scholarly journals An alternative approach to the relation between accretion luminosity and mass accretion rate

2021 ◽  
Author(s):  
Stefano Pezzuto
Keyword(s):  
Radiation Pressure ◽  
Accretion Rate ◽  
Free Fall ◽  
High Mass ◽  
Alternative Form ◽  
Mass Star ◽  
Practical Applications ◽  
Mass Accretion Rate ◽  
Alternative Approach ◽  
Accretion Rates

Abstract In this paper I introduce and discuss an alternative approach to the relation between accretion luminosity, Lacc, and mass accretion rate, ˙M : instead of the universally adopted Lacc = GM ˙M/R, I propose the dynamical definition Lacc = v2f˙M/2 where vf is the final velocity of the infalling matter at the surface of the accreting object of mass M and radius R. Both definitions are based on the energy conservation, but while the former assumes that matter is in free fall, the latter is valid always. By adopting the alternative form for Lacc, I show that the Eddington luminosity Led, when the outward radiation pressure wins on gravity, is never produced with a finite ˙M. Instead, Led is a limit asymptotically reached when ˙M → ¥. My argument is very simple, so I felt the need to find a possible explanation to why no one arrived to this conclusion before. To this aim, I give a brief presentation of the history of accretion, from the pioneer work of Hoyle and collaborators until the ’60s of last century, to show how the perception of the role of the radiation pressure in accretion evolved. I give also some practical applications of the formulae I derived, in the case of high-mass star formation and of the growth of super massive black holes. The study of these two processes, already complex per se, becomes more difficult to solve because of the existence of a limiting ˙M, named Eddington mass accretion rate or ˙Med, that it is supposed to generate a luminosity equal to Led, making it impossible to accrete at rate ˙M > ˙Med. Accretion rates higher than ˙Med are however necessary, as theory and observations show. My definition of Lacc takes naturally into account the work done by radiation pressure to slow down the infalling matter: as a consequence, Lacc does not increase linearly with ˙M and Led is only an asymptotic value.

scholarly journals Filamentary Flows and Clump-fed High-mass Star Formation in G22

2017 ◽  
Vol 13 (S336) ◽  
pp. 299-300 ◽  
Author(s):  
J. Yuan ◽  
J.-Z. Li ◽  
Y. Wu
Keyword(s):  
Star Formation ◽  
Accretion Rate ◽  
Free Fall ◽  
High Mass ◽  
Mass Star ◽  
The Core ◽  
Velocity Gradients ◽  
Molecular Core ◽  
Methanol Masers ◽  
Global Collapse

AbstractG22 is a hub-filament system composed of four supercritical filaments. Velocity gradients are detected along three filaments. A total mass infall rate of 700 M⊙ Myr−1 would double the hub mass in about three free-fall times. The most massive clump C1 would be in global collapse with an infall velocity of 0.26 km s−1 and a mass infall rate of 5 × 10−4M⊙ yr−1, which is supported by the prevalent HCO+ (3-2) and 13CO (3-2) blue profiles. A hot molecular core (SMA1) was revealed in C1. At the SMA1 center, there is a massive protostar (MIR1) driving multipolar outflows which are associated with clusters of class I methanol masers. MIR1 may be still growing with an accretion rate of 7 × 10−5M⊙ yr−1. Filamentary flows, clump-scale collapse, core-scale accretion coexist in G22, suggesting that high-mass starless cores may not be prerequisite to form high-mass stars. In the high-mass star formation process, the central protostar, the core, and the clump can grow in mass simultaneously.

scholarly journals Failed and delayed protostellar outflows with high-mass accretion rates

2020 ◽  
Vol 499 (3) ◽  
pp. 4490-4514
Author(s):  
Masahiro N Machida ◽  
Takashi Hosokawa
Keyword(s):  
Star Formation ◽  
Magnetic Fields ◽  
Accretion Rate ◽  
High Mass ◽  
Mass Star ◽  
Strong Magnetic Fields ◽  
Ram Pressure ◽  
Accretion Rates ◽  
Protostellar Outflows ◽  
Magnetohydrodynamic Simulations

ABSTRACT The evolution of protostellar outflows is investigated under different mass accretion rates in the range ∼10−5–$10^{-2}\, {\rm M}_\odot$ yr−1 with 3D magnetohydrodynamic simulations. A powerful outflow always appears in strongly magnetized clouds with $B_0 \gtrsim B_{\rm 0, cr}\, =10^{-4} (M_{\rm cl}/100\, {\rm M}_\odot)$ G, where Mcl is the cloud mass. When a cloud has a weaker magnetic field, the outflow does not evolve promptly with a high-mass accretion rate. In some cases with moderate magnetic fields B0 slightly smaller than B0, cr, the outflow growth is suppressed or delayed until the infalling envelope dissipates and the ram pressure around the protostellar system is significantly reduced. In such an environment, the outflow begins to grow and reaches a large distance only during the late accretion phase. On the other hand, the protostellar outflow fails to evolve and is finally collapsed by the strong ram pressure when a massive (≳ 100 M⊙) initial cloud is weakly magnetized with B0 ≲ 100 μG. The failed outflow creates a toroidal structure that is supported by magnetic pressure and encloses the protostar and disc system. Our results indicate that high-mass stars form only in strongly magnetized clouds, if all high-mass protostars possess a clear outflow. If we would observe either very weak or no outflow around evolved protostars, it means that strong magnetic fields are not necessarily required for high-mass star formation. In any case, we can constrain the high-mass star formation process from observations of outflows.

scholarly journals X-shooter survey of disk accretion in Upper Scorpius

2020 ◽  
Vol 639 ◽  
pp. A58 ◽  
Author(s):  
C. F. Manara ◽  
A. Natta ◽  
G. P. Rosotti ◽  
J. M. Alcalá ◽  
B. Nisini ◽  
...  
Keyword(s):  
Accretion Rate ◽  
Protoplanetary Disks ◽  
Young Stellar Objects ◽  
High Mass ◽  
Star Forming ◽  
Mass Accretion Rate ◽  
Disk Mass ◽  
Accretion Rates ◽  
Disk Evolution ◽  
Simple Models

Determining the mechanisms that drive the evolution of protoplanetary disks is a necessary step toward understanding how planets form. For this work, we measured the mass accretion rate for young stellar objects with disks at age > 5 Myr, a critical test for the current models of disk evolution. We present the analysis of the spectra of 36 targets in the ∼5–10 Myr old Upper Scorpius star-forming region for which disk masses were measured with ALMA. We find that the mass accretion rates in this sample of old but still surviving disks are similarly high as those of the younger (∼1−3 Myr old) star-forming regions of Lupus and Chamaeleon I, when considering the dependence on stellar and disk mass. In particular, several disks show high mass accretion rates ≳10−9 M⊙ yr−1 while having low disk masses. Furthermore, the median values of the measured mass accretion rates in the disk mass ranges where our sample is complete at a level ∼60−80% are compatible in these three regions. At the same time, the spread of mass accretion rates at any given disk mass is still > 0.9 dex, even at age > 5 Myr. These results are in contrast with simple models of viscous evolution, which would predict that the values of the mass accretion rate diminish with time, and a tighter correlation with disk mass at age > 5 Myr. Similarly, simple models of internal photoevaporation cannot reproduce the observed mass accretion rates, while external photoevaporation might explain the low disk masses and high accretion rates. A possible partial solution to the discrepancy with the viscous models is that the gas-to-dust ratio of the disks at ∼5–10 Myr is significantly different and higher than the canonical 100, as suggested by some dust and gas disk evolution models. The results shown here require the presence of several interplaying processes, such as detailed dust evolution, external photoevaporation, and possibly MHD winds, to explain the secular evolution of protoplanetary disks.

scholarly journals Disc kinematics and stability in high-mass star formation

2019 ◽  
Vol 632 ◽  
pp. A50 ◽  
Author(s):  
A. Ahmadi ◽  
R. Kuiper ◽  
H. Beuther
Keyword(s):  
Star Formation ◽  
Spatial Resolution ◽  
High Mass ◽  
Mass Star ◽  
Gas Temperature ◽  
Centrifugal Barrier ◽  
Gravitational Instabilities ◽  
Linear Resolution ◽  
Accretion Rates ◽  
Single Structure

Context. In the disc-mediated accretion scenario for the formation of the most massive stars, high densities and high accretion rates could induce gravitational instabilities in the disc, forcing it to fragment and produce companion objects. Aims. We investigate the effects of inclination and spatial resolution on the observable kinematics and stability of discs in high-mass star formation. Methods. We studied a high-resolution 3D radiation-hydrodynamic simulation that leads to the fragmentation of a massive disc. Using RADMC-3D we produced 1.3 mm continuum and CH3CN line cubes at different inclinations. The model was set to different distances, and synthetic observations were created for ALMA at ~80 mas resolution and NOEMA at ~0.4′′. Results. The synthetic ALMA observations resolve all fragments and their kinematics well. The synthetic NOEMA observations at 800 pc with linear resolution of ~300 au are able to resolve the fragments, while at 2000 pc with linear resolution of ~800 au only a single structure slightly elongated towards the brightest fragment is observed. The position–velocity (PV) plots show the differential rotation of material best in the edge-on views. A discontinuity is seen at a radius of ~250 au, corresponding to the position of the centrifugal barrier. As the observations become less resolved, the inner high-velocity components of the disc become blended with the envelope and the PV plots resemble rigid-body-like rotation. Protostellar mass estimates from PV plots of poorly resolved observations are therefore overestimated. We fit the emission of CH3CN (12K−11K) lines and produce maps of gas temperature with values in the range of 100–300 K. Studying the Toomre stability of the discs, we find low Q values below the critical value for stability against gravitational collapse at the positions of the fragments and in the arms connecting the fragments for the resolved observations. For the poorly resolved observations we find low Q values in the outskirts of the disc. Therefore, although we could not resolve any of the fragments, we are able to predict that the disc is unstable and fragmenting. This conclusion is valid regardless of our knowledge about the inclination of the disc. Conclusions. These synthetic observations reveal the potential and limitations of studying discs in high-mass star formation with current (millimetre) interferometers. While the extremely high spatial resolution of ALMA reveals objects in extraordinary detail, rotational structures and instabilities within accretion discs can also be identified in poorly resolved observations.

scholarly journals The maximum accretion rate of hot gas in dark matter haloes

2020 ◽  
Vol 492 (4) ◽  
pp. 6042-6058 ◽  
Author(s):  
Jonathan Stern ◽  
Drummond Fielding ◽  
Claude-André Faucher-Giguère ◽  
Eliot Quataert
Keyword(s):  
Accretion Rate ◽  
Formation Rate ◽  
Free Fall ◽  
Mass Relation ◽  
Physical Conditions ◽  
Accretion Rates ◽  
The Galaxy ◽  
Halo Masses ◽  
Mode Versus ◽  
Free Falling

ABSTRACT We revisit the question of ‘hot mode’ versus ‘cold mode’ accretion on to galaxies using steady-state cooling flow solutions and idealized 3D hydrodynamic simulations. We demonstrate that for the hot accretion mode to exist, the cooling time is required to be longer than the free-fall time near the radius where the gas is rotationally supported, Rcirc, i.e. the existence of the hot mode depends on physical conditions at the galaxy scale rather than on physical conditions at the halo scale. When allowing for the depletion of the halo baryon fraction relative to the cosmic mean, the longer cooling times imply that a virialized gaseous halo may form in halo masses below the threshold of $\sim 10^{12}\, {\rm M_{\odot }}$ derived for baryon-complete haloes. We show that for any halo mass there is a maximum accretion rate for which the gas is virialized throughout the halo and can accrete via the hot mode of ${\dot{M}}_{\rm crit}\approx 0.7(v_{\rm c}/100\, \rm km\ s^{-1})^{5.4}(R_{\rm circ}/10\, {\rm kpc})(Z/\, {\rm Z_{\odot }})^{-0.9}\, {\rm M_{\odot }}\, {\rm yr}^{-1}$, where Z and vc are the metallicity and circular velocity measured at Rcirc. For accretion rates $\gtrsim {\dot{M}}_{\rm crit}$ the volume-filling gas phase can in principle be ‘transonic’ – virialized in the outer halo but cool and free-falling near the galaxy. We compare ${\dot{M}}_{\rm crit}$ to the average star formation rate (SFR) in haloes at 0 < z < 10 implied by the stellar-mass–halo-mass relation. For a plausible metallicity evolution with redshift, we find that ${\rm SFR}\lesssim {\dot{M}}_{\rm crit}$ at most masses and redshifts, suggesting that the SFR of galaxies could be primarily sustained by the hot mode in halo masses well below the classic threshold of $\sim 10^{12}\, {\rm M_{\odot }}$.

scholarly journals The accretion rates and mechanisms of Herbig Ae/Be stars

2020 ◽  
Vol 493 (1) ◽  
pp. 234-249 ◽  
Author(s):  
C Wichittanakom ◽  
R D Oudmaijer ◽  
J R Fairlamb ◽  
I Mendigutía ◽  
M Vioque ◽  
...  
Keyword(s):  
Accretion Rate ◽  
High Mass ◽  
Be Stars ◽  
Alpha Emission ◽  
T Tauri ◽  
Accretion Model ◽  
Accretion Rates ◽  
Theoretical Considerations ◽  
Gaia Dr2 ◽  
Magnetospheric Accretion

ABSTRACT This work presents a spectroscopic study of 163 Herbig Ae/Be stars. Amongst these, we present new data for 30 objects. Stellar parameters such as temperature, reddening, mass, luminosity, and age are homogeneously determined. Mass accretion rates are determined from $\rm H\alpha$ emission line measurements. Our data is complemented with the X-Shooter sample from previous studies and we update results using Gaia DR2 parallaxes giving a total of 78 objects with homogeneously determined stellar parameters and mass accretion rates. In addition, mass accretion rates of an additional 85 HAeBes are determined. We confirm previous findings that the mass accretion rate increases as a function of stellar mass, and the existence of a different slope for lower and higher mass stars, respectively. The mass where the slope changes is determined tobe $3.98^{+1.37}_{-0.94}\, \rm M_{\odot }$. We discuss this break in the context of different modes of disc accretion for low- and high-mass stars. Because of their similarities with T Tauri stars, we identify the accretion mechanism for the late-type Herbig stars with the Magnetospheric Accretion. The possibilities for the earlier-type stars are still open, we suggest the Boundary Layer accretion model may be a viable alternative. Finally, we investigated themass accretion–age relationship. Even using the superior Gaia based data, it proved hard to select a large enough sub-sample to remove the mass dependence in this relationship. Yet, it would appear that the mass accretion does decline with age as expected from basic theoretical considerations.

scholarly journals Dynamics of cluster-forming hub-filament systems

2019 ◽  
Vol 629 ◽  
pp. A81 ◽  
Author(s):  
S. P. Treviño-Morales ◽  
A. Fuente ◽  
Á. Sánchez-Monge ◽  
J. Kainulainen ◽  
P. Didelon ◽  
...  
Keyword(s):  
Column Density ◽  
Accretion Rate ◽  
Dust Emission ◽  
Free Fall ◽  
High Mass ◽  
Hii Region ◽  
Velocity Gradients ◽  
Density Maps ◽  
The Individual ◽  
Main Filament

Context. High-mass stars and star clusters commonly form within hub-filament systems. Monoceros R2 (hereafter Mon R2), at a distance of 830 pc, harbors one of the closest of these systems, making it an excellent target for case studies. Aims. We investigate the morphology, stability and dynamical properties of the Mon R2 hub-filament system. Methods. We employed observations of the 13CO and C18O 1 →0 and 2 →1 lines obtained with the IRAM-30 m telescope. We also used H2 column density maps derived from Herschel dust emission observations. Results. We identified the filamentary network in Mon R2 with the DisPerSE algorithm and characterized the individual filaments as either main (converging into the hub) or secondary (converging to a main filament). The main filaments have line masses of 30–100 M⊙ pc−1 and show signs of fragmentation, while the secondary filaments have line masses of 12–60 M⊙ pc−1 and show fragmentation only sporadically. In the context of Ostriker’s hydrostatic filament model, the main filaments are thermally supercritical. If non-thermal motions are included, most of them are transcritical. Most of the secondary filaments are roughly transcritical regardless of whether non-thermal motions are included or not. From the morphology and kinematics of the main filaments, we estimate a mass accretion rate of 10−4–10−3 M⊙ yr−1 into the central hub. The secondary filaments accrete into the main filaments at a rate of 0.1–0.4 × 10−4 M⊙ yr−1. The main filaments extend into the central hub. Their velocity gradients increase toward the hub, suggesting acceleration of the gas. We estimate that with the observed infall velocity, the mass-doubling time of the hub is ~2.5 Myr, ten times longer than the free-fall time, suggesting a dynamically old region. These timescales are comparable with the chemical age of the HII region. Inside the hub, the main filaments show a ring- or a spiral-like morphology that exhibits rotation and infall motions. One possible explanation for the morphology is that gas is falling into the central cluster following a spiral-like pattern.

scholarly journals Thermal state of transiently accreting neutron stars with additional heating beyond deep crustal heating

2020 ◽  
Vol 2020 (4) ◽  
Author(s):  
Helei Liu ◽  
Masa-aki Hashimoto ◽  
Guoliang Lü ◽  
Yasuhide Matsuo ◽  
Dehua Wen ◽  
...  
Keyword(s):  
Neutron Star ◽  
Heat Source ◽  
Neutron Stars ◽  
High Temperatures ◽  
Accretion Rate ◽  
Thermal State ◽  
X Ray ◽  
Mass Accretion Rate ◽  
Accretion Rates ◽  
Theoretical Predictions

Abstract As some neutron star transients require an additional unknown heat source (referred to as “shallow heating”) to explain their high temperatures at the beginning of quiescence, we investigate the effect of shallow heating as well as compressional heating on the thermal state of transiently accreting neutron stars with the use of evolutionary calculations in the present work. Through comparing our theoretical predictions of the equilibrium redshifted luminosities $(L_{\gamma}^{\infty})$ produced by both deep crustal heating and shallow heating/compressional heating for different time-averaged mass-accretion rates $\langle\dot{M}\rangle$ with 35 updated observations of soft X-ray transients, the results show that both shallow heating and compressional heating make significant contributions to the equilibrium redshifted luminosity. The hotter sources (XTE J1701, MAXI J0556, EXO 0748, Aql X-1 etc.) with higher accretion rates are more likely to be explained with the effect of shallow heating or compressional heating. In addition, for a proper shallow heat $q_\mathrm{sh}$ and mass-accretion rate $\dot{M}$, the effect of shallow heating could be simulated by compressional heating.

scholarly journals Resolved millimeter-dust continuum cavity around the very low mass young star CIDA 1

2018 ◽  
Vol 615 ◽  
pp. A95 ◽  
Author(s):  
Paola Pinilla ◽  
Antonella Natta ◽  
Carlo F. Manara ◽  
Luca Ricci ◽  
Aleks Scholz ◽  
...  
Keyword(s):  
Accretion Rate ◽  
Circumstellar Disks ◽  
Young Star ◽  
High Mass ◽  
Close Binary ◽  
Mass Star ◽  
Typical Characteristic ◽  
Gaussian Profile ◽  
Low Mass ◽  
Very High

Context. Transition disks (TDs) are circumstellar disks with inner regions highly depleted in dust. TDs are observed in a small fraction of disk-bearing objects at ages of 1–10 Myr. They are important laboratories to study evolutionary effects in disks, from photoevaporation to planet–disk interactions. Aim. We report the discovery of a large inner dust-empty region in the disk around the very low mass star CIDA 1 (M⋆ ~ 0.1−0.2 M⊙). Methods. We used ALMA continuum observations at 887 µm, which provide a spatial resolution of 0″.21 × 0″.12 (~15 × 8 au in radius at 140 pc). Results. The data show a dusty ring with a clear cavity of radius ~20 au, the typical characteristic of a TD. The emission in the ring is well described by a narrow Gaussian profile. The dust mass in the disk is ~17 M⊕. CIDA 1 is one of the lowest mass stars with a clearly detected millimeter cavity. When compared to objects of similar stellar mass, it has a relatively massive dusty disk (less than ~5% of Taurus Class II disks in Taurus have a ratio of Mdisk/M⋆ larger than CIDA 1) and a very high mass accretion rate (CIDA 1 is a disk with one of the lowest values of Mdisk/Ṁ ever observed). In light of these unusual parameters, we discuss a number of possible mechanisms that can be responsible for the formation of the dust cavity (e.g. photoevaporation, dead zones, embedded planets, close binary). We find that an embedded planet of a Saturn mass or a close binary are the most likely possibilities.

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