An X‐shooter survey of star forming regions: Low‐mass stars and sub‐stellar objects

Manara

Natta

et al. 2017

A&A

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427

698

The mass accretion rate,Ṁ acc , is a key quantity for the understanding of the physical processes governing the evolution of accretion discs around young low-mass (M 2.0M ) stars and substellar objects (YSOs). We present here the results of a study of the stellar and accretion properties of the (almost) complete sample of class II and transitional YSOs in the Lupus I, II, III and IV clouds, based on spectroscopic data acquired with the VLT/X-Shooter spectrograph. Our study combines the dataset from our previous work with new observations of 55 additional objects. We have investigated 92 YSO candidates in total, 11 of which have been definitely identified with giant stars unrelated to Lupus. The stellar and accretion properties of the 81 bona fide YSOs, which represent more than 90% of the whole class II and transition disc YSO population in the aforementioned Lupus clouds, have been homogeneously and self-consistently derived, allowing for an unbiased study of accretion and its relationship with stellar parameters. The accretion luminosity, L acc , increases with the stellar luminosity, L , with an overall slope of ∼1.6, similar but with a smaller scatter than in previous studies. There is a significant lack of strong accretors below L ≈0.1L , where L acc is always lower than 0.01 L . We argue that the L acc -L slope is not due to observational biases, but is a true property of the Lupus YSOs. The logṀ acclogM correlation shows a statistically significant evidence of a break, with a steeper relation for M 0.2 M and a flatter slope for higher masses. The bimodality of theṀ acc -M relation is confirmed with four different evolutionary models used to derive the stellar mass. The bimodal behaviour of the observed relationship supports the importance of modelling self-gravity in the early evolution of the more massive discs, but other processes, such as photo-evaporation and planet formation during the YSO's lifetime, may also lead to disc dispersal on different timescales depending on the stellar mass. The sample studied here more than doubles the number of YSOs with homogeneously and simultaneously determined L acc and luminosity, L line , of many permitted emission lines. Hence, we also refined the empirical relationships between L acc and L line on a more solid statistical basis.

Section: Samplementioning

confidence: 99%

X-shooter spectroscopy of young stellar objects in Lupus

Manara

Natta

et al. 2017

A&A

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427

698

“…The X-Shooter observations presented in this paper were conducted on April 17 2012, on the VLT as part of the INAF X-Shooter guaranteed time observations (GTO) program on star forming regions (Alcalá et al 2011). The single-node exposure time was 850 s, yielding a nominal exposure time of almost 2 h after 2 (ABBA) cycles.…”

Section: Observationsmentioning

confidence: 99%

Accretion-ejection connection in the young brown dwarf candidate ISO-ChaI 217

Whelan

Bacciotti

et al. 2014

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As the number of observed brown dwarf outflows is growing it is important to investigate how these outflows compare to the wellstudied jets from young stellar objects. A key point of comparison is the relationship between outflow and accretion activity and in particular the ratio between the mass outflow and accretion rates (Ṁ out /Ṁ acc ). The brown dwarf candidate ISO-ChaI 217 was discovered by our group, as part of a spectro-astrometric study of brown dwarfs, to be driving an asymmetric outflow with the blueshifted lobe having a position angle of ∼20 • . The aim here is to further investigate the properties of ISO-ChaI 217, the morphology and kinematics of its outflow, and to better constrainṀ out /Ṁ acc . The outflow is spatially resolved in the [S ] λλ6716, 6731 lines and is detected out to ∼1. 6 in the blue-shifted lobe and 1 in the red-shifted lobe. The asymmetry between the two lobes is confirmed although the velocity asymmetry is less pronounced with respect to our previous study. Using thirteen different accretion tracers we measure log (Ṁ acc ) [M /yr] = −10.6 ± 0.4. As it was not possible to measure the effect of extinction on the ISO-ChaI 217 outflowṀ out was derived for a range of values of A v , up to a value of A v = 2.5 mag estimated for the source extinction. The logarithm of the mass outflow (Ṁ out ) was estimated in the range −11.7 to −11.1 for both jets combined. ThusṀ out /Ṁ acc [M /yr] lies below the maximum value predicted by magneto-centrifugal jet launching models. Finally, both model fitting of the Balmer decrements and spectro-astrometric analysis of the Hα line show that the bulk of the H I emission comes from the accretion flow.

“…The data were acquired within the INAF/GTO time (Alcalá et al 2011). With its three spectrograph arms, X-Shooter provides simultaneous wavelength coverage in the spectral range 3000-24 800 Å. Slit widths of 0.5 /0.4 /0.4 were used in the UVB/VIS/NIR arms, respectively, yielding spectral resolutions of 9100/17400/11300.…”

Section: X-shooter Data Analysismentioning

confidence: 99%

The ultracool dwarf DENIS-P J104814.7-395606

Stelzer

Biazzo

et al. 2012

A&A

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Context. Several diagnostics ranging from the radio to the X-ray band are suitable for investigating the magnetic activity of late-type stars. Empirical connections between the emission at different wavelengths place constraints on the nature and efficiency of the emission mechanism and the physical conditions in different atmospheric layers. The activity of ultracool dwarfs, at the low-mass end of the main-sequence, is poorly understood. Aims. We perform a multi-wavelength study of one of the nearest M9 dwarfs, DENIS-P J104814.7-395606 (4 pc), to examine its position within the group of magnetically active ultracool dwarfs, and, in general, advance our understanding of these objects by comparing them to early-M type dwarf stars and the Sun. Methods. We obtained an XMM-Newton observation of DENIS-P J104814.7-395606 and a broad-band spectrum from the ultraviolet to the near-infrared with X-Shooter. From this dataset, we derive the X-ray properties, stellar parameters, kinematics, and the emission-line spectrum tracing chromospheric activity. We integrate these data by compiling the activity parameters of ultracool dwarfs from the literature. Results. Our deep XMM-Newton observation provides the first X-ray detection of DENIS-P J104814.7-395606 (log L x = 25.1), as well as the first measurement of its V band brightness (V = 17.35 mag). The flux-flux relations between X-ray and chromospheric activity indicators are here for the first time extended into the regime of the ultracool dwarfs. The approximate agreement of DENIS-P J104814.7-395606 and other ultracool dwarfs with flux-flux relations for early-M dwarfs suggests that the same heating mechanisms work in the atmospheres of ultracool dwarfs, albeit weaker as judged from their lower fluxes. The observed Balmer decrements of DENIS 1048-3956 are compatible with optically thick plasma in local thermal equilibrium (LTE) at low, nearly photospheric temperature or optically thin LTE plasma at 20 000 K. Describing the decrements with case B recombination requires different emitting regions for Hα and the higher Balmer lines. The high observed Hα/Hβ flux ratio is also poorly fitted by the optically thin models. We derive a similarly high value for the Hα/Hβ ratio of vB 10 and LHS 2065 and conclude that this may be a characteristic of ultracool dwarfs. We add DENIS-P J104814.7-395606 to the list of ultracool dwarfs detected in both the radio and the X-ray band. The Benz-Güdel relation between radio and X-ray luminosity of late-type stars is well-known to be violated by ultracool dwarfs. We speculate on the presence of two types of ultracool dwarfs with distinct radio and X-ray behaviors.