibration to be able to derive v $\sin$ i values from the cross-correlated spectra. Stellar ages are estimated from the $\ion{Li}{i}$ equivalent width at 6708 Å. The equivalent width and width at 10% height of the Hα emission are measured to identify accretors and used to estimate accretion rates $\dot{M}_{\rm acc}$. The $v\sin i$ is then analysed with respect to the evolutionary state of the circumstellar disks to search for indications of disk-braking mechanisms in accretors. Results. We find that the broad v $\sin$ i distribution of our targets extends to rotation velocities of up to more than 100 km s-1 and peaks at a value of 7.8 ± 1.2 km s -1, and that ~70% of our stars show v $\sin$ i < 30 km s-1. Furthermore, we can find indications for disk-braking in accretors and rotational spin-up of stars which are decoupled from their disks. In addition, we show that a number of young stars are suitable for precise radial-velocity measurements for planet-search surveys.
The temperature and density structure of molecular cloud cores are the most important physical quantities that determine the course of the protostellar collapse and the properties of the stars they ...firm. The instruments of the Herschel satellite provide us for the first time with both the spectral coverage and the spatial resolution that is needed to directly measure the dust temperature structure of nearby molecular cloud cores. With the aim of better constraining the initial physical conditions in molecular cloud cores at the onset of protostellar collapse, in particular of measuring their temperature structure, we initiated the guaranteed time key project (GTKP) "The Earliest Phases of Star Formation" (EPoS) with the Herschel satellite. This paper gives an overview of the low-mass sources in the EPoS project, the Herschel and complementary ground-based observations, our analysis method, and the initial results of the survey.
Context. Because of their short evolutionary time-scales, the earliest stages of high-mass star formation prior to the existence of any embedded heating source have barely been characterized until ...today. Aims. We study the fragmentation and dynamical properties of a massive starless gas clump at the onset of high-mass star formation. Methods. Based on Herschel continuum data we identify a massive gas clump that remains far-infrared dark up to 100 μm wavelengths. The fragmentation and dynamical properties are investigated by means of Plateau de Bure Interferometer and Nobeyama 45 m single-dish spectral line and continuum observations. Results. The massive gas reservoir (between ~800 and ~1600 M⊙, depending on the assumed dust properties) fragments at spatial scales of ~18 000 AU in four cores. Comparing the spatial extent of this high-mass region with intermediate- to low-mass starless cores from the literature, we find that linear sizes do not vary significantly over the whole mass regime. However, the high-mass regions squeeze much more gas into these similar volumes and hence have orders of magnitude larger densities. The fragmentation properties of the presented low-to high-mass regions are consistent with gravitational instable Jeans fragmentation. Furthermore, we find multiple velocity components associated with the resolved cores. Recent radiative transfer hydrodynamic simulations of the dynamic collapse of massive gas clumps also result in multiple velocity components along the line of sight because of the clumpy structure of the regions. This result is supported by a ratio between viral and total gas mass for the whole region <1. Conclusions. This apparently still starless high-mass gas clump exhibits clear signatures of early fragmentation and dynamic collapse prior to the formation of an embedded heating source. A comparison with regions of lower mass reveals that the linear size of star-forming regions does not necessarily have to vary much for different masses, however, the mass reservoirs and gas densities are orders of magnitude enhanced for high-mass regions compared to their lower-mass siblings.
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