Gas når unge stjerner langs magnetfeltlinjer

Astronomer har brugt GRAVITY-instrumentet til at studere den umiddelbare nærhed af en ung stjerne mere detaljeret end nogensinde før. Deres observationer bekræfter en tredive år gammel teori om væksten af ​​unge stjerner:Det magnetiske felt, der produceres af stjernen selv, leder materiale fra en omgivende tilvækstskive af gas og støv op på dens overflade. Resultaterne, offentliggjort i dag i tidsskriftet Natur , hjælpe astronomer til bedre at forstå, hvordan stjerner som vores sol dannes, og hvordan jordlignende planeter dannes fra skiverne, der omgiver disse stjernernes babyer.

Når stjerner dannes, de starter forholdsvis lille og er placeret dybt inde i en sky af gas. I løbet af de næste hundredtusinder af år, de trækker mere og mere af den omgivende gas på sig selv, øge deres masse i processen. Ved at bruge GRAVITY-instrumentet, en gruppe forskere, der omfatter astronomer og ingeniører fra Max Planck Institute for Astronomy (MPIA), har nu fundet det mest direkte bevis endnu for, hvordan den gas kan ledes ind på unge stjerner:den ledes af stjernens magnetfelt op på overfladen i en smal søjle.

De relevante længdeskalaer er så små, at selv med de bedste teleskoper, der er tilgængelige i øjeblikket, er ingen detaljerede billeder af processen mulige. Stadig, ved hjælp af den nyeste observationsteknologi, astronomer kan i det mindste indsamle nogle oplysninger. Til den nye undersøgelse, forskerne gjorde brug af den fremragende høje opløsningsevne i instrumentet kaldet GRAVITY. Den kombinerer fire 8-meters VLT-teleskoper fra European Southern Observatory (ESO) ved Paranal-observatoriet i Chile til et virtuelt teleskop, der kan skelne små detaljer såvel som et teleskop med et 100-meter spejl kunne.

Ved at bruge GRAVITY, forskerne var i stand til at observere den indre del af gasskiven omkring stjernen TW Hydrae. "Denne stjerne er speciel, fordi den er meget tæt på Jorden kun 196 lysår væk, og stofskiven, der omgiver stjernen, vender direkte mod os, " siger Rebeca García López (Max Planck Institute for Astronomy, Dublin Institute for Advanced Studies og University College Dublin), hovedforfatter og førende videnskabsmand til denne undersøgelse. "Dette gør det til en ideel kandidat til at undersøge, hvordan stof fra en planetdannende skive kanaliseres videre til stjernens overflade."

Observationen gjorde det muligt for astronomerne at vise, at nær-infrarød stråling, der udsendes af hele systemet, faktisk stammer fra det inderste område, hvor brintgas falder ned på stjernens overflade. Resultaterne peger tydeligt mod en proces kendt som magnetosfærisk tilvækst, det er, indfaldende stof styret af stjernens magnetfelt.

Stjernefødsel og stjernernes vækst

En stjerne bliver født, når et tæt område i en sky af molekylær gas kollapser under sin egen tyngdekraft, bliver betydeligt tættere, opvarmes i processen, indtil densiteten og temperaturen i den resulterende protostjerne til sidst er så høj, at nuklear fusion af brint til helium starter. For protostjerner op til omkring to gange Solens masse, de ti millioner år lige før antændelsen af ​​proton-proton-kernefusion udgør den såkaldte T Tauri-fase (opkaldt efter den første observerede stjerne af denne art, T Tauri i stjernebilledet Tyren).

Stjerner, som vi ser i den fase af deres udvikling, kendt som T Tauri stjerner, skinne ganske klart, især i infrarødt lys. Disse såkaldte "unge stjerneobjekter" (YSO'er) har endnu ikke nået deres endelige masse:de er omgivet af resterne af skyen, hvorfra de blev født, især af gas, der har trukket sig sammen til en cirkumstellar skive, der omgiver stjernen. I de ydre områder af den disk, støv og gas klumper sig sammen og danner stadig større kroppe, som i sidste ende bliver til planeter. Store mængder gas og støv fra det indre skiveområde, på den anden side, bliver trukket på stjernen, øge sin masse. Sidst men ikke mindst, stjernens intense stråling driver en betydelig del af gassen ud som en stjernevind.

Retningslinjer til overfladen:stjernens magnetfelt

Naivt, man kunne tro, at transport af gas eller støv på en massiv, graviterende krop er let. I stedet, det viser sig slet ikke at være så enkelt. På grund af det, fysikere kalder bevarelsen af ​​vinkelmomentum, it is much more natural for any object—whether planet or gas cloud—to orbit a mass than to drop straight onto its surface. One reason why some matter nonetheless manages to reach the surface is a so-called accretion disk, in which gas orbits the central mass. 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. Endnu, 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, for eksempel, 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. Imidlertid, 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. Indtil for nylig, 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.

For nylig, 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. I øvrigt, 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 Astronomi og astrofysik .

The new results, which have now been published in the journal Natur , go one step further. I dette tilfælde, 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. Taken together, what is left as a plausible explanation is the magnetospheric accretion model.

Hvad er det næste?

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. I dette tilfælde, 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.