Context Classical T Tauri Stars (CTTSs) are young low-mass stellar objects that accrete mass from their circumstellar disks. The disks extend internally up to the truncation radius, where the mag- netic field is strong enough to lift up the material from the disk plane and to funnel the mate- rial forming accretion columns (Koenigl 1991). The funneled plasma falls down onto the star and hits the stellar surface. The impacts generate hot shocks. CTTSs are, also, characterized by high levels of coronal activity, as revealed by X-ray observations (e.g. Favata et al. 2005). This coronal activity is mainly produced by energetic flares. Aims of this work In this work we investigated the mass accretion process in CTTSs. We studied if accretion from the disk to the star might occur as a result of a coronal activity, and we analyzed the structure and the dynamics of the accretion column plasma in the impact regions. We de- veloped numerical models that describe: a star-disk system subject to the effects of a coronal activity in proximity of the disk surface; the impact of an accretion column onto the surface of a CTTS. We investigated if an intense coronal activity due to flares that occur close to the accretion disk may perturb the stability of the inner disk, disrupt the inner part of the disk, and possi- bly trigger accretion phenomena with mass accretion rates comparable with those observed in CTTSs (Colombo et al. 2019c). To this end, we modeled a magnetized protostar surrounded by an accretion disk through 3D magnetohydrodynamics simulations. The model takes into account the gravity from the central star, the effects of viscosity in the disk, the thermal con- duction (including the effects of heat flux saturation), the radiative losses from optically thin plasma, and a parameterized heating function to trigger the flares. We explored cases charac- terized by a dipole plus an octupole stellar magnetic field configuration and by either different densities of disk or different levels of flaring activity. As it concerns the study of accretion impacts, we analyzed the effects of radiation emerg- ing from the shock-heated plasma at the base of accretion columns on the structure of the pre- shock downfalling material. To this end, we upgraded a module handling the local thermody- namic equilibrium (LTE) radiation-hydrodynamics (RHD) in the PLUTO code (Mignone et al. 2007, 2012), which we have extended to handle also the non-LTE regime (Colombo et al. 2019a). Then, we investigated if a significant absorption of radiation arising from the shock heated plasma occurs in the unshocked downfalling material, and if it leads to a pre-shock heating of the accreting gas. We developed a radiation hydrodynamics model that describes an accretion column impacting onto the surface of a CTTS (Colombo et al. 2019b). The model takes into account the stellar gravity, the thermal conduction, and the effects of both radia- tive losses and absorption of radiation by matter in non local thermodynamic equilibrium conditions. Results As it concerns the effects of flaring activity on the disk stability, we observed, as a result of the simulated intense flaring activity, the formation of several magnetic loops confining hot plasma that link the star to the disk. All these loops build up a hot extended corona with an X- ray luminosity comparable to typical values observed in CTTSs (Colombo et al. 2019c). The intense flaring activity close to the disk can strongly perturb the disk stability. The flares trig- ger overpressure waves which travel through the disk and modify its configuration. Accretion funnels may be triggered by the flaring activity, thus contributing to the mass accretion rate of the star. Accretion rates derived from the simulations range from 10−10 to 10−9M⊙yr−1 (Colombo et al. 2019c). The accretion columns can be perturbed by the flares and can interact with each other, possibly merging together in larger streams. As a result, the accretion pattern can be rather complex: the streams are highly inhomogeneous, with a complex density struc- ture, and clumped. This inhomogenity may be the origin of the variability observed in the structure of the accretion columns (Alencar et al. 2018). The non-LTE radiation module developed to study the dynamics and structure of the im- pact region of CTTSs has been validated through different tests. In particular, we modeled the structure of a radiative shock, simulating a simple shock case as described by Ensman (1994). The agreement between our solutions and the semi-analytical solutions (when available) is good, with a maximum error of 7%. Moreover, we have proven that a non-LTE approach change significantly the structure and the dynamics of the impact regions, leading to a ra- diative precursor and a greater extension of the post-shock region compared to the LTE case (Colombo et al. 2019a). Our radiative model describing the impact of an accretion column onto the stellar chro- mosphere shows that part of radiation emitted by the post-shock plasma (≈ 70%) is absorbed by the pre-shock accretion column immediately above the slab. The irradiation heats the downfalling unshocked material up to ≈ 105K. This hot material forms a precursor region that emits in the UV band. The results of this PhD project may address some open questions regarding CTTSs. For instance, we proved that an intense flaring activity in proximity of the disk may perturb its stability and may generate accretion columns highly structured in density and characterized by clumps as recently observed, for example, by Alencar et al. (2018). Moreover, with our radiation model we may naturally explain the origin of the complex UV spectra arising from impact regions (Ardila et al. 2013) and the fact that accretion rates derived from UV observations are systematically higher than rates inferred from X-ray ob- servations (Curran et al. 2011). In fact, our model predicts the presence of a precursor region emitting in the UV. This region: 1) would increase the UV flux arising from the impact with- out assuming higher accretion rates and 2) may generate an UV flux produced by plasma at free fall velocity, thus with Doppler shifts stronger than those generated by the post-shock plasma. This may explain the high redshifts and broadening observed in emission lines of UV spectra (Ardila et al. 2013).
Contesto scientifico Le stelle T Tauri classiche (CTTSs) sono oggetti stellari giovani poco massivi che accrescono massa dal loro disco circumstellare. Il disco si estende internamente fino al raggio di tron- camento, ovvero dove il campo magnetico e` abbastanza intenso da sollevare il materiale dal piano del disco e da incanalarlo formando delle colonne di accrescimento (Koenigl 1991). Il materiale incanalato precipita sulla stella e impatta sulla superficie stellare. Gli impatti gener- ano shocks caldi. Le CTTSs sono anche caratterizzate da un alto livello di attivita` coronale, come rivelato dalle osservazioni in banda X (e.g. Favata et al. 2005). Questa attivita` coronale e` prodotta principalmente da flares energetici. Obiettivo di questo lavoro In questo lavoro abbiamo investigato il processo di accrescimento di massa nelle CTTSs. Ab- biamo studiato se l’accrescimento dal disco puo ́ essere il risultato di un’ attivita` coronale e ab- biamo analizzato la struttura e la dinamica della colonna di accrescimento nella regione di im- patto. Abbiamo sviluppato modelli numerici che descrivono: un sistema stella disco sotto gli effetti di un’attivita` coronale in prossimita` della superficie del disco; l’impatto di una colonna di accrescimento su una CTTS. Abbiamo investigato se un’intensa attivita` coronale causata da flares che avvengono in prossimita` del disco di accrescimento puo ́ perturbare la stabilita` del disco interno, distruggere la parte interna del disco e possibilmente innescare fenomeni di accrescimento con tassi di ac- crescimento confrontabili con quelli osservati nelle CTTSs (Colombo et al. 2016). A tal fine, abbiamo sviluppato delle simulazioni 3D magnetoidrodinamiche di una protostella magne- tizzata circondata da un disco di accrescimento. Il modello considera gli effetti della gravita` della stella, della viscosita` nel disco, della conduzione termica (considerando pure gli effetti della saturazione del flusso), delle perdite radiative da parte di plasma otticamente sottile e una funzione parametrizzata per descrivere i flares. Abbiamo esplorato casi caratterizzati da una configurazione di campo magnetico stellare costituita da un ottupolo piu` un dipolo e da diverse densita` o differenti livelli di attivita` di flaring. Per quanto riguarda lo studio degli impatti di accrescimento, abbbiamo analizzato gli ef- fetti della radiazione emergente dal plasma riscaldato dallo shock sulla struttura del materiale pre-shock in caduta. A tal fine abbiamo aggiornato un modulo radiativo implementato nel codice PLUTO (Mignone et al. 2007, 2012) che assume il regime termodinamico locale (LTE). Il modulo e` stato generalizzato anche al caso non-LTE (Colombo et al. 2019a). Abbiamo stu- diato se la radiazione emergente dalla regione di shock viene significativamente assorbita dal materiale ancora non scioccato in caduta libera, e se questo assorbimento puo ́ generare un riscaldamento del materiale pre-shock. Abbiamo sviluppato un modello radiativo magne- toidrodinamico che descrive una colonna di accrescimento che impatta sulla superficie di una CTTS (Colombo et al. 2019b). Il modello considera gli effetti della gravita` della stella, della conduzione termica, delle perdite radiative e anche dell’assorbimento della radiazione dal ma- teriale nel regime non-LTE. Risultati Per quanto riguarda gli effetti sulla stabilita` del disco dell’attivita` di flaring, abbiamo osser- vato, come risultato delle simulazioni di una attivita` flaring intensa, la formazione di diverse loops di plasma caldo confinate dal campo magnetico che collegano la superficie della stella al disco. Tutte queste loops costituiscono una calda corona estesa che produce una luminosita` in banda X confrontabile con i valori tipici osservati nelle CTTSs (Colombo et al. 2019c). L’intensa attivita` di flaring vicina al disco puo ́ perturbarne significativamente la stabilita`. I flares generano un’onda di pressione, la quale viaggia attraverso il disco e ne modifica la con- figurazione. In questo modo, delle colonne di accrescimento vengono generate dall’attivita` di flaring, contribuendo cosi al rate di accrescimento sulla stella. I rates di accrescimento derivati dalle simulazioni assumono valori compresi tra 10−10 e 10−9 M⊙yr−1 (Colombo et al. 2019c). Le colonne di accrescimento possono essere perturbate a loro volta dai flares e interagiscono l’una con l’altra, fondendosi per formare colonne piu` grandi. Il risultato di questa dinamica e` che le colonne di accrescimento hanno una struttura molto disomogenea, costituita da blob densi. Questa disomogeneita` potrebbe essere l’origine della variabilita` osservata nelle strutture di accrescimento (Alencar et al. 2018). Il modulo radiativo in non-LTE che e` stato sviluppato per studiare le dinamica e la strut- tura della regione d’impatto nelle CTTSs e` stato validato utilizzando diversi test. In parti- colare, abbiamo realizzato un modello per analizzare la struttura di un semplice shock radia- tivo come descritto da Ensman (1994). Le nostre soluzioni sono in accordo con le soluzioni semi-analitiche (quando disponibili) con una discrepanza massima del 7%. Inoltre, abbiamo provato che un approccio in non-LTE cambia significativamente la struttura e la dinamica della regione di impatto, dando origine ad un precursore radiativo e ad una maggiore esten- sione della regione post-shock rispetto al caso LTE (Colombo et al. 2019a). Il nostro modello radiativo, che descrive la regione di impatto di una colonna di accresci- mento sulla cromosfera stellare, prova che parte della radiazione emessa dal plasma post-shock (≈ 70%) viene assorbita dal materiale freddo pre-shock in caduta sulla stella. L’irraggiamento riscalda il materiale, in caduta sulla stella, ancora non scioccato fino a temperature di 105K. Il materiale caldo forma un precursore termico che emette nella banda UV. I risultati di questo progetto di dottorato potrebbero aiutare nella soluzione di alcune questioni aperte sulle CTTSs. Per esempio, abbiamo provato che un’intensa attivita` di flaring in prossimita` del disco puo ́ perturbarne la stabilita` e generare colonne di accrescimento con una forte strutturazione in densita` come quelle recentemente osservate da Alencar et al. (2018). Inoltre, utilizzando il nostro modello radiativo possiamo spiegare naturalmente l’origine dei complessi spettri in banda UV provenienti dalle regioni di impatto (Ardila et al. 2013) e an- che il fatto che i rates di accrescimento ottenuti dalle osservazioni in banda UV sono sistemati- camente piu` grandi di quelli ottenuti in banda X (Curran et al. 2011). Infatti, il nostro modello radiativo predice la presenza di un precursore termico che emette in banda UV. Questo pre- cursore: 1) Aumenterebbe il flusso UV prodotto dalla regione di impatto senza assumere un piu` alto rate di accrescimento e 2) potrebbe generare un flusso UV prodotto da plasma a ve- locita` di caduta libera sulla stella, quindi con Doppler shifts molto piu` grandi di quelli generati dalla regione post-shock. Questo potrebbe spiegare le componenti ad alto redshift osservate in banda UV (Ardila et al. 2013).
(2019). Radiation hydrodynamic and magnetohydrodynamic models of plasma flows accreting onto Classical T Tauri Stars.
Radiation hydrodynamic and magnetohydrodynamic models of plasma flows accreting onto Classical T Tauri Stars
COLOMBO, Salvatore
2019-10-28
Abstract
Context Classical T Tauri Stars (CTTSs) are young low-mass stellar objects that accrete mass from their circumstellar disks. The disks extend internally up to the truncation radius, where the mag- netic field is strong enough to lift up the material from the disk plane and to funnel the mate- rial forming accretion columns (Koenigl 1991). The funneled plasma falls down onto the star and hits the stellar surface. The impacts generate hot shocks. CTTSs are, also, characterized by high levels of coronal activity, as revealed by X-ray observations (e.g. Favata et al. 2005). This coronal activity is mainly produced by energetic flares. Aims of this work In this work we investigated the mass accretion process in CTTSs. We studied if accretion from the disk to the star might occur as a result of a coronal activity, and we analyzed the structure and the dynamics of the accretion column plasma in the impact regions. We de- veloped numerical models that describe: a star-disk system subject to the effects of a coronal activity in proximity of the disk surface; the impact of an accretion column onto the surface of a CTTS. We investigated if an intense coronal activity due to flares that occur close to the accretion disk may perturb the stability of the inner disk, disrupt the inner part of the disk, and possi- bly trigger accretion phenomena with mass accretion rates comparable with those observed in CTTSs (Colombo et al. 2019c). To this end, we modeled a magnetized protostar surrounded by an accretion disk through 3D magnetohydrodynamics simulations. The model takes into account the gravity from the central star, the effects of viscosity in the disk, the thermal con- duction (including the effects of heat flux saturation), the radiative losses from optically thin plasma, and a parameterized heating function to trigger the flares. We explored cases charac- terized by a dipole plus an octupole stellar magnetic field configuration and by either different densities of disk or different levels of flaring activity. As it concerns the study of accretion impacts, we analyzed the effects of radiation emerg- ing from the shock-heated plasma at the base of accretion columns on the structure of the pre- shock downfalling material. To this end, we upgraded a module handling the local thermody- namic equilibrium (LTE) radiation-hydrodynamics (RHD) in the PLUTO code (Mignone et al. 2007, 2012), which we have extended to handle also the non-LTE regime (Colombo et al. 2019a). Then, we investigated if a significant absorption of radiation arising from the shock heated plasma occurs in the unshocked downfalling material, and if it leads to a pre-shock heating of the accreting gas. We developed a radiation hydrodynamics model that describes an accretion column impacting onto the surface of a CTTS (Colombo et al. 2019b). The model takes into account the stellar gravity, the thermal conduction, and the effects of both radia- tive losses and absorption of radiation by matter in non local thermodynamic equilibrium conditions. Results As it concerns the effects of flaring activity on the disk stability, we observed, as a result of the simulated intense flaring activity, the formation of several magnetic loops confining hot plasma that link the star to the disk. All these loops build up a hot extended corona with an X- ray luminosity comparable to typical values observed in CTTSs (Colombo et al. 2019c). The intense flaring activity close to the disk can strongly perturb the disk stability. The flares trig- ger overpressure waves which travel through the disk and modify its configuration. Accretion funnels may be triggered by the flaring activity, thus contributing to the mass accretion rate of the star. Accretion rates derived from the simulations range from 10−10 to 10−9M⊙yr−1 (Colombo et al. 2019c). The accretion columns can be perturbed by the flares and can interact with each other, possibly merging together in larger streams. As a result, the accretion pattern can be rather complex: the streams are highly inhomogeneous, with a complex density struc- ture, and clumped. This inhomogenity may be the origin of the variability observed in the structure of the accretion columns (Alencar et al. 2018). The non-LTE radiation module developed to study the dynamics and structure of the im- pact region of CTTSs has been validated through different tests. In particular, we modeled the structure of a radiative shock, simulating a simple shock case as described by Ensman (1994). The agreement between our solutions and the semi-analytical solutions (when available) is good, with a maximum error of 7%. Moreover, we have proven that a non-LTE approach change significantly the structure and the dynamics of the impact regions, leading to a ra- diative precursor and a greater extension of the post-shock region compared to the LTE case (Colombo et al. 2019a). Our radiative model describing the impact of an accretion column onto the stellar chro- mosphere shows that part of radiation emitted by the post-shock plasma (≈ 70%) is absorbed by the pre-shock accretion column immediately above the slab. The irradiation heats the downfalling unshocked material up to ≈ 105K. This hot material forms a precursor region that emits in the UV band. The results of this PhD project may address some open questions regarding CTTSs. For instance, we proved that an intense flaring activity in proximity of the disk may perturb its stability and may generate accretion columns highly structured in density and characterized by clumps as recently observed, for example, by Alencar et al. (2018). Moreover, with our radiation model we may naturally explain the origin of the complex UV spectra arising from impact regions (Ardila et al. 2013) and the fact that accretion rates derived from UV observations are systematically higher than rates inferred from X-ray ob- servations (Curran et al. 2011). In fact, our model predicts the presence of a precursor region emitting in the UV. This region: 1) would increase the UV flux arising from the impact with- out assuming higher accretion rates and 2) may generate an UV flux produced by plasma at free fall velocity, thus with Doppler shifts stronger than those generated by the post-shock plasma. This may explain the high redshifts and broadening observed in emission lines of UV spectra (Ardila et al. 2013).File | Dimensione | Formato | |
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