1. Dust trapping in transition disks

My research focuses primarily on the density structure of gas and dust in transitional disks. Transitional disks are disks with a cavity in their dust distribution. Whereas they were originally identified through a dip in the infrared part of their SED, through the lack of warm dust, interferometric imaging with ALMA has revolutionized the field, revealing the complex structures in these systems. One of the major breakthroughs in Oph IRS 48 received a large amount of media attention.

Gallery of ALMA continuum images of transition disks (credits Nienke van der Marel).

Image of the Oph IRS 48
dust continuum (credits ESO).

The millimeter continuum images show that the millimeter-sized dust grains are distributed in rings and asymmetries, consistent with dust trapping or dust segregation in pressure bumps, as postulated in early dust evolution models. I work in close collaboration with e.g. Paola Pinilla to interpret these structures in the context of dust evolution. The first unambiguous evidence was found in the Oph IRS 48 system, which shows an extreme asymmetry in the submillimeter continuum, whereas the infrared image (smaller dust grains) and gas are ringlike. These results were presented in "A major asymmetric dust trap" in Science in 2013 (see press release page).

The structure of gas and dust in Oph IRS 48 (from van der Marel et al. 2013, Science).

Through multi-wavelength analysis at high spatial resolution we have demonstrated evidence for dust trapping in several other transition disk systems, for example HD 135344B, SR 21 and IRS 48 using the JVLA. In HD135344B we have also found evidence for a combination of a ring with an outer vortex, which may be related to the spiral structures seen in scattered light images.
Millimeter and scattered light images of HD135344B (van der Marel et al. 2016b)

2. Gas densities inside cavities of transition disks

Modeling structure of gas and dust
in DALI (van der Marel et al. 2016a).

The origin of dust cavities has been debated for decades: planets may be responsible, but also dead zones and photoevaporation have been proposed as possible mechanisms. In order to constrain the origin, the gas density distribution inside the cavity has be constrained. Using spatially resolved ALMA images cubes of CO isotopologues and the physical-chemical modeling code DALI developed by Simon Bruderer, we have successfully modeled the density structure for several transition disks. The zero-moment maps of the CO isotopologues already reveal that the gas and dust are distributed differently: the 12CO images show a peak of emission inside the dust cavity, but as the 12CO emission is optically thick we used DALI to demonstrate for 5 disks that the drop in density was much deeper for the dust than for the gas. The 12CO emission in IRS 48 even revealed a gap in the gas, with a smaller radius than the dust.

Radial cuts through the 12CO zero-moment maps and dust continuum (van der Marel et al. 2015)

In follow up studies with CO isotopologues it was found that several transition disks contain gas gaps smaller than their dust counter parts, which is direct evidence for the planet clearing scenario in combination with dust trapping.

Radial cuts through the 13CO zero-moment maps and dust continuum (van der Marel et al. 2016a)

A detailed study of J1604-2130 through 3 CO isotopologues even shows that the slope of the gas gap is not vertical, but has a smooth slope, again consistent with planet clearing. I am currently working on ALMA CO data of several other transition disks to constrain their gas density structure inside the cavity.

A smooth density drop in the gas density is required to fit the visibility curves of 3 CO isotopologues in J1604-2130 (Dong et al. 2017)

3. Disk surveys

In addition to the detailed studies of individual objects, I am also working on large disk surveys, in order to get a better understanding of general disk evolution. During the four years of my PhD I have put together an (almost) complete sample of transition disk SEDs using the Spitzer catalogs. The derived properties are catalogued here. Also, I am part of the Lupus ALMA Disk Survey team, led by Jonathan Williams, which has imaged more than 90 disks at 0.3" resolution in continuum and CO isotopologues. With several follow up projects we aim to get a full picture of disk evolution in young star forming regions. Recently we have also finished a survey on disks in Sigma Orionis.
Gallery of the Lupus disk survey continuum images. (Ansdell et al. 2016)

4. Direct imaging

Through the access to Mauna Kea telescopes, I have started several direct imaging programs of planets inside transition disks, using the Adaptive Optics systems at Keck and Subaru. The combination of constraints from ALMA observations and upper limits on planet detections provides a unique tool to derive properties of planets in disks.

5. Astrochemistry

I am interested in the chemical composition of disks, as they may determine the molecular composition of planets and they can help to trace physical properties in disks. We have detected formaldehyde in the IRS 48 disk and I am currently working on a combination of modeling and observations of CN in disks through ALMA and SMA data. In the past I have also worked on lab spectroscopy of interstellar ice analogs and complex molecules in protostars.

6. Other projects

In addition to disk projects, I have worked and contributed to a large range of projects throughout the years: star formation studies of molecular outflows, instrumentation, debris disks, embedded disks, exoplanet studies, binary systems and discussions on development of DALI and physical-chemical models. For any inquiries on my on-going work and collaborations, please contact me.