scholarly journals A study of accretion and disk diagnostics in the NGC 2264 cluster

2019 ◽  
Vol 629 ◽  
pp. A67
Author(s):  
Alana P. Sousa ◽  
Silvia H. P. Alencar ◽  
Luisa M. Rebull ◽  
Catherine C. Espaillat ◽  
Nuria Calvet ◽  
...  
Keyword(s):  
Spectral Energy Distribution ◽  
Spectral Energy ◽  
Infrared Observations ◽  
T Tauri ◽  
Stellar Radiation ◽  
Accretion Rates ◽  
Uv Excess ◽  
Stellar Properties ◽  
Full Disk ◽  
Transition Disks

Context. Understanding disk dissipation is essential for studying how planets form. Disk gaps and holes, which almost correspond to dust-free regions, are inferred from infrared observations of T Tauri stars (TTS), indicating the existence of a transitional phase between thick accreting disks and debris disks. Transition disks are usually referred to as candidates for newly formed planets. Aims. We searched for transition disk candidates belonging to NGC 2264. Using stellar and disk parameters obtained in the observational multiwavelength campaign CSI 2264, we characterized accretion, disk, and stellar properties of transition disk candidates and compared them to systems with a full disk and diskless stars. Methods. We modeled the spectral energy distribution (SED) of a sample of 401 TTS, observed with both CFHT equipped with MegaCam and IRAC instrument on the Spitzer, with Hyperion SED fitting code using photometric data from the U band (0.3 μm) to the Spitzer/MIPS 24 μm band. We used the SED modeling to distinguish transition disk candidates, full disk systems, and diskless stars. Results. We classified ∼52% of the sample as full disk systems, ∼41% as diskless stars, and ∼7% of the systems as transition disk candidates, among which seven systems are new transition disk candidates belonging to the NGC 2264 cluster. The sample of transition disk candidates present dust in the inner disk similar to anemic disks, according to the αIRAC classification, which shows that anemic disk systems can be candidate transition disks. We show that the presence of a dust hole in the inner disk does not stop the accretion process since 82% of transition disk candidates accrete and show Hα, UV excess, and mass accretion rates at the same level as full disk systems. We estimate the inner hole sizes, ranging from 0.1 to 78 AU, for the sample of transition disk candidates. In only ∼18% of the transition disk candidates, the hole size could be explained by X-ray photoevaporation from stellar radiation.

scholarly journals DZ Chamaeleontis: a bona fide photoevaporating disc

2018 ◽  
Vol 610 ◽  
pp. A13 ◽  
Author(s):  
H. Canovas ◽  
B. Montesinos ◽  
M. R. Schreiber ◽  
L. A. Cieza ◽  
C. Eiroa ◽  
...  
Keyword(s):  
Accretion Rate ◽  
Spectral Energy Distribution ◽  
Line Emission ◽  
Spectral Energy ◽  
T Tauri ◽  
Stellar Radiation ◽  
Infrared Spectral ◽  
Chromospheric Emission ◽  
Imaging Polarimetry ◽  
Bona Fide

Context. DZ Cha is a weak-lined T Tauri star (WTTS) surrounded by a bright protoplanetary disc with evidence of inner disc clearing. Its narrow Hα line and infrared spectral energy distribution suggest that DZ Cha may be a photoevaporating disc. Aims. We aim to analyse the DZ Cha star + disc system to identify the mechanism driving the evolution of this object. Methods. We have analysed three epochs of high resolution optical spectroscopy, photometry from the UV up to the sub-mm regime, infrared spectroscopy, and J-band imaging polarimetry observations of DZ Cha. Results. Combining our analysis with previous studies we find no signatures of accretion in the Hα line profile in nine epochs covering a time baseline of ~20 yr. The optical spectra are dominated by chromospheric emission lines, but they also show emission from the forbidden lines [SII] 4068 and [OI] 6300Å that indicate a disc outflow. The polarized images reveal a dust depleted cavity of ~7 au in radius and two spiral-like features, and we derive a disc dust mass limit of Mdust< 3 MEarth from the sub-mm photometry. No stellar (M⋆> 80 MJup) companions are detected down to 0.̋07 (~8 au, projected). Conclusions. The negligible accretion rate, small cavity, and forbidden line emission strongly suggests that DZ Cha is currently at the initial stages of disc clearing by photoevaporation. At this point the inner disc has drained and the inner wall of the truncated outer disc is directly exposed to the stellar radiation. We argue that other mechanisms like planet formation or binarity cannot explain the observed properties of DZ Cha. The scarcity of objects like this one is in line with the dispersal timescale (≲105 yr) predicted by this theory. DZ Cha is therefore an ideal target to study the initial stages of photoevaporation.

scholarly journals Accretion and Outflow Activity in the Earliest Phases of Star-Formation: CO, Sub-mm Continuum, and Near-Infrared Observations

1997 ◽  
Vol 163 ◽  
pp. 725-726
Author(s):  
K.-W. Hodapp ◽  
E. F. Ladd
Keyword(s):  
Near Infrared ◽  
Spectral Energy Distribution ◽  
Far Infrared ◽  
Point Sources ◽  
Spectral Energy ◽  
Infrared Observations ◽  
Circumstellar Material ◽  
Dense Cores ◽  
Wavelength Radiation ◽  
Main Component

Stars in the earliest phases of their formation, i.e., those accreting the main component of their final mass, are deeply embedded within dense cores of dust and molecular material. Because of the high line-of-sight extinction and the large amount of circumstellar material, stellar emission is reprocessed by dust into long wavelength radiation, typically in the far-infrared and sub-millimeter bands. Consequently, the youngest sources are strong submillimeter continuum sources, and often undetectable as point sources in the near-infrared and optical. The most deeply embedded of these sources have been labelled “Class 0” sources by André, Ward-Thompson, & Barsony (1994), in an extension of the spectral energy distribution classification scheme first proposed by Adams, Lada, & Shu (1987).

scholarly journals Constraining X-ray emission in HBL blazars using multiwavelength observations

2020 ◽  
Vol 496 (2) ◽  
pp. 1295-1306
Author(s):  
Alicja Wierzcholska ◽  
Stefan J Wagner
Keyword(s):  
Spectral Energy Distribution ◽  
Broad Band ◽  
Neutral Hydrogen ◽  
High Energy ◽  
Host Galaxy ◽  
Spectral Energy ◽  
Additional Absorption ◽  
X Ray ◽  
Uv Excess ◽  
Survey Results

ABSTRACT The X-ray spectrum of extreme high-energy peaked BL Lac-type blazars is located in the synchrotron branch of the broad-band spectral energy distribution (SED), at energies below the peak. A joint fit of the extrapolated X-ray spectra together with a host galaxy template allows characterizing the synchrotron branch in the SED. The X-ray spectrum is usually characterized either with a pure or a curved power-law model. In the latter case, however, it is hard to distinguish an intrinsic curvature from excess absorption. In this paper, we focus on five well-observed blazars: 1ES 0229+200, PKS 0548−322, RX J 1136+6737, 1ES 1741+196, and 1ES 2344+514. We constrain the infrared to X-ray emission of these five blazars using a model that is characterized by the host galaxy, spectral curvature, absorption, and ultraviolet (UV) excess to separate these spectral features. In the case of four sources, namely 1ES 0229+200, PKS 0548−322, 1ES 1741+196, and 1ES 2344+514, the spectral fit with the atomic neutral hydrogen from the Leiden Argentina Bonn Survey results in a significant UV excess present in the broad-band SED. Such excess can be interpreted as an additional component, for example, a blue bump. However, in order to describe spectra of these blazars without such excess, additional absorption to the atomic neutral hydrogen from the Leiden Argentina Bonn Survey is needed.

scholarly journals Sporadic and intense accretion in a 1 Myr-old brown dwarf candidate

2020 ◽  
Vol 634 ◽  
pp. A128
Author(s):  
D. Nguyen-Thanh ◽  
N. Phan-Bao ◽  
S. J. Murphy ◽  
M. S. Bessell
Keyword(s):  
Spectral Energy Distribution ◽  
Brown Dwarfs ◽  
Photometric Data ◽  
Spectral Energy ◽  
Brown Dwarf ◽  
M Dwarfs ◽  
Trigonometric Parallax ◽  
Accretion Process ◽  
Accretion Rates ◽  
Low Mass

Context. Studying the accretion process in very low-mass objects has important implications for understanding their formation mechanism. Many nearby late-M dwarfs that have previously been identified in the field are in fact young brown dwarf members of nearby young associations. Some of them are still accreting. They are therefore excellent targets for further studies of the accretion process in the very low-mass regime at different stages. Aims. We aim to search for accreting young brown dwarf candidates in a sample of 85 nearby late-M dwarfs. Methods. Using photometric data from DENIS, 2MASS, and WISE, we constructed the spectral energy distribution of the late- M dwarfs based on BT-Settl models to detect infrared excesses. We then searched for lithium and Hα emission in candidates that exhibit infrared excesses to confirm their youth and the presence of accretion. Results. Among the 85 late-M dwarfs, only DENIS-P J1538317−103850 (M5.5) shows strong infrared excesses in WISE bands. The detection of lithium absorption in the M5.5 dwarf and its Gaia trigonometric parallax indicate an age of ~1 Myr and a mass of 47 MJ. The Hα emission line in the brown dwarf shows significant variability that indicates sporadic accretion. This 1 Myr-old brown dwarf also exhibits intense accretion bursts with accretion rates of up to 10−7.9 M⊙ yr−1. Conclusions. Our detection of sporadic accretion in one of the youngest brown dwarfs might imply that sporadic accretion at early stages could play an important role in the formation of brown dwarfs. Very low-mass cores would not be able to accrete enough material to become stars, and thus they end up as brown dwarfs.

scholarly journals Exploring the dimming event of RW Aurigae A through multi-epoch VLT/X-shooter spectroscopy

2019 ◽  
Vol 625 ◽  
pp. A49 ◽  
Author(s):  
M. Koutoulaki ◽  
S. Facchini ◽  
C. F. Manara ◽  
A. Natta ◽  
R. Garcia Lopez ◽  
...  
Keyword(s):  
Energy Distribution ◽  
Physical Properties ◽  
Near Infrared ◽  
Accretion Rate ◽  
Spectral Energy Distribution ◽  
Spectral Energy ◽  
Infrared Excess ◽  
Mass Accretion Rate ◽  
Near Ir ◽  
T Tauri

Context. RW Aur A is a classical T Tauri star that has suddenly undergone three major dimming events since 2010. The reason for these dimming events is still not clear. Aims. We aim to understand the dimming properties, examine accretion variability, and derive the physical properties of the inner disc traced by the CO ro-vibrational emission at near-infrared wavelengths (2.3 μm). Methods. We compared two epochs of X-shooter observations, during and after the dimming. We modelled the rarely detected CO bandhead emission in both epochs to examine whether the inner disc properties had changed. The spectral energy distribution was used to derive the extinction properties of the dimmed spectrum and compare the infrared excess between the two epochs. Lines tracing accretion were used to derive the mass accretion rate in both states. Results. The CO originates from a region with physical properties of T = 3000 K, NCO = 1 × 1021 cm−2 and vk sin i = 113 km s−1. The extinction properties of the dimming layer were derived with the effective optical depth ranging from τeff ~2.5−1.5 from the UV to the near-IR. The inferred mass accretion rate Ṁacc is ~1.5 × 10−8 M⊙ yr−1 and ~2 × 10−8 M⊙ yr−1 after and during the dimming respectively. By fitting the spectral energy distribution, additional emission is observed in the infrared during the dimming event from dust grains with temperatures of 500–700 K. Conclusions. The physical conditions traced by the CO are similar for both epochs, indicating that the inner gaseous disc properties do not change during the dimming events. The extinction curve is flatter than that of the interstellar medium, and large grains of a few hundred microns are thus required. When we correct for the observed extinction, the mass accretion rate is constant in the two epochs, suggesting that the accretion is stable and therefore does not cause the dimming. The additional hot emission in the near-IR is located at about 0.5 au from the star and is not consistent with an occulting body located in the outer regions of the disc. The dimming events could be due to a dust-laden wind, a severe puffing-up of the inner rim, or a perturbation caused by the recent star-disc encounter.

scholarly journals The Origin and Evolution of Transition Discs: Successes, Problems, and Open Questions

2016 ◽  
Vol 33 ◽  
Author(s):  
James E. Owen
Keyword(s):  
Spectral Energy Distribution ◽  
Theoretical Work ◽  
Spectral Energy ◽  
Theoretical Understanding ◽  
Origin And Evolution ◽  
Hidden Population ◽  
Open Questions ◽  
Accretion Rates ◽  
Show Evidence ◽  
Dust Component

AbstractTransition discs are protoplanetary discs that show evidence for large holes or wide gaps (with widths comparable to their radii) in their dust component. These discs could be giving us clues about the disc destruction mechanism or hints about the location and time-scales for the formation of planets. However, at the moment there remain key gaps in our theoretical understanding. The vast majority of transition discs are accreting onto their central stars, indicating that—at least close to the star—dust has been depleted from the gas by a very large amount. In this review, we discuss evidence for two distinct populations of transition discs: mm-faint—those with low mm-fluxes, small holes (≲ 10 AU), and low accretion rates (~ 10−10 − 10−9 M⊙ yr−1) and mm-bright—discs with large mm-fluxes, large holes (≳ 20 AU), and high accretion rates ~ 10−8 M⊙ yr−1. MM-faint transition discs are consistent with what would naively be expected from a disc undergoing dispersal; however, mm-bright discs are not, and are likely to be rare and long-lived objects. We discuss the two commonly proposed mechanisms for creating transition discs: photoevaporation and planet–disc interactions, with a particular emphasis on how they would evolve in these models, comparing these predictions to the observed population. More theoretical work on explaining the lack of optically thick, non-accreting transition discs is required in both the photoevaporation and planetary hypothesis, before we can start to use transition discs to constrain models of planet formation. Finally, we suggest that the few discs with primordial looking spectral energy distribution, but serendipitously imaged showing large cavities in the mm (e.g. MWC758 and WSB 60) may represent a hidden population of associated objects. Characterising and understanding how these objects fit into the overall paradigm may allow us to unravel the mystery of transition discs.

scholarly journals Disks Around T Tauri Stars with SPHERE (DARTTS-S)

2020 ◽  
Vol 633 ◽  
pp. A82 ◽  
Author(s):  
A. Garufi ◽  
H. Avenhaus ◽  
S. Pérez ◽  
S. P. Quanz ◽  
R. G. van Holstein ◽  
...  
Keyword(s):  
Scattered Light ◽  
Spectral Energy Distribution ◽  
Surrounding Medium ◽  
Circumstellar Disks ◽  
T Tauri Stars ◽  
Spectral Energy ◽  
Near Ir ◽  
Nir Imaging ◽  
T Tauri ◽  
Disk Size

Context. Near-IR polarimetric images of protoplanetary disks enable us to characterize substructures that might be due to the interaction with (forming) planets. The available census is strongly biased toward massive disks around old stars, however. Aims. The DARTTS program aims at alleviating this bias by imaging a large number of T Tauri stars with diverse properties. Methods. DARTTS-S employs VLT/SPHERE to image the polarized scattered light from disks. In parallel, DARTTS-A provides ALMA images of the same targets for a comparison of different dust components. In this work, we present new SPHERE images of 21 circumstellar disks, which is the largest sample released to date. We also recalculated some relevant stellar and disk properties following Gaia DR2. Results. The targets of this work are significantly younger than those published thus far with polarimetric near-IR (NIR) imaging. Scattered light is unambiguously resolved in 11 targets, and some polarized unresolved signal is detected in 3 additional sources. Some disk substructures are detected. However, the paucity of spirals and shadows from this sample reinforces the trend according to which these NIR features are associated with Herbig stars, either because they are older or more massive. Furthermore, disk rings that are apparent in ALMA observations of some targets do not appear to have corresponding detections with SPHERE. Inner cavities larger than ~15 au are also absent from our images, even though they are expected from the spectral energy distribution. On the other hand, 3 objects show extended filaments at larger scale that are indicative of strong interaction with the surrounding medium. All but one of the undetected disks are best explained by their limited size (≲20 au), and the high occurrence of stellar companions in these sources suggests an important role in limiting the disk size. One undetected disk is massive and very large at millimeter wavelengths, implying that it is self-shadowed in the NIR. Conclusions. This work paves the way toward a more complete and less biased sample of scattered-light observations, which is required to interpret how disk features evolve throughout the disk lifetime.

scholarly journals Star formation and the circumstellar matter of young stellar objects

1987 ◽  
Vol 122 ◽  
pp. 7-22
Author(s):  
Frank H. Shu ◽  
Fred C. Adams
Keyword(s):  
Stellar Wind ◽  
Binary Star ◽  
Spectral Energy Distribution ◽  
Theoretical Models ◽  
Circumstellar Matter ◽  
Young Stellar Objects ◽  
Spectral Energy ◽  
Second Phase ◽  
Infrared Excess ◽  
T Tauri

We propose that the formation of low mass stars in molecular clouds takes place in four stages. The first stage is the formation of slowly rotating cloud cores through the slow leakage of magnetic (and turbulent) support by ambipolar diffusion. These cores asymptotically approach quasistatic states resembling singular isothermal spheres, but such end states cannot actually be reached because they are unstable. The second phase begins when a condensing cloud core passes the brink of instability and collapses dynamically from “inside-out,” building up a central protostar and nebular disk. The emergent spectral energy distributions of theoretical models in the infall stage are in close agreement with those of recently found infrared sources with steep spectra. As the rotating protostar gains mass, deuterium will eventually ignite in the central regions and drive the star nearly completely convective if its mass is less than about 2 M⊙. This initiates the next step of evolution - the bipolar outflow phase - in which a stellar wind pushes outward and breaks through the infalling envelope. The initial breakout is likely to occur along the rotational poles, leading to collimated jets and bipolar outflows. The intense stellar wind eventually widens to sweep out gas in nearly all 4π steradian, revealing the fourth stage - a T Tauri star with a surrounding remnant nebular disk. Radiation from a disk adds an infrared excess to the expected spectral energy distribution of the revealed source. The detailed shape of this infrared excess depends on whether the disk is largely passive and merely reprocesses stellar photons, or is relatively massive and actively accreting. Both extremes of spectral shapes are observed in T Tauri stars; the amount of circumstellar material in the form of disks around nearly formed stars may be related to the dual issues of the origins of binary-star and planetary systems.

scholarly journals Fraction of bolometric luminosity absorbed by dust in DustPedia galaxies

2018 ◽  
Vol 620 ◽  
pp. A112 ◽  
Author(s):  
S. Bianchi ◽  
P. De Vis ◽  
S. Viaene ◽  
A. Nersesian ◽  
A. V. Mosenkov ◽  
...  
Keyword(s):  
Star Formation ◽  
Galaxy Evolution ◽  
Star Formation Rate ◽  
Spectral Energy Distribution ◽  
Formation Rate ◽  
Spectral Energy ◽  
Stellar Content ◽  
Late Type ◽  
Bolometric Luminosity ◽  
Stellar Radiation

Aims. We aim to study the fraction of stellar radiation absorbed by dust, fabs, in 814 galaxies of different morphological types. The targets constitute the vast majority (93%) of the DustPedia sample, including almost all large (optical diameter larger than 1′), nearby (v ≤ 3000 km s−1) galaxies observed with the Herschel Space Observatory. Methods. For each object, we modelled the spectral energy distribution from the ultraviolet to the sub-millimetre using the dedicated, aperture-matched DustPedia photometry and the Code Investigating GALaxy Evolution (CIGALE). The value of fabs was obtained from the total luminosity emitted by dust and from the bolometric luminosity, which are estimated by the fit. Results. On average, 19% of the stellar radiation is absorbed by dust in DustPedia galaxies. The fraction rises to 25% if only late-type galaxies are considered. The dependence of fabs on morphology, showing a peak for Sb-Sc galaxies, is weak; it reflects a stronger, yet broad, positive correlation with the bolometric luminosity, which is identified for late-type, disk-dominated, high-specific-star-formation rate, gas-rich objects. We find no variation of fabs with inclination, at odds with radiative transfer models of edge-on galaxies. These results call for a self-consistent modelling of the evolution of the dust mass and geometry along the build-up of the stellar content. We also provide template spectral energy distributions in bins of morphology and luminosity and study the variation of fabs with stellar mass and specific star-formation rate. We confirm that the local Universe is missing the high fabs, luminous and actively star-forming objects necessary to explain the energy budget in observations of the extragalactic background light.

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