# Linear extent of an accretion disk

I have a homework assignment question on accretions discs (essentially an estimation of the number of electron scatterings, but this is just for background).

There are a few parameters, one of them being $L$, which is the linear size of the medium (the medium in this case being an accretion disc around a blackhole)

Now, I have been given the mass of the black hole. Other than that, nothing else which could give me the linear size of the accretion disc.

Could I assume that the linear extent of the accretion disc is perhaps of the order of a few Schwarzschild radii? Which could be calculated from the mass, which is given.

If anyone could shed some light on this I would be very appreciative. I need a nudge in the right direction on this.

I think the outer edge of an accretion disk is not well-defined, and observationally the radius will depend on which wavelength you consider, since the farther you get from the BH, the softer the radiation will be. But if you look in the UV, then Morgan et al. (2010) find the following relation between $R_{2500}$ (the radius when observed at $lambda = 2500$ Å) and the mass $M_mathrm{BH}$ of the black hole: $$logleft( frac{R_{2500}}{mathrm{cm}} ight) = 15.78 + 0.80 logleft( frac{M_mathrm{BH}}{10^9M_odot} ight),$$ (modulo some uncertainties that you can look up in the paper).

That is, if your BH has a mass of $10^8 M_odot$, its radius will be $R_{2500}sim64,mathrm{AU}$, or roughly 1/3 lightdays.

For comparison, its Schwarzschild radius is $sim2,mathrm{AU}$, so your estimate was actually pretty good.

This result is consistent with Edelson et al. (2015), who find 0.35 lightdays, also in the UV. However, in you look in longer wavelengths, the disk is much, much larger. If you're interested beyond your homework assignment, take a look at the accretion disk theory review by Armijo (2013), who shows that in the radio regime, the disk is thousands of AU, and even up to ~100 pc.

$10^{8} M_{odot}$ SMBH. Eddington Luminosity (dodgy estimate, since assumes spherical accretion) is $L/L_{odot} simeq 3 imes 10^{4} (M/M_{odot}) = 3 imes 10^{12} L_{odot}$.

Let's assume we are seeing all this emerge from the face of a flat disc of total area (front and back) $pi R^2$ and temperature $T simeq 10^{5}$ K.

Assume black body emission, so $L = 2pi R^2 sigma T^4$. And thus $$R simeq left[frac{3 imes 10^{12} L_{odot}}{2 pi sigma T^4} ight]^{1/2}$$

Putting in the numbers I get $R simeq 40$ au. However it is very sensitive to the assumed temperature (much smaller for $T = 10^{6}$ K).

## Accretion geometry of the black-hole binary Cygnus X-1 from X-ray polarimetry

Black hole binary (BHB) systems comprise a stellar-mass black hole and a closely orbiting companion star. Matter is transferred from the companion to the black hole, forming an accretion disk, corona and jet structures. The resulting release of gravitational energy leads to the emission of X-rays 1 . The radiation is affected by special/general relativistic effects, and can serve as a probe for the properties of the black hole and surrounding environment, if the accretion geometry is properly identified. Two competing models describe the disk–corona geometry for the hard spectral state of BHBs, based on spectral and timing measurements 2,3 . Measuring the polarization of hard X-rays reflected from the disk allows the geometry to be determined. The extent of the corona differs between the two models, affecting the strength of the relativistic effects (such as enhancement of the polarization fraction and rotation of the polarization angle). Here, we report observational results on the linear polarization of hard X-ray emission (19–181 keV) from a BHB, Cygnus X-1 4 , in the hard state. The low polarization fraction, <8.6% (upper limit at a 90% confidence level), and the alignment of the polarization angle with the jet axis show that the dominant emission is not influenced by strong gravity. When considered together with existing spectral and timing data, our result reveals that the accretion corona is either an extended structure, or is located far from the black hole in the hard state of Cygnus X-1.

## Greek/Ελληνικά

Adapted to Greek by Dimitrios Irodotou / Μεταφράστηκε στα Ελληνικά από τον Δημήτριο Ηροδότου

Το σημερινό άρθρο μας πηγαίνει 47 χρόνια πίσω στον χρόνο, τότε όπου ξεκίνησε να αναπτύσσεται ένα μοντέλο περιγραφής των δίσκων επιπρόσθεσης, το οποίο όχι μόνο έφερε την επανάσταση στην κατανόηση αυτών, αλλά και στην κατανόηση της επίδραση τους στην συνολική εξέλιξη του γαλαξία.

Αντικείμενα τα οποία περιστρέφονται (αρκετά) γρήγορα τείνουν να σχηματίζουν δίσκους. Αυτό το φαινόμενο παρατηρείται στους σπειροειδής γαλαξίες αλλά και κατά την διαδικασία σχηματισμού άστρων και πλανητών. Από τις αρχές της δεκαετίας του 1940, οι αστρονόμοι είχαν φτιάξει μοντέλα βασισμένα σε θεμελιώδεις νόμους της φυσικής για να μπορέσουν να κατανοήσουν τους δίσκους επιπρόσθεσης. Μέχρι προσφάτως, οι αστρονομικές παρατηρήσεις δεν ήταν αρκετά λεπτομερείς ώστε να μπορέσουν να “δουν” τους δίσκους επιπρόσθεσης, αλλά μπορούσαν να εντοπίσουν φαινόμενα μεγαλύτερης κλίμακας όπως ενεργούς γαλαξιακούς πυρήνες και πίδακες εκροής υλικού, τα οποία σχετίζονται άμεσα με τους δίσκους επιπρόσθεσης.

Γράφημα 1: Καλλιτεχνική αποτύπωση ενός διπλού αστρικού συστήματος όπου μια μαύρη τρύπα περιστρέφεται μαζί με έναν δευτερεύων αστέρα ο οποίος την τροφοδοτεί με μάζα. Στην μαύρη τρύπα φαίνονται και ο δίσκος επιπρόσθεσης και ο πίδακας εκροής υλικού. Πηγή: ESA/NASA/Felix Mirabel

Η βασική αρχή δημιουργίας/λειτουργίας των δίσκων επιπρόσθεσης έχει να κάνει με την στροφορμή του υλικού. Φανταστείτε ένα διπλό αστρικό σύστημα όπως αυτό του Γραφήματος 1. Βλέπουμε υλικό να διαφεύγει από τον αστέρα και να κατευθύνεται προς την μαύρη τρύπα λόγω της βαρυτικής της έλξης. Αν αυτό το υλικό βρισκόταν σε ηρεμία, τότε θα έπεφτε ευθύγραμμα προς την μαύρη τρύπα χωρίς να σχηματίσει πρώτα τον δίσκο επιπρόσθεσης, επομένως δεν θα έχανε/εξέπεμπε ενέργεια και δεν θα μπορούσαμε να το παρατηρήσουμε. Όμως, το συγκεκριμένο αστέρι περιστρέφεται, επομένως το υλικό του δεν βρίσκεται σε ηρεμία. Αυτό το υλικό διαθέτει στροφορμή την οποία πρέπει να “ξεφορτωθεί” για να μπορέσει εν τέλει να πέσει στην μαύρη τρύπα. Καθώς το υλικό του αστέρα προσεγγίζει την μαύρη τρύπα η φυγόκεντρος δύναμη εξισορροπείται από την βαρυτική με αποτέλεσμα το υλικό να εκτελεί κυκλική κίνηση και να σχηματίζει τον δίσκο επιπρόσθεσης, όπως φαίνεται στο Γράφημα 1. Για να μπορέσει το υλικό στον δίσκο να πέσει στην μαύρη τρύπα θα πρέπει να μεταφέρει στην στροφορμή του. Αυτή η διαδικασία ονομάζεται επιπρόσθεση υλικού.

### Μια μικρή παρένθεση για να αναφερθούμε στην ακτινοβολία των μαύρων τρυπών

“Οι μαύρες τρύπες δεν εκπέμπουν ηλεκτρομαγνητική κύματα” είχαν γράψει οι Shakura & Sunyaev το 1973. Μόλις έναν χρόνο αργότερα, ο Stephen Hawking δημοσίευσε μια επιστημονική εργασία σχετικά με την ακτινοβολία Hawking. Αν και πρόκειται για μια εξαιρετική θεωρητική δουλειά, είναι πρακτικά αδύνατο να παρατηρήσουμε την ακτινοβολία Hawking από υπερμαζικές ή ενδιάμεσων μαζών μαύρες τρύπες. Η ακτινοβολία Hawking είναι αντιστρόφως ανάλογη της μάζας, επομένως η καλύτερη επιλογή θα είναι να ψάξουμε για μαύρες τρύπες με εξαιρετικά χαμηλή μάζα. Όμως ακόμα και η πιο ενεργή μαύρη τρύπα εκπέμπει σε τέσσερεις μήνες ενέργεια μέσω ακτινοβολίας Hawking η οποία είναι συγκρίσιμη με την ενέργεια ενός πρωτονίου κατάλοιπου από την Μεγάλη Έκρηξη. Επομένως το σήμα αυτό είναι εντελώς μη ανιχνεύσιμο. Οπότε μπορούμε να τροποποιήσουμε την αρχική μας δήλωση σε “Οι μαύρες τρύπες δεν εκπέμπουν ανιχνεύσιμα ηλεκτρομαγνητική κύματα”. Αντ’ αυτού η ακτινοβολία την οποία παρατηρούμε προέρχεται αποκλειστικά από το υλικό γύρω από το ουράνιο αντικείμενο: δηλαδή από τον δίσκο επιπρόσθεσης.

### Το μοντέλο μεταφοράς στροφορμής των Shakura-Sunyaev

Ας πάμε τώρα πίσω στο Γράφημα 1 και τον δίσκο επιπρόσθεσης. Το υλικό στον δίσκο αυτό εκτελεί κυκλικές τροχιές όμως για να μπορέσει να προσληφθεί από την μαύρη τρύπα θα πρέπει κάπως να χάσει στροφορμή.

Ένα σωματίδιο το οποίο περιστρέφεται γύρω από την μαύρη τρύπα δεν είναι απομονωμένο. Η “τριβή” η οποία αναπτύσσεται μεταξύ των σωματιδίων τα αναγκάζει να μεταφέρουν στροφορμή σε γειτονικά σωματίδια και επομένως να χάσουν ένας μέρος της στροφορμή τους. Αυτή η “τριβή” ονομάζεται ιξώδες. Η αντίληψη για το ιξώδες ήταν (και ακόμα είναι) υπό συζήτηση, όμως οι Shakura & Sunyaev πρότειναν ότι το ιξώδες σχετίζεται με τις μαγνητικές δυνάμεις. Έκαναν αυτήν την υπόθεση με πλήρη έλλειψη παρατηρησιακών δεδομένων, όμως φαίνεται ότι ήταν σωστή.

Οι Shakura & Sunyaev είδαν ότι η δομή του δίσκου επιπρόσθεσης και η εκπεμπόμενη ενέργεια είναι σχεδόν απολύτως εξαρτημένες από τον ρυθμό εισροής υλικού. Επίσης ανέπτυξαν μια αναλυτική έκφραση για την ενεργό θερμοκρασία του δίσκου συναρτήσει της ακτίνας του. Η ενεργός θερμοκρασία είναι η θερμοκρασία ενός σώματος εάν αυτό ήταν μέλαν σώμα. Η μετέπειτα εξέλιξη των παρατηρησιακών οργάνων μπόρεσε να επιβεβαιώσει αυτήν την αναλυτική έκφραση.

Γράφημα 2: Μαύρη τρύπα στο κέντρο του γαλαξία M87. Η εικόνα δείχνει ένα λαμπρό δαχτυλίδι το οποίο σχηματίζεται λόγω της στρέβλωσης του φωτός από το ισχυρό βαρυτικό πεδίο της μαύρης τρύπας η οποία είναι 6.5 δισεκατομμύρια φορές πιο μαζική από τον Ήλιο. Πηγή: Event Horizon Telescope Collaboration.

Από την δημοσίευση του άρθρου το 1973 μέχρι και σήμερα η θεωρία των δίσκων επιπρόσθεσης είναι ελλιπής. Ο κυρίως λόγος για τον οποίο πρέπει να συνεχιστεί η έρευνα στο συγκεκριμένο θέμα είναι η πρόσφατη μεγάλη τεχνολογική εξέλιξη των παρατηρησιακών οργάνων. Η γνωστή φωτογραφία της μαύρης τρύπας η οποία δημοσιεύτηκε πέρυσι (φαίνεται στο Γράφημα 2) είναι στην πραγματικότητα η εικόνα του δίσκου επιπρόσθεσης. Τώρα όσο ποτέ είμαστε σε θέση να μπορέσουμε να συνδέσουμε την νέα γενιά παρατηρησιακών δεδομένων με την προϋπάρχουσα θεωρία των τελευταίων 50 ετών και να δημιουργήσουμε ένα καθοριστικό μοντέλο περιγραφής των δίσκων επιπρόσθεσης.

Δήλωση: Όταν μεταφράζεται κάποιο κείμενο, η κυριολεκτική μετάφραση δεν είναι πάντοτε ικανή να συλλάβει την έννοια των ιδιωμάτων και των εκφράσεων. Σε τέτοιες περιπτώσεις, ως μεταφραστές/μεταφράστριες κάνουμε το καλύτερο δυνατό για να διατηρήσουμε το ύφος του κειμένου παρά την κυριολεκτική σημασία των λέξεων. Επίσης, προσπαθούμε να παραθέσουμε συνδέσεις μεταξύ εννοιών στην γλώσσα μετάφρασης παρά στην πρωτότυπη, όπου αυτό είναι δυνατό. Ως εκ τούτου, αναγνωρίζουμε ότι η φύση των μεταφράσεων μας αποτελεί μια συνεργασία μεταξύ των συγγραφέων του πρωτότυπου κειμένου και των μεταφραστών.

Το πρωτότυπο άρθρο γράφτηκε από την Jessica May Hislop και επιμελήθηκε από την Huei Sears.
Πηγή κεντρικής εικόνας: Γράφημα 1 από το
σημερινό άρθρο.

H συγγραφέας αυτού του Astrobite εργάζεται επίσης στο σημερινό ινστιτούτο του Rashid Sunyaev, στο Max Planck Institute for Astrophysics στο Garching της Γερμανίας, αλλά σε διαφορετική ερευνητική ομάδα.

## Astronomers confirm extended atmosphere on accretion disk of X-ray binary

Figure 1: Artist impression. Astronomers propose that stars form an accretion disk around them while stealing material from their companion star. Credit: Dana Berry/NASA Goddard Space Flight Center

Astronomers use stellar eclipses to study the atmosphere of accretion disks around compact stars. SRON-researchers observed this method on a low-mass X-ray binary. They find a thicker atmosphere than predicted and distinguish two different gas components. The research was published in Astronomy & Astrophysics.

Almost half of observable star systems actually consist of binary star systems. The stars in these systems hold each other captive with their gravitational pull. The one with the higher gravity 'steals' material from its companion star and forms an accretion disk (see figure 1).

Currently, the exact size and geometry of accretion disks are not clear. New models and X-ray observations propose that the vertical size of the disk is bigger than older theoretical models predict. There might be an extended atmosphere above the disk. But how do you see this without the X-ray bright disk overwhelming the observation? The solution is to find a suitable X-ray binary system at such a viewing angle that the companion star eclipses the bright disk (see figure 2).

SRON-astronomers Ioanna Psaradaki, Elisa Costantini and Missagh Mehdipour, together with Maria Diaz Trigo from ESO, selected the eclipsing binary system EXO 0748-676 and studied it with the X-ray space observatory XMM-Newton. The team chose a binary of two low-mass stars for their research, as more massive stars have strong outflowing winds that are hard to distinguish from accretion flows. At times, the accreting star and its disk were completely eclipsed by the companion star, so the researchers managed to obtain a spectrum of the intriguing disk atmosphere.

Figure 2: As seen from Earth, EXO 0748-676 is tilted at such an angle that the companion star sometimes blocks the primary star and its accretion disk. This provides astronomers with a view of the disk’s atmosphere, without the star and disk outshining it.

The eclipse method enabled the astronomers to observe the atmosphere more directly than previous studies. They confirm that the atmosphere must be thicker than predicted and that the gas in the extended atmosphere appears in two different phases. The first gas component is hot, with a temperature close to that of the lower part of the disk. The second gas component is cooler and smaller in size, and comes from the outer part of the disk. The researchers propose that the latter component is clumpy material created by the impact of the accretion stream on the disk.

"The most likely explanation for such an extended disk atmosphere is that the accreting star photoionizes the outer parts of the disk due to strong X-ray radiation," Psaradaki says. "This phenomenon causes thermal instabilities, while the gas tries to find a stable solution. This is made possible if the disk increases its volume and hence creates an extended atmosphere, as we saw in our research."

## VLA Focuses on Inner Region of Massive Protostellar Jet

Astronomers using NSF’s Karl G. Jansky Very Large Array (VLA) have observed the fast-moving jet of material ejected by Cep A HW2, a massive protostar located 2,283 light-years away in the Cepheus A star-forming region.

An artist’s conception of Cep A HW2, showing a wide-angle wind originating close to the star and an accretion disk, with a much narrower jet farther away. Image credit: Bill Saxton, NRAO, AUI & NSF.

Low- and high-mass protostars propel jets outward perpendicular to a disk of material closely orbiting the star.

In stars with masses similar to the Sun, these jets are narrowed, or focused, relatively tightly near to the star in a process called collimation.

Because most high-mass protostars are more distant, studying the regions close to them has been more difficult, so astronomers were unclear if this was the case with them.

Cep A HW2 is expected to develop into a new star about 10 times more massive than the Sun.

The new VLA images of the system showed the finest detail yet seen in such an object, giving the astronomers their first view of the innermost portion of the jet, a portion roughly as long as the diameter of the Solar System.

“What we saw is very different from what usually is seen in the jets from low-mass stars,” said Dr. Adriana Rodriguez-Kamenetzky, an astronomer at the National Autonomous University of Mexico.

“In lower-mass protostars, observations have shown the jets to be collimated as close to the star as only a few times the Earth-Sun distance.”

“In Cep A HW2, however, we see not a single jet, but two things: a wide-angle wind originating close to the star, then a highly-collimated jet some distance away,” added Dr. Alberto Sanna, an astronomer at the Osservatorio Astronomico di Cagliari.

The collimated jet starts at a distance from the star comparable to the distance from the Sun to Uranus or Neptune.

VLA image of the jet from Cep A HW2, with surrounding area shown in the image from the NASA/ESA Hubble Space Telescope circles indicate location of the accretion disk, not seen in this image. Image credit: Carrasco-Gonzalez et al. / Bill Saxton, NRAO, AUI & NSF / STScI.

“The discovery raises two main possibilities,” the astronomers said.

“First, the same mechanism could be at work in both high-mass and low-mass protostars, but the collimation distance could be determined by the mass, occurring farther away in more-massive systems.”

“The second possibility is that high-mass stars might produce only the wide-angle wind seen in Cep A HW2, with collimation only coming when physical conditions around the star restrict the flow.”

“That case would point to a major difference in the mechanisms at work in protostars of different masses,” said Dr. Carlos Carrasco-Gonzalez, an astronomer at the National Autonomous University of Mexico.

“Answering this question is important to understanding how stars of all masses form.”

Carlos Carrasco-González et al. 2021. Zooming into the Collimation Zone in a Massive Protostellar Jet. ApJL, in press arXiv: 2106.01235

## Star Formation

Stars are formed deep within giant molecular clouds in the galaxy, shrouding star formation in a fundamental yet unsolved mystery. It is a process that spans magnitudes in scale and is strongly coupled to the cloud's dynamics. The cloud is influenced by gravity, the interstellar magnetic field, supersonic turbulence, and mechanical and radiative feedback from the newborn stars themselves. The primary challenge to both theorists and observers is to determine the role each plays in the star formation process as these relate to the fraction of a cloud's mass converted into stars, the formation of massive stars and young stellar clusters, the distribution of angular momentum, and the quenching of star formation.

With its sensitivity to point sources and low surface brightness emission coupled with its imaging array instruments in the 1-3mm bands, the LMT can make significant contributions to this effort by measuring both the large scale low-density envelopes of giant molecular clouds and the high density cores from which stars and clusters condense.

### Magneto-Turbulence in Molecular Clouds

Turbulent gas flows and the magnetic properties involved are key to regulating star formation and configuring the mass distribution of cores within them. By studying the molecular line emission of giant molecular clouds, measurements made by the LMT can assess the conditions in which the turbulent energy spectrum departs from the norm, which may signal zones of energy dissipation or injection, and may also help in determining the role of the magnetic fields.

### Properties of Protostellar and Protocluster Cores

The protostellar and protocluster cores that emerge within the cloud are the precise sites of star formation. These cores strongly radiate in the 1mm band from cold dust within them. Imaging the thermal emission from dust grains over the extent of a molecular cloud using the LMT's millimeter-wavelength cameras provides a direct census of active or potential sites of star formation. The emission can be used to derive radial profiles of density for individual cores that can be compared to theoretical predictions and compile the core mass distribution function. Insight to the star formation process is further revealed by observations that probe the chemistry and kinematics of dense gas, as these trace the initial conditions prior to protostellar collapse.

### Protostellar Disks

The gravitational collapse of dense rotating cores within a molecular cloud results in the creation of a central protostar surrounded by a flattened spinning disk of gaseous material. In this accretion disk, mass is transported inward toward the star and angular momentum is transported outward. Eventually, around the time newly formed planets inhibit further growth of the star, the disk moves into a phase known as a debris disk, where it resembles something not so different from our own asteroid belt, with lots of dust and planetismals.

The accretion phase for low mass protostars that will become sun-like stars is intriguing, as it is always accompanied by the simultaneous presence of a high velocity ejection of material into bipolar jets that emerge perpendicular to the plane of the disk. Although we know that accretion disks and jets of expelled material are always seen together, exactly how this pairing happens is a mystery. It is possible to solve this mystery by further exploring both the intersection of the disk and the star, where the jets are probably formed, and the innermost part of the disk, which spinning rapidly and has a magnetic field.

Simulated observation of a protoplanetary disk. Generated with AzTEC operating at 1.1 mm on the LMT. The inner-hole could be created by the processes that sweep up material during the aggregation of planetesimals and early formation of planets.

## Astronomy Picture of the Day

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2002 September 27
Accretion Disk Simulation
Credit: Michael Owen, John Blondin (North Carolina State Univ.)

Explanation: Don't be fooled by the familiar symmetry. The graceful spiral structure seen in this computer visualization does not portray winding spiral arms in a distant galaxy of stars. Instead, the graphic shows spiral shock waves in a three dimensional simulation of an accretion disk -- material swirling onto a compact central object that could represent a white dwarf star, neutron star, or black hole. Such accretion disks power bright x-ray sources within our own galaxy. They form in binary star systems which consist of a donor star (not shown above), supplying the accreting material, and a compact object whose strong gravity ultimately draws the material towards its surface. For known x-ray binary systems the size of the accretion disk itself might fall somewhere between the diameter of the Sun (about 1,400,000 kilometers) and the diameter of the Moon's orbit (800,000 kilometers). One interesting result of the virtual reality astrophysics illustrated here is that the simulated disk develops instabilities which tend to smear out the pronounced spiral shocks.

We are investigating disks around WD accretors in close binary systems (CVs). CVs are the origin of nova and dwarf-nova phenomena. Particularly interesting are AM CVn systems, which have (almost) pure helium disks. Determining their metal abundances allows us to conclude on the nature of the donor star, which could be a helium white dwarf. Such white-dwarf binaries are of general interest because they are potential SN Ia progenitors and primary targets for future gravitational wave detectors.

We also model the temporal evolution of dwarf-nova outbursts of CVs, which are thermal instabilities in the disk, resulting in a strong increase of the mass-accretion rate.

Many CV spectra exhibit prominent P Cygni line profiles in the UV, indicative for strong mass-loss away from the disk. We are modeling the disk-wind spectra in order to conclude on mass-loss rates, wind structure and wind-acceleration processes. Depending on the mass-loss rate, the evolution of the binary system can be significantly affected.

Like CVs, Symbiotic Stars also consist of WD accretors in binaries, however, with long orbital periods. In contrast to CVs, mass-transfer does not occur by Roche-lobe overflow but through accretion of wind-matter from the red-giant companion. Our disk plus WD atmosphere models can be used to compute the spectral energy distribution of such systems.

## Discovery of Interaction between Jet and Disk Wind from a Star-Forming Accretion Disk

(Left) ALMA composite image of dust emission (gray image), SO emission (orange), and SiO emission (green) towards the center of the HH 212 star-forming system. Accretion disk is seen in dust emission, jet is seen in SiO and SO emission along the symmetric axis, bow shocks are seen in SiO emission at large distances from the protostar. Faint wind is seen in SO emission, fanning out from the disk. The shells produced by the jet-wind interaction is also seen in SO emission, connecting to the bow shocks at large distances. Credit: ALMA (ESO/NAOJ/NRAO)/Lee et al. (Right) An artistic conception showing the disk, jet, wind (greenish), and shells in the system.
Credit: Ya-Ling Huang/ASIAA

“Thanks to the powerful ALMA, we spatially resolve a previously detected disk wind in the HH 212 star-forming system and confirm it to be a magnetic wind launched from an accretion disk”, says Chin-Fei Lee at ASIAA with excitement. “In addition, we also detect its interaction with the jet, providing the first evidence of jet and disk wind interaction in star formation. A thin shell produced by the interaction can be clearly seen, forming an inner boundary of the disk wind and connecting to the large bow shocks driven by the jet at large distance.”
Benoit Tabone at Leiden Observatory, who provided the theoretical model to this study, said “It is amazing to see how well our magnetic disk wind models can match the observed morphology and kinematics of the HH 212 wind. Our model initially reproduced low spatial resolution ALMA observations, but with these new high angular resolution observations we are able to robustly test the magnetic disk wind models and infer the angular momentum carried away by the wind.”

“The observations and modeling of the jet-wind interaction open an entirely new and promising avenue to constrain the large-scale magnetic field in accretion disks, which can have fundamental impact on the early process of planet formation”, commented also Sylvie Cabrit at Observatoire de Paris.

Schematic diagram showing the launching of the jet and disk wind from an accretion disk, driving the accretion process for star formation.
Credit: Ya-Ling Huang/ASIAA

HH 212 is a nearby star-forming system in Orion at a distance of about 1300 ly. The central protostar (baby star) is very young with an age of only

40,000 yrs (which is about 10 millionth of the age of Our Sun) and a mass of

0.25 Msun. It accretes material actively through an accretion disk. A powerful bipolar jet is ejected from the center of the disk, allowing disk material there to be accreted to the central protostar.

Previous search in SO molecular emission at a resolution of 60 au detected a disk wind around the jet. Now with a resolution of 13 au (i.e. about 5 times higher resolution) and an unprecedented high sensitivity, ALMA resolved the disk wind and detected its interaction with the jet (see Figure 1). Quantitative modeling indicates (see Figure 2): (1) the wind is consistent with an extended magnetic disk wind launched from ≃ 4 to 40 au, extracting angular momentum to drive disk accretion (2) the jet is launched from the dust-free zone of the disk, allowing material there to fall onto the baby star and (3) the jet drives large bow shocks interacting with the disk wind and producing a cavity, with a thin SO shell forming its boundary. This interaction provides unique first clues to the unknown magnetic field strength and distribution in young accretion disks.

This research was presented in a paper “First Detection of Interaction between a Magnetic Disk Wind and an Episodic Jet in a Protostellar System,” by Lee et al. appeared in the Astrophysical Journal Letters on Feb 2nd, 2021.

## Accretion disk

An accretion disk is rapidly spiraling matter that is in the process of falling into an astronomical object. In principle, any star could have an accretion disk, but in practice, accretion disks are often associated with highly collapsed stars such as black holes or neutron stars.

The matter that feeds the accretion disk can be obtained when a star passes through a region where the interstellar matter is thicker than normal. Usually, however, a star obtains matter for an accretion disk from a companion star. When two stars orbit each other, there is an invisible surface around each of the stars, called the Roche lobe. Each star is more or less at the center of one of these two teardrop-shaped surfaces, which touch at their points. The two Roche lobes represent all points where the gravitational potential of both stars is equal. Any matter on a Roche lobe can just as easily fall into either star. If one star in a binary system becomes larger than its Roche lobe, matter will fall from it onto the other star, forming an accretion disk.

The matter falling into a collapsing star hole tends to form a disk because a spherical mass of gas that is spinning will tend to flatten out. The faster it is spinning, the flatter it gets. So, if the falling material is orbiting the central mass, the spinning flattens the matter into an accretion disk.

Black holes are objects that have collapsed to the point that nothing, not even light, can escape their gravity. Because no light can escape, there is no way to directly observe a black hole. However, if the black hole has an accretion disk, we can observe the black hole indirectly by observing the behavior of the accretion disk, which will emit x rays.

Accretion disks can also occur with a white dwarf in a binary system. A white dwarf is a collapsed star that is the final stage in the evolution of stars similar to the Sun. White dwarfs contain as much mass as the Sun, compressed to about the size of Earth. Normally the nuclear reactions in a white dwarf have run out of fuel, but additional nuclear reactions may be fueled by hydrogen from the accretion disk falling onto the white dwarf. White dwarfs have some unusual properties that do not allow them to expand slowly to release the heat pressure generated by these nuclear reactions. This heat pressure therefore builds up until the surface of the white dwarf explodes. This type of explosion is called a nova (not the same as a supernova), and typically releases as much energy in the form of protons in less than a year as the Sun does in 100,000 years.