Studies in Sarcoidosis

Norberg, Renée

doi:10.1111/j.0954-6820.1964.tb00584.x

Cited by 47 publications

(53 citation statements)

References 10 publications

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“…The slope of this power law is close to the value of 1.5 suggested by Norberg & Maeder (2000) as the one that best matches the distribution of intermediate and high-mass YSOs in the H-R diagram. In that study the authors concentrate on relatively evolved objects (like Herbig Ae/Be stars) for which an estimate of the effective temperature is possible, and it is interesting that we obtain a similar result for objects still heavily embedded in their parental cocoon.…”

Section: The Ir-p Strip As the Zams Location In The L Bol -M Env Diagramsupporting

confidence: 81%

“…There is however a fundamental difference. Norberg & Maeder (2000) use their parametrization of the accretion rate throughout the accretion process, while our Eq. (9) is valid only at the end of the main accretion phase when the object is very close to or on the ZAMS.…”

Section: The Ir-p Strip As the Zams Location In The L Bol -M Env Diagrammentioning

confidence: 99%

“…The difference in power law exponent for the expression of the instaneous accretion rate has a dramatic impact on the evolution of a YSO in the L bol −M env diagram. If we adopt the Norberg & Maeder (2000) prescription in the simulations of Fig. 9 we find that the YSO would spend almost all its accretion time at a luminosity below ∼100 L and then cover the final portion of the ascending tracks, between the MM and the IR-P sources, in just few 10 3 years or about ten times faster than using the turbulent core formulation (independently of the initial envelope mass, i.e.…”

Section: The Ir-p Strip As the Zams Location In The L Bol -M Env Diagrammentioning

confidence: 99%

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The evolution of the spectral energy distribution in massive young stellar objects

Molinari

Pezzuto

Cesaroni

et al. 2008

A&A

248

475

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Context. The mechanism of formation of massive stars is still a matter of debate. It is not yet clear if it can be considered to be a scaled-up analogue of the low-mass star regime, or if there are additional agents like merging of lower-mass forming objects or accretion from initially unbound material. Most of the uncertainties come from the lack of diagnostic tools to evolutionarily classify large samples of candidate massive protostellar objects that can then be studied in more detail. Aims. We want to verify whether diagnostic tools like the SED shape and the relationship between envelope mass and bolometric luminosity can be extended to the study of high-mass star formation. Methods. The 8−1200 µm SED of YSOs in 42 regions of massive star formation has been reconstructed using MSX, IRAS, and submm data partly available from previous works. Apart from IRAS catalogue fluxes, the fluxes in the Mid-IR and sub-mm/mm were derived directly from the images. The SEDs were fitted to an extensive grid of envelope models with embedded ZAMS stars, available from the literature. Sources that could not be fitted with a single model were then fitted with a two-component model composed of an embedded ZAMS for the mid-IR part and a single-temperature optically thin greybody for the longer wavelength emitting component. Sources were classified as "IR" if they were fitted with an embedded ZAMS envelope, and "MM" if they could only be fitted with a greybody with a peak at high λ; further subclassification was based on being the most massive object in the field ("P", for primary) or not ("S", for secondary). Results. The different classes of sources identified in our analysis have very different SEDs and occupy distinct areas in the L bol −M env diagram; by analogy with the low-mass regime, we see that MM-P, IR-P and IR-S objects could be interpreted as the high-mass analogue of Class 0-I-II. Evolutionary tracks obtained from a simple model based on the turbulent core prescriptions show that the three classes of sources possibly mark different periods in the formation of a massive YSO. The IR-P objects are consistent with being at the end of the main accretion phase, while MM-P sources are probably in an earlier evolutionary stage. The timescales for the formation decrease from ∼4 × 10 5 to ∼1 × 10 5 years with stellar mass increasing from ∼6 to ∼40 M ; these timescales, and the association with young clusters with median stellar age of a few 10 6 years suggest that the most massive objects are among the last ones to form. Conclusions. Our results are consistent with the high-mass star formation being a scaled-up analogue of the traditional accretiondominated paradigm valid for the low-mass regime.

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Section: The Ir-p Strip As the Zams Location In The L Bol -M Env Diagramsupporting

confidence: 81%

Section: The Ir-p Strip As the Zams Location In The L Bol -M Env Diagrammentioning

confidence: 99%

Section: The Ir-p Strip As the Zams Location In The L Bol -M Env Diagrammentioning

confidence: 99%

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The evolution of the spectral energy distribution in massive young stellar objects

Molinari

Pezzuto

Cesaroni

et al. 2008

A&A

248

475

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“…The chemical composition is solar (Z = 0.014) with the abundances of Asplund et al (2005) and Cunha et al (2006). For deuterium, we take a mass fraction of 5 × 10 −5 as in Bernasconi & Maeder (1996), Norberg & Maeder (2000), Behrend & Maeder (2001), and Haemmerlé et al (2013). (7)) accreting at the same rate (10 −4 M yr −1 ), but computed with the two different versions of the code.…”

Section: Improvements In the Codementioning

confidence: 99%

“…Bernasconi & Maeder (1996), Norberg & Maeder (2000), Behrend & Maeder (2001), and Haemmerlé et al (2013) have computed stellar models with accretion using the Geneva Stellar Evolution code. The accretion rates used in these works were increasing functions of time, i.e.…”

Section: Introductionmentioning

confidence: 99%

Massive star formation by accretion

Haemmerlé

Eggenberger

Meynet

et al. 2017

A&A

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Context. Massive stars likely form by accretion and the evolutionary track of an accreting forming star corresponds to what is called the birthline in the Hertzsprung-Russell (HR) diagram. The shape of this birthline is quite sensitive to the evolution of the entropy in the accreting star. Aims. We first study the reasons why some birthlines published in past years present different behaviours for a given accretion rate. We then revisit the question of the accretion rate, which allows us to understand the distribution of the observed pre-main-sequence (pre-MS) stars in the HR diagram. Finally, we identify the conditions needed to obtain a large inflation of the star along its pre-MS evolution that may push the birthline towards the Hayashi line in the upper part of the HR diagram. Methods. We present new pre-MS models including accretion at various rates and for different initial structures of the accreting core. We compare them with previously published equivalent models. From the observed upper envelope of pre-MS stars in the HR diagram, we deduce the accretion law that best matches the accretion history of most of the intermediate-mass stars. Results. In the numerical computation of the time derivative of the entropy, some treatment leads to an artificial loss of entropy and thus reduces the inflation that the accreting star undergoes along the birthline. In the case of cold disc accretion, the existence of a significant swelling during the accretion phase, which leads to radii > ∼ 100 R and brings the star back to the red part of the HR diagram, depends sensitively on the initial conditions. For an accretion rate of 10 −3 M yr −1 , only models starting from a core with a significant radiative region evolve back to the red part of the HR diagram. We also obtain that, in order to reproduce the observed upper envelope of pre-MS stars in the HR diagram with an accretion law deduced from the observed mass outflows in ultra-compact HII regions, the fraction of the mass that is accreted onto the star should represent a decreasing fraction of the mass outflows when the mass of the accreting object increases. In other words, the accretion efficiency (mass effectively accreted onto the star with respect to the total in falling matter) decreases when the mass of the star increases.

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Spectropolarimetry and the Study of Circumstellar Disks

Oudmaijer

Diffuse Matter From Star Forming Regions to Active Galaxies

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Circumstellar disks play an important role in many stages of the evolution of stars. However, it is only possible to directly image circumstellar disks for a few of the nearest stars. For massive stars, the situation is even more difficult, as they are on average further away than the more numerous low mass stars. Here, we discuss spectropolarimetry which is a technique that can reveal the presence of disks on spatial scales of order stellar radii. It is therefore extremely useful in studying disks around young stars, while the method has prospects for quantitative modelling. In this review we will discuss the status on the study of (accretion) disks around intermediate-to high mass pre-main sequence stars and will concentrate on the linear spectropolarimetry aspect. We wil make a brief excursion to observations of massive evolved stars where spectropolarimetry has revealed the clumpy structure of stellar winds, and conclude with a brief outlook. 1

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Studies in Sarcoidosis

Cited by 47 publications

References 10 publications

The evolution of the spectral energy distribution in massive young stellar objects

The evolution of the spectral energy distribution in massive young stellar objects

Massive star formation by accretion

Spectropolarimetry and the Study of Circumstellar Disks

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