New evidence for weak magnetic fields in Herbig Ae/Be stars

Järvinen, S. P.; Carroll, T. A.; Hubrig, S.; Ilyin, I.; Schöller, M.

doi:10.1093/mnras/stz2190

Cited by 8 publications

(5 citation statements)

References 35 publications

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“…Nonetheless, other measurements of the same star suggested that this could indeed be one of the HAeBes with the strongest magnetic field of several kG (see [96] and references therein), which combined with uncertainties in stellar parameters and accretion rates could be potentially consistent with the minimum magnetic field necessary to drive MA. Finally, HD 190073 is a massive HBe star and the most recent measurements in Järvinen et al (2019) [91] confirm that its magnetic field is below ∼100 G, a factor >6 smaller than the value of B min estimated here and ruling out MA as the driving mechanism for this source.…”

Section: Is Magnetospheric Accretion Plausible For Herbig Ae/be Stars?supporting

confidence: 46%

“…Those strong magnetic fields are enough to sustain MA in CTTs (e.g., [20][21][22]). On the opposite, the vast majority of HAeBes show magnetic fields ≤ hundreds of G or below detection limits, as inferred from wide samples including dozens of sources (e.g., [86][87][88][89][90][91]). The contrast between both previous results is probably behind the strongest arguments against MA working in HAeBes, and it certainly motivates the question on whether this scenario is plausible for the HAeBe regime or not (see e.g., the corresponding discussions in [63,72,81,88,92]).…”

Section: Is Magnetospheric Accretion Plausible For Herbig Ae/be Stars?mentioning

confidence: 99%

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On the Mass Accretion Rates of Herbig Ae/Be Stars. Magnetospheric Accretion or Boundary Layer?

Mendigutía

2020

Galaxies

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Understanding how young stars gain their masses through disk-to-star accretion is of paramount importance in astrophysics. It affects our knowledge about the early stellar evolution, the disk lifetime and dissipation processes, the way the planets form on the smallest scales, or the connection to macroscopic parameters characterizing star-forming regions on the largest ones, among others. In turn, mass accretion rate estimates depend on the accretion paradigm assumed. For low-mass T Tauri stars with strong magnetic fields there is consensus that magnetospheric accretion (MA) is the driving mechanism, but the transfer of mass in massive young stellar objects with weak or negligible magnetic fields probably occurs directly from the disk to the star through a hot boundary layer (BL). The intermediate-mass Herbig Ae/Be (HAeBe) stars bridge the gap between both previous regimes and are still optically visible during the pre-main sequence phase, thus constituting a unique opportunity to test a possible change of accretion mode from MA to BL. This review deals with our estimates of accretion rates in HAeBes, critically discussing the different accretion paradigms. It shows that although mounting evidence supports that MA may extend to late-type HAes but not to early-type HBes, there is not yet a consensus on the validity of this scenario versus the BL one. Based on MA and BL shock modeling, it is argued that the ultraviolet regime could significantly contribute in the future to discriminating between these competing accretion scenarios.

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Section: Is Magnetospheric Accretion Plausible For Herbig Ae/be Stars?supporting

confidence: 46%

Section: Is Magnetospheric Accretion Plausible For Herbig Ae/be Stars?mentioning

confidence: 99%

On the Mass Accretion Rates of Herbig Ae/Be Stars. Magnetospheric Accretion or Boundary Layer?

Mendigutía

2020

Galaxies

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“…Stellar magnetic field measurements in pre-mainsequence stars do support a strong stellar mass dependence in the strength and evolution of magnetic fields, with weaker fields below ≈ 200 G in Herbig Ae/Be stars compared to kG in T Tauri stars (e.g. Alecian et al 2013;Villebrun et al 2019;Järvinen et al 2019). In a scenario where CO traces an inner disk wind that is MHD in nature, the internal stellar structure and duration of magnetic fields could be related to the T eff dependence of line shapes, where solar-mass, convective stars typically have triangular lines tracing an inner MHD wind supported by their stronger magnetic field, while (older) Herbig Ae/Be stars with their radiative structure do not support inner MHD winds and typically have Keplerian lines tracing the disk only.…”

Section: Why a T Eff Dependence For Line Shapes?mentioning

confidence: 99%

Scanning disk rings and winds in CO at 0.01-10 au: a high-resolution $M$-band spectroscopy survey with IRTF-iSHELL

Banzatti,

Abernathy,

Brittain

et al. 2022

Preprint

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We present an overview and first results from a M-band spectroscopic survey of planet-forming disks performed with iSHELL on IRTF, using two slits that provide resolving power R ≈ 60,000-92,000 (5-3.3 km/s). iSHELL provides a nearly complete coverage at 4.52-5.24 µm in one shot, covering > 50 lines from the R and P branches of 12 CO and 13 CO for each of multiple vibrational levels, and providing unprecedented information on the excitation of multiple emission and absorption components. Some of the most notable new findings of this survey are: 1) the detection of two CO Keplerian rings at < 2 au (in HD 259431), 2) the detection of H 2 O ro-vibrational lines at 5 µm (in AS 205 N), and 3) the common kinematic variability of CO lines over timescales of 1-14 years. By homogeneously analyzing this survey together with a previous VLT-CRIRES survey of cooler stars, we discuss a unified view of CO spectra where emission and absorption components scan the disk surface across radii from a dust-free region within dust sublimation out to ≈ 10 au. We classify two fundamental types of CO line shapes interpreted as emission from Keplerian rings (double-peak lines) and a disk surface plus a low-velocity part of a wind (triangular lines), where CO excitation reflects different emitting regions (and their gas-to-dust ratio) rather than just the irradiation spectrum. A disk+wind interpretation for the triangular lines naturally explains several properties observed in CO spectra, including the line blue-shifts, line shapes that turn into narrow absorption at high inclinations, and the frequency of disk winds as a function of stellar type.

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“…We consider field strengths B * ≈ 100 -300 G as have been reported for some HAeBe stars (e.g. Hubrig et al 2004Hubrig et al , 2013Bagnulo et al 2015;Järvinen et al 2019). Taking Ṁacc ∼ 10 −8 M ⊙ yr −1 and B * ≈ 100 G gives r i ≈ 1.3 R * , v s ≈ v f f = 341 km s −1 (ignoring centrifugal forces), T s = 1.6 MK, kT s = 0.14 keV, and log L acc = 32.6 ergs s −1 .…”

Section: Hd 100546 X-ray Emission Processesmentioning

confidence: 99%

X-Ray Emission and Disk Irradiation of HL Tau and HD 100546

Skinner

Güdel

2019

ApJ

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We present new X-ray observations of the optically-obscured protostar HL Tau and the intermediate mass Herbig Be star HD 100546. Both objects are surrounded by spectacular disks showing complex morphology including rings and gaps that may have been sculpted by protoplanets. HL Tau was detected as a variable hard X-ray source by Chandra, typical of late-type magnetically-active coronal sources. No extended X-ray emission was seen along the HL Tau jet, or along the jet of the T Tauri binary system XZ Tau located 23 ′′ to its east. In contrast, HD 100546 was detected by XMM-Newton as a soft X-ray source (kT < ∼ 1 keV) with no short-term (<1 day) variability. Its X-ray properties are remarkably similar to the Herbig stars AB Aur and HD 163296, strongly suggesting that their X-ray emission arises from the same mechanism and is intrinsic to the Herbig stars themselves, not due to unseen late-type companions. We consider several possible emission mechanisms and conclude that the X-ray properties of HD 100546 are consistent with an accretion shock origin, but higher resolution grating spectra capable of providing information on individual emission lines are needed to more reliably distinguish between accretion shocks and alternatives. We show that X-ray ionization and heating are mainly confined to the upper disk layers in both HL Tau and HD 100546, and any exoplanets near the midplane at distances ≥1 au are well-shielded from X-rays produced by the central star. 2The X-ray luminosity of low and intermediate mass PMS stars is typically in the range L x ∼ 10 28 -10 31 ergs s −1 , about 10 1 -10 4 times that of the quiet Sun. As young solar-like stars age and spin down, their X-ray luminosity declines (Güdel, Guinan, & Skinner 1997). Stellar X-rays heat and ionize disk gas and the gaseous atmospheres of any close-in protoplanets, and must be taken into account in realistic models of planet formation. X-ray ionization of disk gas also promotes the magnetorotational instability (Balbus & Hawley 1991) and can thereby affect accretion onto the central star.In magnetically-active class I protostars obscured by surrounding envelopes and optically-revealed T Tauri stars (TTS), very high plasma temperatures (T ∼ 100 MK) can be reached during powerful short-duration (minutes to hours) magnetic reconnection flares. In addition to flash-heating, such flares can produce high energy particles that bombard the inner disk and any close-in protoplanets (reviewed by Feigelson & Montmerle 1999; Feigelson 2010).The photoelectric cross section for absorption of X-rays by gas scales inversely with photon energy as σ ∝ E −p , where p ≈ 2.5 for solar-abundance gas. Soft X-rays (E < ∼ 1 keV) are thus heavily absorbed in outer disk layers, whereas harder X-rays with energies of several keV penetrate deeper and can potentially reach the disk midplane, as can cosmic rays. As a young star evolves and its inner disk gas disperses, close-in protoplanets become more heavily exposed to stellar X-rays. Detailed discussions of X-ray heating and ionization ...

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New evidence for weak magnetic fields in Herbig Ae/Be stars

Cited by 8 publications

References 35 publications

On the Mass Accretion Rates of Herbig Ae/Be Stars. Magnetospheric Accretion or Boundary Layer?

On the Mass Accretion Rates of Herbig Ae/Be Stars. Magnetospheric Accretion or Boundary Layer?

Scanning disk rings and winds in CO at 0.01-10 au: a high-resolution $M$-band spectroscopy survey with IRTF-iSHELL

X-Ray Emission and Disk Irradiation of HL Tau and HD 100546

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