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The Accretion Rates and Mechanisms of Herbig Ae/Be Stars 1

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The Accretion Rates and Mechanisms of Herbig Ae/Be Stars 1 The accretion rates and mechanisms of Herbig Ae/Be stars 1 The accretion rates and mechanisms of Herbig Ae/Be stars Downloaded from https://academic.oup.com/mnras/advance-article-abstract/doi/10.1093/mnras/staa169/5709940 by University of Leeds user on 29 January 2020 C. Wichittanakom,1;2? R. D. Oudmaijer,2 J. R. Fairlamb,3 I. Mendigut´ıa,4 M. Vioque,2 and K. M. Ababakr5 1Department of Physics, Faculty of Science and Technology, Thammasat University, Rangsit Campus, Pathum Thani 12120, Thailand 2School of Physics and Astronomy, EC Stoner Building, University of Leeds, Leeds LS2 9JT, UK 3Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI, 96822, USA 4Centro de Astrobiolog´ıa(CSIC-INTA), Departamento de Astrof´ısica, ESA-ESAC Campus, PO Box 78, 28691 Villanueva de la Ca~nada, Madrid, Spain 5Erbil Polytechnic University, Kirkuk Road, Erbil, Iraq Accepted 2020 January 07. Received 2020 January 01; in original form 2019 October 09 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 Hα 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 +1:37 mass stars respectively. The mass where the slope changes is determined to be 3:98−0:94 M . We discuss this break in the context of different modes of disk 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 the mass accretion - age relationship. Even using the superior Gaia based data, it proved hard to select a large enough sub-sample to remove the mass dependency in this relationship. Yet, it would appear that the mass accretion does decline with age as expected from basic theoretical considerations. Key words: accretion, accretion discs { stars: formation { stars: fundamental param- eters { stars: pre-main-sequence { stars: variables: T Tauri, Herbig Ae/Be { techniques: spectroscopic 1 INTRODUCTION A long standing problem has also been that their brightness is very high, such that radiation pressure can in principle Herbig Ae/Be stars (HAeBes) are optically visible interme- stop accretion onto the stellar surface (Kahn 1974). More- diate pre-main sequence (PMS) stars whose masses range over, they form very quickly and reach the main sequence from about 2 to 10 M . These PMS stars were first identi- before their surrounding cloud disperses (Palla & Stahler fied by Herbig(1960), using three criteria: \Stars with spec- 1993). This suggests that their evolutionary processes are tral type A and B with emission lines; lie in an obscured very different from T Tauri stars. region; and illuminate fairly bright nebulosity in its imme- The accretion onto classical T Tauri low-mass stars is diate vicinit". HAeBes play an important role in understand- magnetically controlled. The magnetic field from the star ing massive star formation, because they bridge the gap be- truncates material in the disc and from this point, material tween low-mass stars whose formation is relatively well un- falls onto the star along the field line. After matter hits the derstood, and high-mass stars whose formation is still un- photosphere, it produces X-ray radiation that is absorbed by clear. The study of their formation is not well understood as surrounding particles. Then, these particles heat up and re- massive stars are very rare and as a consequence on average radiate at ultraviolet wavelengths, producing an UV-excess far away, and often optically invisible (Lumsden et al. 2013). that can be observed and from which the accretion luminos- ity can be calculated (Bouvier et al. 2007; Hartmann et al. 2016). It was found later, that a correlation between the ? E-mail: [email protected] (CW) line luminosity and the accretion luminosity exists, allowing 2 C. Wichittanakom et al. accretion rates of classical T Tauri stars to be determined ing the distances determined from the Gaia DR2 parallaxes from the UV-excess and absorption line veiling (Calvet et al. and a redetermined extinction towards the objects. As such, 2004; Ingleby et al. 2013). this becomes the largest homogeneous spectroscopic anal- Downloaded from https://academic.oup.com/mnras/advance-article-abstract/doi/10.1093/mnras/staa169/5709940 by University of Leeds user on 29 January 2020 HAeBes have similar properties as classical T ysis of HAeBes to date. In addition, the accretion rates of Tauri stars, for instance having emission lines, UV- HAeBes in the V18 study are determined for the 104 objects excess, a lower surface gravity than main-sequence stars for which Hα emission line equivalent widths are collected (Hamann & Persson 1992; Vink et al. 2005) and they are from the literature or determined from archival spectra. usually identified by an infrared (IR) excess from circumstel- The structure of this paper is as follows. Section 2 lar discs (Van den Ancker et al. 2000; Meeus et al. 2001). presents the spectroscopic data observation of all targets. Because their envelopes are radiative, no magnetic field Details of observations, instrumental setups and reduction is expected to be generated in HAeBe stars as this usu- procedures are given in this section. Section 3 and 4 detail ally happens in stars by convection. Indeed, magnetic fields the determination of stellar parameters and mass accretion have rarely been detected towards them (Catala et al. 2007; rates. Sections 5 and 6 focus on the analysis and a discus- Alecian et al. 2013). As a result magnetically controlled ac- sion respectively. Section 7 provides a summary of the main cretion is not necessarily expected to apply in the case of conclusions of this paper. the most massive objects, requiring another accretion mech- anism. However, how the material arrives at the stellar sur- face in the absence of magnetic fields is still unclear. 2 OBSERVATIONS AND DATA REDUCTION Several lines of evidence have indicated a difference be- 2.1 IDS and sample selection tween the lower mass Herbig Ae and higher mass Herbig Be stars. Spectropolarimetric studies suggest that Herbig The data were collected in June 2013 using the Intermediate Ae stars and T Tauri stars may form in the same process Dispersion Spectrograph (IDS) instrument and the RED+2 (Vink et al. 2005; Ababakr et al. 2017). Accretion rates of CCD detector with 2048 × 4096 pixels (pixel size 24 µm), HAeBes are determined from the measurement of UV-excess which is attached to the Cassegrain focus of the 2.54-m Isaac (Mendigut´ıa et al. 2011b) who have found that the relation- Newton Telescope (INT) at the Observatorio del Roque de ship between the line luminosity and the accretion luminos- los Muchachos, La Palma, Spain. The observations spanned ity is similar to the classical T Tauri stars. A spectroscopic six nights between the 20th and the 25th June 2013. Bias variability study suggests that a Herbig Ae star is undergo- frames, flat field frames, object frames and arc frames of ing magnetospheric accretion in the same manner as classical Cu-Ar and Cu-Ne comparison lamp were taken each night T Tauri stars (Sch¨oller et al. 2016) while Mendigut´ıa et al. to prepare for the data reduction. (2011a) find the Herbig Be stars to have different Hα vari- In the first two nights the spectrograph was set up with ability properties than the Herbig Ae stars. Therefore, mag- a 1200 lines mm−1 R1200B diffraction grating and a 0.9 or netically influenced accretion in HAeBes would still be pos- 1.0 arcsecond wide slit in order to obtain spectra across the sible. A study employing X-shooter spectra for 91 HAeBes Balmer discontinuity. The wavelength coverage was 3600{ was carried out by Fairlamb et al.(2015, 2017, hereafter F15 4600 A,˚ centred at 4000.5 A.˚ In this range the hydrogen lines and F17 respectively). They determined the stellar param- of Hγ;Hδ, H(7-2), H(8-2), H(9-2) and H(10-2) can be anal- eters in a homogenous fashion, derived mass accretion rates ysed. This combination provides a reciprocal dispersion of from the UV-excess and found the relationship between ac- 0.53 A˚ pixel−1 with a spectral resolution of ∼ 1 A˚ and a re- cretion luminosity and line luminosity for 32 emission lines solving power of R ∼ 4000. in the range 0.4{2.4 µm. 47 objects in their sample were For the next two nights the R1200Y grating was used to taken from Th´eet al.(1994, table 1), which to this date observe spectra in the visible. Its spectral range covers 5700 contains the strongest (candidate) members of the group. to 6700 A,˚ centred at 6050.4 A.˚ This range includes the He i ˚ ˚ The Fairlamb papers were published before the Gaia par- line at 5876 A, the [O iλ6300] line and the Hα line at 6562 A. allaxes became available, and the distances used may need This setup results in a reciprocal dispersion of 0.52 A˚ pixel−1 revision as these can affect parameters such as the radius, and a resolving power of R ∼ 6500.
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