❭❭ DDiT - Debris disks tool
I just released my Python code to make images of debris disks, in scattered or polarized light. If interested, click the title of this note to get to the github page, where you should find some basic instructions to install the module and run some first models quickly. Hope this will be useful to someone.

"Coder view" of a debris disk
I had previously done an "artist view" of a debris disk, here is another attempt, with a different approach, that actually includes some physics. The image is done with Python, and is just a bunch of particles, each of them with their own orbital parameters (driven by their beta ratio due to the radiation pressure), and I trace back the orbits along some short period of time to give a sense of movement.

The idea was to illustrate most of the research areas that we have here in the Max Planck Tandem Group, so there is dust, a planet, well, a star, and a secondary star outside of the disk. The only thing that is missing is gas, but I could not find a nice way to represent it. Maybe there will a new version at some point.

Nothing too fancy overall, but it was interesting to write the code in a (relatively) flexible way so that I can easily change the color, transparency, length of the trails, etc. Feel free to use it if you find it useful.

❭❭ The debris disk around HR4796
Our paper on the debris disk around HR 4796 A has just been accepted by A&A (click the title for the astro-ph version). We obtained polarimetric observations with the SPHERE/ZIMPOL instrument, which revealed the disk with exquisite details. The main motivation of the paper is to try to trace back the production of the small dust grains that we see in those observations. I wrote a code that can account for some of the dynamical effects such as radiation pressure from the central star (which acts differently on grains with different sizes), and with our best fit model, we are able to not only reproduce the SPHERE observations, but also to reproduce mid-IR and sub-mm observations.

We find that small dust grains should be preferentially released close to the pericenter of the disk (which is slightly eccentric), while this is not expected from a dynamical point view. The planetesimals spend most of their time near the apocenter, and therefore the natural expectation is that the collisions releasing dust grains should take place there. One of the possible explanation is that the relative velocities between the planetesimals are large near the pericenter and therefore collisions are more destructive and releasing more dust grains. An alternative explanation is that a catastrophic collision between two massive planetesimals took place near the pericenter of the disk. This point of colision becomes a "pinch" point for all the other bodies, and it as a consequence it becomes a place of preferential dust production.

A video explaining the basics of the model can be found here.

Discovery of a planetary mass companion in a transition disk
Very exciting results by Keppler et al. and Müller et al.. These two papers present the detection and characterization of a newly discovered planetary-mass companion, right in the gap of the transition disk PDS70. The gap in this disk has been known for quite some time now, but no point sources had been discovered yet. New SPHERE observations revealed an intriguing candidate companion that turned out to be of planetary mass. In the paper by M. Keppler, of which I am part, we used observations from other instruments (VLT/NaCo and Gemini/NICI) in which we also retrieve the companion. With the observations at different dates, we could confirm that the companion is actually co-moving with the central star, and it is not a background or foreground object in the line of sight of PDS70. Very fascinating discovery, and the PDS70 will be studied in great detail in the years to come!

See also the very nice ESO Press Release (from which I took the following image).

❭❭ SPHERE reveals the faint debris disk around TWA7
Debris disks are usually quite faint compared to younger proto-planetary disks, and they are relatively rare around low-mass stars. Therefore, any opportunity to study them is extremely interesting. For instance the debris disk around the low-mass star AU Mic has shown some very intriguing features that are moving outward of the system, at high velocities. AU Mic being seen edge-on, we are losing a lot of information due to projection effect.

TWA7 is another debris disk around a young low-mass star, and observations presented in Choquet et al. (2016) revealed that the disk is seen close to face-on. We obtained open time and GTO observations with the VLT/SPHERE instruments to observe this interesting disk. To suppress the light of the star, we used the Differential Polarimetric Imaging technique and we recovered a nice detection of the disk.

To our surprise, in the two epochs that we have (separated by a year) we see the beginning of a spiral arm (in the lower right side), which is quite unexpected in a debris disk (spiral are often supported by gas, and debris disks are expected to be free of gas). We also detected a very faint halo of dust beyond the main ring, but deeper observations are most likely required to confirm its existence.

❭❭ Postdoc opportunity
I am now in the process of finding a post-doc to join the Max Planck Tandem Group in Valparaiso (comprised of Nicolas and myself). Nicolas, a first year PhD student, is working on direct imaging, trying to find planets in systems with debris disks. I would like the post-doc to work with me on the debris disks themselves, either on observations or numerical simulations. Click on the title to find the AAS announcement.
Paper software
I will very soon switch from Mac OS to Linux. I had been using the "Papers 3" software quite intensively, but to my knowledge there is no Linux version available. Other alternatives did not really appeal to me, for a variety of reasons. So I thought I could "design" my own application, and I am really happy with what I have at the moment. Everything is in Python, connected to an SQL database, and the interface is done with QT. I have my sets of collections, the software can try to update papers that were taken from arXiv to the version from the journal, I can export the bibtex, and go to the url of the paper.

There are still some issues, and it is very suited the setup of my computer (some properties are hard coded, I'd like to make a preferences system in the near future), so it is most certainly not to be distributed yet, but I'll keep working on that on my spare time. Definitely learned a lot doing it, and I am already using it quite intensively.

❭❭ ALMA detection of the disk around a free-floating planet
Amelia just got a Letter accepted on our ALMA observations of the free-floating planet OTS44. We detected the circum-planetary disk and we could derive some estimates of the disk dust mass. It seems that the correlation between the (sub) stellar mass and the disk mass prevails down to the planetary regime. This has important implications on the formation mechanism of (very) low mass objects. We are also having two press releases (MPIA and Valparaiso).

❭❭ SPHERE observations of the disk around TCha
The very nice paper by Adriana Pohl was accepted by A&A. We resolved the transition disk around TCha, with the SPHERE instrument (scattered and polarized ligth images). Now, if you have a look at my publications, you will see that I've been particularly interested about this disk. I was very happy to see that the inclination and position angle that we had found using VLTI are in the end not too far off what we see with SPHERE.

Unfortunately, we did not detect any companions inside the gap, with relatively stringent upper limits (~9 Jupiter masses). The modeling led by Adriana suggests that the dust grains are fairly big in the outer disk, which would explain the strong forward scattering phase function that we observe with IRDIS in scattered light.

Artist view of a debris disk
And here is my attempt at making a debris disk with Blender. It was a different challenge as the transition disk one. I tried to use the results of N-body simulations, but it was a bit too patchy (not real "continuum"), and in the end, I am quite happy with the final result.

Artist view of a transition disk
Having artist impressions is always helpful for talks and outreach, and I was curious if I could manage to have a decent representation of a transition disk. I used to be quite familiar with Blender but never got to use the dynamic features it offers (smoke or fluid simulations). Took me a little bit of time experimenting and testing things, but I am quite happy with the end result. Next stop, a debris disk.

Database is online
The database of debris disks is online (see the link in the top bar). This is the result of some work that has been in preparation for quite some time now and that I still have to publish. But I find it quite useful to have the SEDs accessible from more or less anywhere. Since there was the Gaia first release not so long ago, I will have to update the distances, and I also would like to revise some of the stellar parameters as the photosphere is sometimes a bit off. For these reasons I chose not to provide the full results (e.g., a CSV file with the SED). But once the paper will be published, I will probably make them available.

On top of the SED, a little while ago I went through the literature for disks that have been spatially resolved. I had a very basic offline database, and decided it would be nice to include the images on the website. This is a completely independent database as the one for the SEDs. It may not be super complete, but I'll do my best to keep it up to date with the literature.
❭❭ Concentric rings in the disk around HIP73145
In Feldt et al. we present new VLT/SPHERE images of the interesting disk around HIP 73145 (HD 131835). We resolve several concentric rings within ~1 arcsec from the star. The disk had been resolved before, but this is the first time that we see hints for such a detailed morphology for this object. The disk is also known to have CO gas, so maybe it has to do something with these narrow rings that we observe ?

Star trail in Valparaiso - v2
Another attempt at a startrail from our balcony. Looks much smoother after tweaking a bit the settings. Interestingly, you can see the effect of the twilight, making the trails appearing as a "brush stroke". I may do some frame selection to see how it looks without including the twilight.

Star trail in Valparaiso
A quick exercise done over the week-end. I set up my camera to take 10 sec exposures from our balcony, with a 5 sec delay between exposure. I set up the exposure correction on the camera that basically takes a dark after taking the exposure. Clicking on the image below will show the full picture, and as you can see the trails are dotted. I suspect the dark frames add too much delay on top of the 5 sec and therefore the trails look discontinued. I may try another time, with a shorter exposure time and less delay to avoid this. It was also interesting to combine all the images together. Instead of taking the median (as I thought I should at first), for each pixels, I had to take the maximum value among all the images.

NACO observing run
In Paranal for a bit more than a week, to observe with NACO. This is part of the "ISPY" program in collaboration with Heidelberg and Geneva. We are searching for planets around young and nearby stars, using the AGPM coronagraph. So far the conditions have been pretty favorable, and we've observed some pretty interesting debris disks. It would be a nice bonus if some of these disks show up in the final data reduction, but L-band is not ideal for such detections.

New version of the website
It has been a while that I wanted to update this website, so here it is. I finally had a bit of time to implement the "articles" and publications in a proper SQL database, which should allow me to edit the website from anywhere. In the old version, I had to upload text files basically as the website was hosted at the MPIA and it was not possible to use databases, and write permissions were very limited. Anyways, new version, new design, I tried to get rid of the things I was barely using before, and I think I like the more minimalistic design.

In the "near future" I will try to implement the new version of the debris disk database which is long overdue. I have a few ideas on what I'd like to show and all, but I'm not sure I know PHP and java well enough to do that. We will see I guess.
❭❭ Azimuthal asymmetries in the debris disk around HD 61005
I recently submitted my latest paper, and that was quite an interesting study. In the past two years I spent a lot of time working on the debris disk tool (first entry about the SED modeling was actually in Aug. 2013, and the first images were probably produced end of 2013), and I improved the code in many different places. I could use it to model VLT/SPHERE data (scattered and polarized light) as well as ALMA millimetric observations. The code seems to behave properly and is actually quite fast, meaning I could do a relatively in-depth study of the disk around HD 61005.

We could explain the known brightness asymmetry by including an azimuthal dependance of the dust density distribution, which would be located at the pericenter of the eccentric disk (eccentricity of about 0.1). We proposed that there could have been a collision of massive planetesimals in the main dust belt, at about 60 AU from the star. Such a collision would last for about 0.5 Myr at this distance from the star, and we may be witnessing the in situ formation of Super-Earth around this young solar type star.

Overall, I learned a lot doing this study, about direct imaging, about radio interferometry and I also developped a suite of tools that can enable me to perform such modeling relatively rapidly. I am already collaborating with Markus Feldt on another object and the modeling is actually finished. Can't wait to have more data !

❭❭ A narrow edge-on disk around HD106906
Things are moving fast with another VLT/SPHERE result this month ! We resolved a narrow edge-on debris disk around the star HD 106906. The disk appears to be quite asymmetric, with a radius of about 65 AU. The departure from centro-symmetry could be explained by an eccentric disk (I did not get the chance to try my code on the data, need to publish it first :)).

Interestingly, the disk was also resolved by the GPI instrument, and both publications were on arXiv the same day. Both SPHERE and GPI appear to provide new observations of debris disks at high angular resolution and I couldn't be happier about that :)

❭❭ Fast moving structures around AU Mic
During the commissioning of the SPHERE instrument at the VLT, we had the opportunity to observe the edge-on debris disk around the M dwarf AU Mic. Besides the impressive angular resolution of the observations that beautifully resolve the disk, we found some intriguing "arch"-like structures along the East side of the disk (see Fig. below).

What was even more intriguing was that compared to archival images from the Hubble Space Telescope, these structures appear to have moved quite significantly. We could trace back the movement of these structures and out of the five structures that we detected, three have projected velocities larger than the escape velocity of the star.

The mechanism responsible for these structures is quite puzzling and the interpretation is not straightforward. I invite you to read the Nature paper for further details on the possible interpretation.

Eccentric disks
One of the last update I could think of for the debris disk code was to be able to produce eccentric disks. Some of the SPHERE observations I've seen would suggest that the disks are off-centered compared to the stellar's position and that the surface brightness profiles are not symmetric between the East and West sides.

So I spent a little bit of time over the past days to improve the code, and it seems to be working fairly well. Now the thing is, this adds two more free parameters (the eccentricity and the rotation of the disk). And that may complicate a bit the modeling, but we'll see.

Below are examples of eccentric disks. It may be hard to visualise it straight from the image, but the brightness profile cannot be explained by anisotropic scattering: there are geometric effect to take into account (the star is at the position 0, 0). Pretty happy about that !

❭❭ Polarimetric observations of LkCa 15
Christian Thalmann was granted Science Verification time to observe the transitional disk around LkCa 15 with SPHERE and we just got a letter accepted by ApJ. The object was observed with the ZIMPOL instrument, and the disk was detected in polarimetric light. The gap in the disk is well resolved and we even detect what could well be the inner disk, closer to the star. This indicates at least that the gap is not devoid of dust at all.

The Figure below shows the polarized intensity of the disk at a wavelength of about 0.6 µm. The crescent shape traces the outer disk while there is a significant amount of emission in the center, closer to the central star.

The two main results are that the candidate companion previously proposed based on SAM observations lies where we detect the inner disk. This means that the signal from the inner disk could be a source of contamination for the SAM observations. Nonetheless, more recent results presented at the Lyot conference a few weeks ago would suggest that the story is far from being over. Exciting times !

The second thing is that the disk appears off-centered in scattered light image, while it looks perfectly centered on the star position in sub-mm thermal images. And that is quite unexpected and counter-intuitive at first. Christian and collaborators thought of something that could well explain such differences (summarized in the Fig. 4 of the paper): if the inner and outer disks are not coplanar the inner disk could cast a shadow on the outer disk. Depending where the shadow is, it would trick the observer into thinking the disk is off-centered.

❭❭ Asymmetric features in the protoplanetary disk MWC 758
Last December, Myriam Benisty was awarded observing time for the Science Verification of the VLT/SPHERE instrument and she obtained a beautiful image of the proto-planetary disk around MWC 758. She kindly offered me to participate to this collaboration and we recently got our A&A Letter accepted !

The Figure on the right shows the polarised intensity of the disk at a wavelength of 1.04 µm. Two spiral arms are amazingly showing up, at an unprecedented detail and resolution, and the observations reveal the disk down to an inner working angle of about 26 AU. In the paper we discuss what could be launching these spiral structures and propose that two planets could explain the observed morphology.

SPHERE really is an impressive instrument and hopefully we'll see some other extremely nice results coming up in the coming months. Can't wait to see similar images of debris disks.

Edge-on debris disks
The previous version of the raytracing code was not really suited for edge-on disks. As soon as the line of sight would go below the upper layers of the disk, it would not work anymore. Since several interesting debris disks observed in scattered light are edge-on, or close to edge-on, it was a quite severe limitation of the code.

So I took the time to improve the code so that now it is possible to produce images of edge-on disks. It was actually quite a funny experience. First I really had to clean up the code because it was turning out to be a bit messy. And next I had to think of how to find the intercept between the disk geometry and the line of sight, which took me a bit of time but I am actually quite happy with the way it turned out. Below you can find an example of the kind of images I can produce (log-scale, H-band image, the star is not included).

❭❭ Sculpting the disk around T Cha: an interferometric view
Circumstellar disks are believed to be the birthplace of planets and are expected to dissipate on a timescale of a few Myr. In particular, a young planet will open a gap, resulting in an inner disk dominating the near-IR emission and an outer disk emitting mostly in the far-IR. The structure of the transition disk around T Cha is being constrained by a full set of data involving new interferometric data, as well as literature photometric data and archival data from VLT/NaCo/Sam. We find that the dust responsible for the strong emission in excess in the near-IR must have a narrow temperature distribution with a maximum close to the silicate sublimation temperature. We find that the outer disk starts at about 12 AU and is partially resolved by the Pionier, Sam, and Midi instruments. We show that the Sam closure phases, interpreted as a candidate companion, may actually trace the asymmetry generated by forward scattering by the outer disk. We stress the difficulty to detect point sources in bright massive disks, and the consequent need to account for disk asymmetries in model dependent search for companions.

❭❭ The twofold debris disk around HD113766A
Warm debris disks are a sub-sample of the large population of debris disks, and display excess emission in the mid-infrared. Around solar-type stars, very few objects show emission features in mid-IR spectroscopic observations, that are attributed to small, warm silicate dust grains. The origin of this warm dust can possibly be explained either by a recent catastrophic collision between several bodies or by transport from an outer belt similar to the Kuiper belt in the Solar System. We present and analyse new far-IR Herschel/Pacs photometric observations, for one of these rare systems: the 10–16 Myr old debris disk around HD 113766 A. First, we underline the limitations of SED modeling and the need for spatially resolved observations. We improve existing models and push further our understanding of the disk around HD 113766 A. We find that the system is best described by an inner disk located within the first AU, well constrained by Midi data, and an outer disk located between 9–13 AU. In the inner dust belt, our previous finding of Fe-rich crystalline olivine grains still holds. We discuss possible scenarios concerning the origin of the warm dust observed around HD 113766 A. We suggest that we are witnessing the outcomes of one collision between partially differentiated bodies, in an environment possibly rendered unstable by terrestrial planetary formation.

❭❭ Transient dust in warm debris disks
We develop and present a new radiative transfer code (DEBRA) dedicated to SED modeling of optically thin disks. The DEBRA code is designed such as it can determine dust composition and disk properties simultaneously. We make use of this code on the SEDs of eight warm debris disks, in combination with recent laboratory experiments on dust optical properties. We find that most, if not all, debris disks in our sample are experiencing a transient phase, suggesting a production of small dust grains on relatively short timescales. Dust replenishment should be efficient on timescales of months for at least three sources. From a mineralogical point of view, we find that crystalline pyroxene grains (enstatite) have small abundances compared to crystalline olivine grains. The main result of our study is that we find evidences for Fe-rich olivine crystalline grains (Mg / [Mg + Fe] ∼ 80%) for several debris disks. This finding contrasts with studies of gas-rich protoplanetary disks, where Fe-bearing crystalline grains are usually not observed.