The magnetic obliquity of accreting T Tauri stars

Classical T Tauri stars (CTTS) accrete material from their discs through their magnetospheres. The geometry of the accretion flow strongly depends on the magnetic obliquity, i.e., the angle between the rotational and magnetic axes. We aim at deriving the distribution of magnetic obliquities in a sample of 10 CTTSs. For this, we monitored the radial velocity variations of the HeI$\lambda$5876 line in these stars' spectra along their rotational cycle. HeI is produced in the accretion shock, close to the magnetic pole. When the magnetic and rotational axes are not aligned, the radial velocity of this line is modulated by stellar rotation. The amplitude of modulation is related to the star's projected rotational velocity, $v\sin i$, and the latitude of the hotspot. By deriving $v\sin i$ and HeI$\lambda$5876 radial velocity curves from our spectra we thus obtain an estimate of the magnetic obliquities. We find an average obliquity in our sample of 11.4$^{\circ}$ with an rms dispersion of 5.4$^{\circ}$. The magnetic axis thus seems nearly, but not exactly aligned with the rotational axis in these accreting T Tauri stars, somewhat in disagreement with studies of spectropolarimetry, which have found a significant misalignment ($\gtrsim 20^{\circ}$) for several CTTSs. This could simply be an effect of low number statistics, or it may be due to a selection bias of our sample. We discuss possible biases that our sample may be subject to. We also find tentative evidence that the magnetic obliquity may vary according to the stellar interior and that there may be a significant difference between fully convective and partly radiative stars.Comment: 28 pages (including online material), 25 figure

Investigating episodic accretion in a very low-mass young stellar object

Very low-mass Class I protostars have been investigated very little thus far. Variability of these young stellar objects (YSOs) and whether or not they are capable of strong episodic accretion is also left relatively unstudied. We investigate accretion variability in IRS54, a Class I very low-mass protostar with a mass of M$_{\star}$ ~ 0.1 - 0.2 M$_{\odot}$. We obtained spectroscopic and photometric data with VLT/ISAAC and VLT/SINFONI in the near-infrared ($J$, $H$, and $K$ bands) across four epochs (2005, 2010, 2013, and 2014). We used accretion-tracing lines (Pa$\beta$ and Br$\gamma$) and outflow-tracing lines (H$_2$ and [FeII] to examine physical properties and kinematics of the object. A large increase in luminosity was found between the 2005 and 2013 epochs of more than 1 magnitude in the $K$ band, followed in 2014 by a steep decrease. Consistently, the mass accretion rate ($\dot{M}_{acc}$) rose by an order of magnitude from ~ 10$^{-8}$ M$_{\odot}$ yr$^{-1}$ to ~ $10^{-7}$ M$_{\odot}$ yr$^{-1}$ between the two early epochs. The visual extinction ($A_V$) has also increased from ~ 15 mag in 2005 to ~ 24 mag in 2013. This rise in $A_V$ in tandem with the increase in $\dot{M}_{acc}$ is explained by the lifting up of a large amount of dust from the disc of IRS54, following the augmented accretion and ejection activity in the YSO, which intersects our line of sight due to the almost edge-on geometry of the disc. Because of the strength and timescales involved in this dramatic increase, this event is believed to have been an accretion burst possibly similar to bursts of EXor-type objects. IRS54 is the lowest mass Class I source observed to have an accretion burst of this type, and therefore potentially one of the lowest mass EXor-type objects known so far

Inner disk dynamics of classical T Tauri stars in NGC 2264

Exportado OPUSMade available in DSpace on 2019-08-14T10:21:18Z (GMT). No. of bitstreams: 1 tese_pauline_mcginnis_final.pdf: 34225723 bytes, checksum: 484e3f4905ffb2c9f0bf66f30497331b (MD5) Previous issue date: 31O aglomerado jovem NGC 2264 é uma região de formação estelar rica, contendo centenas de estrelas jovens de baixa massa com indícios de acreção, chamadas de estrelas T Tauri clássicas (ETTC). Em Dezembro de 2011, este aglomerado foi observado simultaneamente com os satélites CoRoT e Spitzer, assim como quase-simultaneamente com vários telescópios no solo, como o CFHT e o VLT, usando o espectrógrafo de multi-objetos FLAMES na faixa espectral que cobre a linha H (forte indicador de acreção). NGC 2264 também já foi observado previamente com o CoRoT em Março de 2008 e foi alvo de uma nova campanha do FLAMES em 2014, centrada na linha de emissão proibida [OI] 6300Å, conhecida por traçar jatos e ventos em estrelas jovens. O objetivo desta tese é aproveitar da riqueza de dados disponíveis de NGC 2264 para investigar a dinâmica da parte interna de discos circumestelares de ETTCs e, com isso, buscar entender melhor os processos de acreção e ejeção de massa que ali ocorrem. Na primeira parte deste trabalho, investigamos um grupo de ETTCs que apresentam variabilidade fotométrica devido à extinção por material da parte interna do disco. Estes sistemas são observados com altas inclinações e representam uma oportunidade de estudarmos a interação entre a magnetosfera estelar e a parte interna do disco de acreção através de medidas indiretas. Analisamos as curvas de luz simultâneas do CoRoT e do Spitzer destes sistemas para investigarmos a distribuição de poeira na região interna do disco e encontramos indícios da presença de grãos maiores que grãos interestelares típicos. Analisamos a variabilidade de velamento nos espectros FLAMES e no índice de cor u - r, encontrando em vários casos evidência de manchas quentes associadas a ocultações da fotosfera por material circum-estelar, o que indica que as estruturas responsáveis pelas ocultações podem estar localizadas na base de colunas de acreção. Um modelo de ocultação por uma deformação na parte interna do disco, proposto inicialmente para explicar o comportamento fotométrico da ETTC AA Tau, conseguiu reproduzir os aspectos gerais da maioria das curvas de luz quase periódicas da nossa amostra, encontrando deformações de altura máxima e extensão azimutal semelhantes àquela que foi encontrada para AA Tau. Atribuímos a variabilidade do tipo de AA Tau a um regime de acreção estável e variabilidade semelhante, porém não-periódica, a um regime de acreção instável. Vemos indícios de que uma transição, e até mesmo certa coexistência, entre os dois regimes pode ser comum. Na segunda parte deste trabalho, investigamos os processos de ejeção através do estudo da linha de emissão proibida de [OI] 6300Å. Separamos a linha de [OI] em três componentes: uma componente de alta velocidade (CAV), uma componente larga de baixa velocidade (CLBV) e uma componente estreita de baixa velocidade (CEBV). Acredita-se que a CAV seja originada em jatos bipolares colimados, porém a origem das componentes de baixa velocidade (CBV) ainda está em debate. Nossas CLBV mostram desvios para o azul consistentes com uma origem em ventos da parte interna do disco. Já foi sugerido que a CEBV seja formada em ventos de fotoevaporação causados pela emissão de raios-X ou radiação UV extrema provenientes do objeto central. Isso é consistente com alguns dos nosso resultados, porém não há evidência suciente para conrmar este fenômeno. Mostramos que as luminosidades de todas as componentes da linha de [OI] correlacionam positivamente com a luminosidade estelar e de acreção. Também vemos que ETTCs que sofrem acreção no regime instável tendem a apresentam CBV mais largas e CAV mais fortes que aquelas que sofrem acreção no regime estável. Detectamos a CAV apenas entre sistemas com discos internos opticamente espessos, enquanto que a CLBV é encontrada entre alguns sistemas com discos anêmicos e a CEBV é comum entre sistemas com discos internos nos. A razão entre o número de CEBV e CLBV detectadas parece reetir a idade da região de formação estelar, indicando que a CEBV tende a dominar cada vez mais a emissão de [OI] com a evolução do sistema. Também mostramos que propriedades estelares como temperatura efetiva, massa ou rotação estelar não parecem inuenciar fortemente a velocidade de propagação ou luminosidade dos jatos, sugerindo que possa ser o disco, e não a estrela, que controla o mecanismo de lançamento dos mesmos.The young open cluster NGC 2264 is a rich site of star formation, containing hundreds of young, accreting, low-mass stars known as classical T Tauri stars (CTTS). In December 2011, this cluster was observed simultaneously by the CoRoT and Spitzer satellites, as well as quasi-simultaneously by a number of ground-based telescopes, such as the CFHT and the VLT with the FLAMES multi-object spectrograph in a spectral range covering the H line (a strong indicator of accretion). NGC 2264 had also been observed previously by CoRoT in March 2008, and it became the target of a new FLAMES campaign in 2014 centered on the forbidden [OI] 6300Å line, a well-known tracer of jets and winds in young stars. The goal of this thesis is to explore the richness of data available for NGC 2264 in order to investigate the dynamics of inner circumstellar disks of CTTSs and, with this, to better understand the processes of mass accretion and mass ejection that occur therein. In the rst part of this work, we investigate a group of CTTSs that present photometric variability indicative of extinction from inner disk material. These systems are viewed at high inclinations and represent an opportunity to study the interaction between the stellar magnetosphere and the inner accretion disk through indirect measurements. We analyze their CoRoT light curves alongside their simultaneous Spitzer light curves in order to better understand the dust distribution in the inner disk region, and in this way nd indications of the presence of grains larger than typical interstellar grains. We investigate veiling variability in their FLAMES spectra and u¿¿r color variations, and in many cases nd evidence of hot spots that are spatially associated with occultations of the stellar photosphere by circumstellar material, indicating that the occulting structures may be located at the base of accretion columns. We nd that an occultation model of an inner disk warp, initially proposed to explain the photometric behavior of the CTTS AA Tau, is successful at reproducing the general aspects of most of the quasi-periodic light curves in our sample, nding warps of similar maximum height and azimuthal extension as had been found for AA Tau. We ascribe this AA Tau-like variability to a stable accretion regime and similar but non-periodic variability to an unstable accretion regime. We see indications that a transition, and even certain coexistence, between the two regimes may be common. In the second part of this work we investigate ejection processes through the forbidden [OI] 6300Å emission line. We separate the [OI] line proles into three distinct features: a high-velocity component (HVC), a broad low-velocity component (BLVC), and a narrow low-velocity component (NLVC). The HVC is believed to originate in bipolar, colimated jets, while the origin of the low-velocity components (LVC) is still under debate. Our BLVCs show blueshifts consistent with an origin in inner disk winds. The NLVC has been suggested to arise in X-ray or EUV driven photoevaporative disk winds, which is consistent with some of our ndings, but we have insucient evidence to conrm this. We nd that the luminosities of all components correlate positively with the stellar and accretion luminosity. We also nd that CTTSs accreting in the unstable regime tend to show broader LVCs and stronger HVCs than stars accreting in the stable regime. We only detect the HVC among systems with optically thick inner disks, while the BLVC is found in a few systems with an anemic disk and the NLVC is common among systems with thin inner disks. The ratio of detected NLVCs to BLVCs seems to reect the age of the star forming region, indicating that as a system evolves, the NLVC tends to dominate the [OI] emission. We also show that stellar properties such as eective temperature, mass, and stellar rotation, do not seem to inuence in a signicant way the propagation velocity or luminosity of protostellar jets, and it appears that it is the disk, rather than the star, that governs their launching mechanism

Accretion dynamics and star-disk interaction in NCG 2264

International audienc