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Title: OBSERVED LUMINOSITY SPREAD IN YOUNG CLUSTERS AND FU Ori STARS: A UNIFIED PICTURE

Abstract

The idea that non-steady accretion during the embedded phase of protostar evolution can produce the observed luminosity spread in the Herzsprung-Russell diagram (HRD) of young clusters has recently been called into question. Observations of FU Ori, for instance, suggest an expansion of the star during strong accretion events, whereas the luminosity spread implies a contraction of the accreting objects, decreasing their radiating surface. In this paper, we present a global scenario based on calculations coupling episodic accretion histories derived from numerical simulations of collapsing cloud prestellar cores of various masses and subsequent protostar evolution. Our calculations show that, assuming an initial protostar mass M{sub i} {approx} 1 M{sub Jup}, typical of the second Larson's core, both the luminosity spread in the HRD and the inferred properties of FU Ori events (mass, radius, accretion rate) can be explained by this scenario, providing two conditions. First, there must be some variation within the fraction of accretion energy absorbed by the protostar during the accretion process. Second, the range of this variation should increase with increasing accretion burst intensity and thus with the initial core mass and final star mass. The numerical hydrodynamics simulations of collapsing cloud prestellar cores indeed show that themore » intensity of the accretion bursts correlates with the mass and initial angular momentum of the prestellar core. Massive prestellar cores with high initial angular momentum are found to produce intense bursts characteristic of FU Ori-like events. Our results thus suggest a link between the burst intensities and the fraction of accretion energy absorbed by the protostar, with some threshold in the accretion rate, of the order of 10{sup -5} M{sub Sun} yr{sup -1}, delimitating the transition from 'cold' to 'hot' accretion. Such a transition might reflect a change in the accretion geometry with increasing accretion rate, i.e., a transition from magnetospheric or thin-disk to thick-disk accretion, or in the magnetospheric interaction between the star and the disk. Conversely, the luminosity spread can also be explained by a variation of the initial protostar mass within the {approx}1-5 M{sub Jup} range, although it is unclear for now whether such a spread among the second Larson's core can be produced during the prestellar core second collapse. This unified picture confirms the idea that early accretion during protostar and proto-brown dwarf formation/evolution can explain the observed luminosity spread in young clusters without invoking any significant age spread, and that the concept of a well-defined birthline does not apply for low-mass objects. Finally, we examine the impact of accretion on the determination of the initial mass function in young clusters.« less

Authors:
;  [1]
  1. Astrophysics Group, University of Exeter, EX4 4QL Exeter (United Kingdom)
Publication Date:
OSTI Identifier:
22092402
Resource Type:
Journal Article
Journal Name:
Astrophysical Journal
Additional Journal Information:
Journal Volume: 756; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 0004-637X
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ACCRETION DISKS; ANGULAR MOMENTUM; ASTRONOMY; ASTROPHYSICS; COMPUTERIZED SIMULATION; COUPLING; DWARF STARS; HERTZSPRUNG-RUSSELL DIAGRAM; HYDRODYNAMICS; LUMINOSITY; MASS; PROTOSTARS; STAR CLUSTERS; STAR EVOLUTION; STELLAR MAGNETOSPHERES; SURFACES; VARIATIONS

Citation Formats

Baraffe, I., Chabrier, G., and Vorobyov, E., E-mail: i.baraffe@ex.ac.uk, E-mail: eduard.vorobiev@univie.ac.at, E-mail: gilles.chabrier@ens-lyon.fr. OBSERVED LUMINOSITY SPREAD IN YOUNG CLUSTERS AND FU Ori STARS: A UNIFIED PICTURE. United States: N. p., 2012. Web. doi:10.1088/0004-637X/756/2/118.
Baraffe, I., Chabrier, G., & Vorobyov, E., E-mail: i.baraffe@ex.ac.uk, E-mail: eduard.vorobiev@univie.ac.at, E-mail: gilles.chabrier@ens-lyon.fr. OBSERVED LUMINOSITY SPREAD IN YOUNG CLUSTERS AND FU Ori STARS: A UNIFIED PICTURE. United States. https://doi.org/10.1088/0004-637X/756/2/118
Baraffe, I., Chabrier, G., and Vorobyov, E., E-mail: i.baraffe@ex.ac.uk, E-mail: eduard.vorobiev@univie.ac.at, E-mail: gilles.chabrier@ens-lyon.fr. 2012. "OBSERVED LUMINOSITY SPREAD IN YOUNG CLUSTERS AND FU Ori STARS: A UNIFIED PICTURE". United States. https://doi.org/10.1088/0004-637X/756/2/118.
@article{osti_22092402,
title = {OBSERVED LUMINOSITY SPREAD IN YOUNG CLUSTERS AND FU Ori STARS: A UNIFIED PICTURE},
author = {Baraffe, I. and Chabrier, G. and Vorobyov, E., E-mail: i.baraffe@ex.ac.uk, E-mail: eduard.vorobiev@univie.ac.at, E-mail: gilles.chabrier@ens-lyon.fr},
abstractNote = {The idea that non-steady accretion during the embedded phase of protostar evolution can produce the observed luminosity spread in the Herzsprung-Russell diagram (HRD) of young clusters has recently been called into question. Observations of FU Ori, for instance, suggest an expansion of the star during strong accretion events, whereas the luminosity spread implies a contraction of the accreting objects, decreasing their radiating surface. In this paper, we present a global scenario based on calculations coupling episodic accretion histories derived from numerical simulations of collapsing cloud prestellar cores of various masses and subsequent protostar evolution. Our calculations show that, assuming an initial protostar mass M{sub i} {approx} 1 M{sub Jup}, typical of the second Larson's core, both the luminosity spread in the HRD and the inferred properties of FU Ori events (mass, radius, accretion rate) can be explained by this scenario, providing two conditions. First, there must be some variation within the fraction of accretion energy absorbed by the protostar during the accretion process. Second, the range of this variation should increase with increasing accretion burst intensity and thus with the initial core mass and final star mass. The numerical hydrodynamics simulations of collapsing cloud prestellar cores indeed show that the intensity of the accretion bursts correlates with the mass and initial angular momentum of the prestellar core. Massive prestellar cores with high initial angular momentum are found to produce intense bursts characteristic of FU Ori-like events. Our results thus suggest a link between the burst intensities and the fraction of accretion energy absorbed by the protostar, with some threshold in the accretion rate, of the order of 10{sup -5} M{sub Sun} yr{sup -1}, delimitating the transition from 'cold' to 'hot' accretion. Such a transition might reflect a change in the accretion geometry with increasing accretion rate, i.e., a transition from magnetospheric or thin-disk to thick-disk accretion, or in the magnetospheric interaction between the star and the disk. Conversely, the luminosity spread can also be explained by a variation of the initial protostar mass within the {approx}1-5 M{sub Jup} range, although it is unclear for now whether such a spread among the second Larson's core can be produced during the prestellar core second collapse. This unified picture confirms the idea that early accretion during protostar and proto-brown dwarf formation/evolution can explain the observed luminosity spread in young clusters without invoking any significant age spread, and that the concept of a well-defined birthline does not apply for low-mass objects. Finally, we examine the impact of accretion on the determination of the initial mass function in young clusters.},
doi = {10.1088/0004-637X/756/2/118},
url = {https://www.osti.gov/biblio/22092402}, journal = {Astrophysical Journal},
issn = {0004-637X},
number = 2,
volume = 756,
place = {United States},
year = {Mon Sep 10 00:00:00 EDT 2012},
month = {Mon Sep 10 00:00:00 EDT 2012}
}