Los astrónomos han utilizado el instrumento GRAVITY para estudiar las inmediaciones de una estrella joven con más detalle que nunca. Sus observaciones confirman una teoría de treinta años sobre el crecimiento de estrellas jóvenes:el campo magnético producido por la propia estrella dirige el material de un disco de acreción de gas y polvo circundante hacia su superficie. Los resultados, publicado hoy en la revista Naturaleza , ayudar a los astrónomos a comprender mejor cómo se forman las estrellas como nuestro Sol y cómo se producen los planetas similares a la Tierra a partir de los discos que rodean a estos bebés estelares.

Cuando se forman las estrellas comienzan siendo comparativamente pequeños y están ubicados en lo profundo de una nube de gas. En el transcurso de los próximos cientos de miles de años, atraen cada vez más el gas circundante hacia sí mismos, aumentando su masa en el proceso. Usando el instrumento GRAVITY, un grupo de investigadores que incluye astrónomos e ingenieros del Instituto Max Planck de Astronomía (MPIA), ahora ha encontrado la evidencia más directa hasta ahora de cómo ese gas se canaliza hacia las estrellas jóvenes:es guiado por el campo magnético de la estrella hacia la superficie en una columna estrecha.

Las escalas de longitud relevantes son tan pequeñas que incluso con los mejores telescopios disponibles actualmente no es posible obtener imágenes detalladas del proceso. Todavía, utilizando la última tecnología de observación, los astrónomos pueden al menos obtener alguna información. Para el nuevo estudio, los investigadores hicieron uso del poder de resolución magníficamente alto del instrumento llamado GRAVITY. Combina cuatro telescopios VLT de 8 metros del Observatorio Europeo Austral (ESO) en el observatorio Paranal en Chile en un telescopio virtual que puede distinguir pequeños detalles, así como un telescopio con un espejo de 100 metros.

Usando GRAVEDAD, los investigadores pudieron observar la parte interior del disco de gas que rodea a la estrella TW Hydrae. "Esta estrella es especial porque está muy cerca de la Tierra a solo 196 años luz de distancia, y el disco de materia que rodea a la estrella está directamente frente a nosotros, "dice Rebeca García López (Instituto Max Planck de Astronomía, Instituto de Estudios Avanzados de Dublín y University College Dublin), autor principal y científico principal de este estudio. "Esto lo convierte en un candidato ideal para investigar cómo se canaliza la materia de un disco que forma un planeta hacia la superficie estelar".

La observación permitió a los astrónomos demostrar que la radiación infrarroja cercana emitida por todo el sistema se origina en la región más interna. donde el gas hidrógeno cae sobre la superficie de la estrella. Los resultados apuntan claramente hacia un proceso conocido como acreción magnetosférica, es decir, materia que cae guiada por el campo magnético de la estrella.

Nacimiento estelar y crecimiento estelar

Una estrella nace cuando una región densa dentro de una nube de gas molecular colapsa bajo su propia gravedad. se vuelve considerablemente más denso, se calienta en el proceso, hasta que finalmente la densidad y la temperatura en la protoestrella resultante son tan altas que comienza la fusión nuclear de hidrógeno en helio. Para protoestrellas de hasta aproximadamente dos veces la masa del Sol, los diez o más millones de años directamente antes de la ignición de la fusión nuclear protón-protón constituyen la llamada fase T Tauri (llamada así por la primera estrella observada de este tipo, T Tauri en la constelación de Tauro).

Estrellas que vemos en esa fase de su desarrollo, conocidas como estrellas T Tauri, brillar muy intensamente, en particular en luz infrarroja. Estos llamados "objetos estelares jóvenes" (YSO) aún no han alcanzado su masa final:están rodeados por los restos de la nube de la que nacieron, en particular por el gas que se ha contraído en un disco circunestelar que rodea a la estrella. En las regiones exteriores de ese disco, el polvo y el gas se agrupan y forman cuerpos cada vez más grandes, que eventualmente se convertirán en planetas. Grandes cantidades de gas y polvo de la región del disco interno, por otra parte, se dibujan en la estrella, aumentando su masa. Por último, si bien no menos importante, La intensa radiación de la estrella expulsa una parte considerable del gas en forma de viento estelar.

Pautas hacia la superficie:el campo magnético de la estrella

Ingenuamente, uno podría pensar que transportar gas o polvo a un gravitar el cuerpo es fácil. En lugar de, resulta no ser tan simple en absoluto. Debido a lo que los físicos llaman la conservación del momento angular, Es mucho más natural que cualquier objeto, ya sea un planeta o una nube de gas, orbite una masa que caiga directamente sobre su superficie. Una razón por la que algo de materia, no obstante, logra llegar a la superficie es el llamado disco de acreción, en el que el gas orbita la masa central. There is plenty of internal friction inside that continually allows some of the gas to transfer its angular momentum to other portions of gas and move further inward. Todavía, at a distance from the star of less than 10 times the stellar radius, the accretion process gets more complex. Traversing that last distance is tricky.

Thirty years ago, Max Camenzind, at the Landessternwarte Königstuhl (which has since become a part of the University of Heidelberg), proposed a solution to this problem. Stars typically have magnetic fields—those of our Sun, por ejemplo, regularly accelerate electrically charged particles in our direction, leading to the phenomenon of Northern or Southern lights. In what has become known as magnetospheric accretion, the magnetic fields of the young stellar object guide gas from the inner rim of the circumstellar disk to the surface in distinct column-like flows, helping them to shed angular momentum in a way that allows the gas to flow onto the star.

In the simplest scenario, the magnetic field looks similar to that of the Earth. Gas from the inner rim of the disk would be funneled to the magnetic North and to the magnetic South pole of the star.

Checking up on magnetospheric accretion

Having a model that explains certain physical processes is one thing. Sin embargo, it is important to be able to test that model using observations. But the length scales in question are of the order of stellar radii, very small on astronomical scales. Hasta hace poco, such length scales were too small, even around the nearest young stars, for astronomers to be able to take a picture showing all relevant details.

First indication that magnetospheric accretion is indeed present came from examining the spectra of some T Tauri stars. Spectra of gas clouds contain information about the motion of the gas. For some T Tauri stars, spectra revealed disk material falling onto the stellar surface with velocities as high as several hundred kilometers per second, providing indirect evidence for the presence of accretion flows along magnetic field lines. In a few cases, the strength of the magnetic field close to a T Tauri star could be directly measured by a combining high-resolution spectra and polarimetry, which records the orientation of the electromagnetic waves we receive from an object.

Más recientemente, instruments have become sufficiently advanced—more specifically:have reached sufficiently high resolution, a sufficiently good capability to discern small details—so as to allow direct observations that provide insights into magnetospheric accretion.

The instrument GRAVITY plays a key role here. It was developed by a consortium that includes the Max Planck Institute for Astronomy, led by the Max Planck Institute for Extraterrestrial Physics. In operation since 2016, GRAVITY links the four 8-meter-telescopes of the VLT, located at the Paranal observatory of the European Southern Observatory (ESO). The instrument uses a special technique known as interferometry. The result is that GRAVITY can distinguish details so small as if the observations were made by a single telescope with a 100-m mirror.

Catching magnetic funnels in the act

In the Summer of 2019, a team of astronomers led by Jerome Bouvier of the University of Grenobles Alpes used GRAVITY to probe the inner regions of the T Tauri Star with the designation DoAr 44. It denotes the 44th T Tauri star in a nearby star forming region in the constellation Ophiuchus, catalogued in the late 1950s by the Georgian astronomer Madona Dolidze and the Armenian astronomer Marat Arakelyan. The system in question emits considerable light at a wavelength that is characteristic for highly excited hydrogen. Energetic ultraviolet radiation from the star ionizes individual hydrogen atoms in the accretion disk orbiting the star.

The magnetic field then influences the electrically charged hydrogen nuclei (each a single proton). The details of the physical processes that heat the hydrogen gas as it moves along the accretion current towards the star are not yet understood. The observed greatly broadened spectral lines show that heating occurs.

For the GRAVITY observations, the angular resolution was sufficiently high to show that the light was not produced in the circumstellar disk, but closer to the star's surface. Es más, the source of that particular light was shifted slightly relative to the centre of the star itself. Both properties are consistent with the light being emitted near one end of a magnetic funnel, where the infalling hydrogen gas collides with the surface of the star. Those results have been published in an article in the journal Astronomía y Astrofísica .

The new results, which have now been published in the journal Naturaleza , go one step further. En este caso, the GRAVITY observations targeted the T Tauri star TW Hydrae, a young star in the constellation Hydra. They are based on GRAVITY observations of the T Tauri star TW Hydrae, a young star in the constellation Hydra. It is probably the best-studied system of its kind.

Too small to be part of the disk

With those observations, Rebeca García López and her colleagues have pushed the boundaries even further inwards. GRAVITY could see the emissions corresponding to the line associated with highly excited hydrogen (Brackett-γ, Brγ) and demonstrate that they stem from a region no more than 3.5 times the radius of the star across (about 3 million km, or 8 times the distance the distance between the Earth and the Moon).

This is a significant difference. According to all physics-based models, the inner rim of a circumstellar disk cannot possibly be that close to the star. If the light originates from that region, it cannot be emitted from any section of the disk. At that distance, the light also cannot be due to a stellar wind blown away by the young stellar object—the only other realistic possibility. Tomados en conjunto, what is left as a plausible explanation is the magnetospheric accretion model.

¿Que sigue?

In future observations, again using GRAVITY, the researchers will try to get data that allows them a more detailed reconstruction of physical processes close to the star. "By observing the location of the funnel's lower endpoint over time, we hope to pick up clues as to how distant the magnetic North and South poles are from the star's axis of rotation, " explains Wolfgang Brandner, co-author and scientist at MPIA. If North and South Pole directly aligned with the rotation axis, their position over time would not change at all.

They also hope to pick up clues as to whether the star's magnetic field is really as simple as a North Pole–South Pole configuration. "Magnetic fields can be much more complicated and have additional poles, " explains Thomas Henning, Director at MPIA. "The fields can also change over time, which is part of a presumed explanation for the brightness variations of T Tauri stars."

All in all, this is an example of how observational techniques can drive progress in astronomy. En este caso, the new observational techniques embody in GRAVITY were able to confirm ideas about the growth of young stellar objects that were proposed as long as 30 years ago. And future observations are set to help us understand even better how baby stars are being fed.