Ragnhild Lunnan Assistant Professor

Contact

Name and title: Ragnhild LunnanAssistant Professor

Mail: ragnhild.lunnan@astro.su.se

Visiting address Room C6:3031Roslagstullsbacken 21, C 6 & D 6

Postal address Institutionen för astronomi106 91 Stockholm

Research group

The supernova group

About me

I am an assistant professor and docent in the Department of Astronomy at Stockholm University. Starting 2023, my group is funded by an ERC starting grant, TransPIre. I am a member of the Young Academy of Sweden for the period 2024-2029.

Before my current position, I held a Marie Skłodowska-Curie Individual Fellowship, also at SU and the Oskar Klein Centre. Prior to starting my fellowship, I was a postdoctoral scholar at the OKC, working with Prof. Claes Fransson and the larger intermediate Palomar Transient Factory and Zwicky Transient Facility group. This position was joint with the Caltech Astronomy department, where I stayed between September 2015-March 2017 working with Prof. Mansi Kasliwal.

I completed my PhD at Harvard University in 2015, supervised by Prof. Edo Berger. My thesis work focused on the properties and environments of superluminous supernovae, mainly from the Pan-STARRS1 Medium Deep Survey.

I did my undergraduate work at Princeton University majoring in Astrophysical Sciences with a certificate in Applied and Computational Mathematics. I was fortunate to attend Princeton as a Davis United World College Scholar, after spending the last two years of high school at Waterford Kamhlaba UWC in Eswatini.

I am originally from Heggenes, Norway.

Teaching

I am currently teaching the undergraduate Cosmology course, AS5003, which is given in the second half of the fall semester.

I am always interested in chatting with potential master or bachelor students who want to do a degree project on supernovae - feel free to contact me or stop by my office!

Research

I am broadly interested in unusual transients, and what they tell us both about massive star evolution and about star formation in extreme environments. Much of my work focuses on “superluminous” supernovae, a rare class of transients 10-100 times brighter than ordinary core-collapse and Type Ia supernovae. A typical superluminous supernova radiates more energy than the entire kinetic energy of a canonical core-collapse explosion (10^51 erg), and therefore cannot be easily explained by the same physical mechanisms. It is still debated what powers these extreme luminosities, with candidates including tapping the rotational energy of a newborn neutron star with a strong magnetic field (“magnetar”), a pair-instability explosion of an extremely massive star, or interaction between the supernova ejecta and dense circumstellar material.

I am an observational astronomer, and have worked with ground-based optical and infrared data from VLT, Magellan, MMT, Gemini and Keck, as well as UV, optical and infrared data from the Hubble Space Telescope, Spitzer Space Telescope, and Swift.

Understanding Superluminous Supernovae

Studying the supernova explosions themselves allows us to characterize the energies, timescales and velocities involved, and compare to the various physical models proposed to explain superluminous supernovae. I have worked on several studies of individual supernovae, as well as a compilation of objects from the Pan-STARRS Medium Deep Survey. I am currently working with the Zwicky Transient Facility, which is turning out to be a fantastic experiment for finding large numbers of superluminous supernovae and exploring their diversity. I explain some of our early results in this AAS Journal Author chat on Youtube; this work was also part of my Marie Curie project "SUPERS".

As part of a late-time spectroscopy campaign, we serendipitously discovered the presence of a fast-moving circumstellar shell around the superluminous supernova iPTF16eh through light echo emission from the Mg II resonance lines. This is exciting both because it provides conclusive evidence that some superluminous supernovae experience significant mass-loss episodes close to explosion, and because the shell properties were best explained by a pulsational pair-instability mass ejection. This regime of stellar evolution is also of great importance for understanding the black hole populations probed by gravitational wave experiments like LIGO/Virgo, and few observational constraints exist. You can read more details in the blog post I wrote for Nature Astronomy Community. I recently led a campaign using ESO's Very Large Telescope to constrain how common this phenomenon is, which forms the basis of Anamaria Gkini's PhD thesis.

In addition to finding nearby SLSNe that allows for late-time follow-up, ZTF (as well as its predecessor PTF) is also great for finding all kinds of rare transients. While at Caltech, I supervised the bachelor thesis of Lindsey Whitesides to analyze an exciting object falling somewhere in between a superluminous supernova and a typical gamma-ray burst supernova, iPTF16asu.

Host Galaxy Environments of Superluminous Supernovae

Since SLSNe are so rare, we don’t expect any to occur close enough for a direct progenitor detection. However, studying the galaxy-scale environments gives us a handle on the stellar populations they come from, and thus an indirect probe of the progenitor population. A systematic sample study of the host galaxies of SLSNe reveal that they show a strong preference for low-mass, low-metallicity dwarf galaxy hosts. Many of these dwarf galaxies are also star-bursting, with properties that would be classified as “extreme emission line galaxies” or “green peas” in a galaxy survey. One interpretation of these findings is that superluminous supernova explosions require a low metallicity progenitor, or at least that the rate is suppressed at higher metallicities. It has also been suggested that the intensity of star formation plays a role - this can be tested with resolved HST UV imaging through measuring the star formation at the supernova sites.

One can also use SLSNe as a tool to study the galaxies they occur in, through host galaxy absorption lines in the supernova spectrum. Similar techniques are used with long gamma-ray burst afterglows. In Pan-STARRS we used this technique successfully to study the ISM of a galaxy at z=1.566.

Calcium-Rich Gap Transients

Calcium-rich gap transients are another rare class of explosions, which unlike superluminous supernovae are fainter and faster-evolving than ordinary supernovae, and predominantly found in old galaxy environments. This suggests that the progenitor is a binary system, but the origin of these transients is not well understood. With Mansi Kasliwal at Caltech, I took a detailed look at two of these transients discovered by PTF, as well as a broader look at the host galaxy environments of Ca-rich gap transients in general. Read more here:

Two New Calcium-rich Gap Transients in Group and Cluster Environments

Other Projects

In my first two years of graduate school, I worked with Prof. Lars Hernquist and Dr. Anna Frebel on various aspects of near-field cosmology and galactic archeology. Read about it here:

The Effects of Patchy Reionization on Satellite Galaxies of the Milky Way

The 300 km s-1 Stellar Stream near Segue 1: Insights from High-resolution Spectroscopy of Its Brightest Star

I wrote my senior thesis with Prof. J. Richard Gott, III at Princeton, exploring the extent to which the genus statistic could be used as a probe of the equation of state of dark energy. Read more here:

Using the topology of large-scale structure to constrain dark energy