Welcome!

I am a Benoziyo prize postdoctoral fellow at the Benoziyo Center for Astrophysics of the Weizmann Institute of Science. I completed my Ph.D. in astrophysics in 2019, working under the supervision of Prof. Dan Maoz in the Astrophysics Department of Tel-Aviv University. During my Ph.D. I spent a couple of years with an ESO Studentship at the European Southern Observatory in Garching. Before that I got an M.Sc. (working with Prof. Lev Vaidman) and a B.Sc. in physics from Tel-Aviv University, and a B.A. in history from The Open University of Israel. I was also one of the organizers of the TAU AstroClub, a public outreach organization.

Contact

Department of particle physics and astrophysics, Weizmann Institute of Science, Rehovot, 7610001, Israel
naama.hallakoun@weizmann.ac.il
Benoziyo Physics building, room 271
+972-8-9346505

My Research

My main research interest is white dwarfs (WDs) and their immediate surroundings, which basically means that I am looking for stuff around WDs. The fact that more than 97% of all stars, our Sun included, end their lives as WDs, makes them an interesting research subject. The immediate surroundings of WDs are key to our understanding of a number of puzzles. Observations of WDs can reveal the presence of stellar, substellar, and stellar-remnant companions, planets, dust, atmospheric heavy elements, and planetary debris, each of relevance to several important questions. In my research I acquire and analyze new observational data, as well as publicly available archival data, in order to provide clues and maybe even answers to some of these questions.
Scroll down to read about some of the things we have found, related and unrelated to WDs.

Characterizing the local double white dwarf population

Binary systems consisting of two WDs are important in a broad range of astrophysical contexts, from stellar evolution, through Type-Ia supernova (SN Ia) progenitors, to sources of gravitational waves. SNe Ia—supernova explosions of WDs—are a major source of heavy elements, and, as ‘standard candles’, they have provided one of the fundamental methods for estimating distances in the Universe. However, the nature of the progenitor systems of SNe Ia is still unclear. A progenitor scenario that has been long considered is the double-degenerate scenario, in which a double WD binary loses energy and angular momentum to gravitational waves, until merger and possible explosion as a SN Ia. If most SN Ia explosions are the result of double WD mergers, then the observed double WD merger rate should be high enough to account for the observed SN Ia rate. This motivates us to ask: are there enough double WDs to explain the observed SN Ia rate?

A review paper:

The binary fraction, separation distribution, and merger rate of white dwarfs

Double WD parameter space
1σ and 2σ likelihood contours in the plane of fbin, the fraction of WDs in binaries with separations less than 4 au, and α, the power-law index of the initial double WD separation distribution, for the SDSS WD sample in blue, for the SPY sample in green, and joint likelihood contours from combining the two sets of results in red. Straight lines are loci of constant double WD merger rate, as marked in units of mergers per year per WD (Maoz, Hallakoun, & Badenes 2018).

From a sample of spectra of 439 WDs from the ESO-VLT Supernova-Ia Progenitor Survey (SPY), we measured the maximal changes in radial velocity (ΔRVmax) between epochs, and model the observed ΔRVmax statistics via Monte Carlo simulations, to constrain the population characteristics of double WDs. We then combined these constraints with those obtained by Badenes and Maoz 2012 for a sample of ~4000 WDs from the Sloan Digital Sky Survey (SDSS). We found that about 10% of WDs are double WDs with separations up to 4 AU, and that the Galactic WD merger rate per WD is about 10-11 per year. Integrated over the Galaxy lifetime, this implies that 8.5–11% of all WDs ever formed have merged with another WD. If most double WD mergers end as more-massive WDs, then some 10% of WDs are double WD-merger products, consistent with the observed fraction of WDs in a ‘high-mass bump’ in the WD mass function. The double WD merger rate is 4.5–7 times the Milky Way’s specific SN Ia rate. If most SN Ia explosions stem from the mergers of some double WDs (say, those with massive-enough binary components) then ∼15% of all WD mergers must lead to a SN Ia.

For further reading:

SDSS J1152+0248: the sixth known eclipsing double white dwarf

SDSS J1152+0248
Light curve of the eclipsing double WD, SDSS J1152+0248, obtained at the APO and McDonald observatories (Hallakoun et al. 2016).

Looking at Kepler K2 data we discovered an eclipsing double WD, SDSS J1152+0248, only the sixth known at the time. With an orbital period of 2.4 hours, it consists of two WDs with masses of about 0.4 M, making it a future verification source for LISA (Korol et al. 2017).

For further reading:

  • N. Hallakoun, D. Maoz, M. Kilic, T. Mazeh, A. Gianninas, E. Agol, K. J. Bell, S. Bloemen, W. R. Brown, J. Debes, S. Faigler, I. Kull, T. Kupfer, A. Loeb, B. M. Morris, and F. Mullally
    “SDSS J1152+0248: an eclipsing double white dwarf from the Kepler K2 campaign"
    2016, Monthly Notices of the Royal Astronomical Society, 458, 845

Limiting the population of collisional triples using Gaia DR2

SDSS J1152+0248
Gaia colour-magnitude diagram for resolved double WDs within 120 pc, with projected separations <300 au. Pairs are connected by black solid lines, where the photometric primary (secondary) is marked by a blue triangle (red circle). None of the pairs have a tertiary companion with a projected separation <9000 au. The number density in this parameter space of the full 120 pc WD sample from Gaia (17,395 sources) is shown in grayscale for reference (Hallakoun and Maoz 2019).

The collisional-triple SN Ia progenitor model posits that SNe Ia result from head-on collisions of double WDs, driven by dynamical perturbations by the tertiary stars in mild-hierarchical triple systems. To reproduce the Galactic SN Ia rate, at least ∼30−55% of all WDs would need to be in triple systems of a specific architecture. We tested this scenario by searching the Gaia DR2 database for the postulated progenitor triples. Within a volume out to 120 pc, we searched around Gaia-resolved double WDs with projected separations up to 300 au, for physical tertiary companions at projected separations out to 9000 au. At 120 pc, Gaia can detect faint low-mass tertiaries down to the bottom of the main sequence and to the coolest WDs. Around 27 double WDs, we identified zero tertiaries at such separations, setting a 95% confidence upper limit of 11% on the fraction of binary WDs that are part of mild hierarchical triples of the kind required by the model. As only a fraction (likely ∼10%) of all WDs are in <300 au WD binaries, the potential collisional-triple progenitor population appears to be at least an order of magnitude (and likely several) smaller than required by the model.

For further reading:

Planets and debris around white dwarfs

The remains of the pre-WD-phase solar systems are revealed in the form of heavy element ‘pollution’ in WD atmospheres, excess emission from dust discs, and – only recently – in transits of planetary debris. In principle, WDs can host not only debris, but also whole planetary systems.

Once in a blue moon: detection of ‘bluing’ during debris transits in the white dwarf WD 1145+017

WD 1145+017 color difference
Demonstration of the circumstellar gas-induced bluing using VLT/X-SHOOTER spectra: photometric (error bars) and ‘spectral’ (star- shape symbols) color differences in and out of transit, for various color indices. The error bars are color-coded by the corresponding mean transit depth (the bluer the deeper). The lower panels show the g'-band ULTRACAM light curves from 2016 April 21 (middle) and 2016 April 26 (bottom). The highlighted areas correspond to the integrated time intervals, with colors matching those of the top panel error bars. The photometry derived from the spectra broadly reproduces the bluing and the trends seen in the photometric transit data (Hallakoun et al. 2017).

The first transiting planetesimal orbiting a WD was detected in K2 data of WD 1145+017 by Vanderburg et al. 2015 and has been followed up intensively. The multiple, long and variable transits suggest the transiting objects are dust clouds, probably produced by a disintegrating asteroid. In addition, the system contains circumstellar gas, evident by broad absorption lines, and a dust disc, indicated by an infrared excess. Using simultaneous multiband fast-photometry ULTRACAM measurements over the u'g'r'i' bands, we detected for the first time a change in the color of WD 1145+017 during transits. The observations reveal what appears to be ‘bluing’ during transits; transits are deeper in the redder bands, with a u'−r' color difference of up to ∼−0.05 mag. We explored various possible explanations for the bluing, including limb darkening or peculiar dust properties. ‘Spectral’ photometry obtained by integrating over bandpasses in the spectroscopic data in and out of transit, compared to the photometric data, shows that the observed color difference is most likely the result of reduced circumstellar absorption in the spectrum during transits. This indicates that the transiting objects and the gas share the same line of sight and that the gas covers the white dwarf only partially.

For further reading:

  • N. Hallakoun, S. Xu, D. Maoz, T. R. Marsh, V. D. Ivanov, V. S. Dhillon, M. C. P. Bours, S. G. Parsons, P. Kerry, S. Sharma, K. Su, S. Rengaswamy, P. Pravec, P. Kušnirák, H. Kučáková, J. D. Armstrong, C. Arnold, N. Gerard, and L. Vanzi
    “Once in a blue moon: detection of ‘bluing’ during debris transits in the white dwarf WD 1145+017"
    2017, Monthly Notices of the Royal Astronomical Society, 469, 3213
  • Siyi Xu, Na'ama Hallakoun, Bruce Gary, Paul A. Dalba, John Debes, Patrick Dufour, Maude Fortin-Archambault, Akihiko Fukui, Michael A. Jura, Beth Klein, Nobuhiko Kusakabe, Philip S. Muirhead, Norio Narita, Amy Steele, Kate Y. L. Su, Andrew Vanderburg, Noriharu Watanabe, Zhuchang Zhan, and Ben Zuckerman
    “Shallow Ultraviolet Transits of WD 1145+017"
    2019, The Astronomical Journal, 157, 255

Periodic optical variability and debris accretion in white dwarfs

WD 1145+017 color difference
(a) HST/COS spectrum of WD J1949+4734 (black) and model fit (red). Unmodelled absorption lines are of interstellar origin. Airglow of O I is visible around 1302–1306 Å. (b) Interstellar and photospheric C II lines. (c) Molecular hydrogen absorption lines (unbinned spectrum, grey; binned every six data points, black). The blue ticks mark the theoretical wavelengths of some H2 (Hallakoun et al. 2018).

Recent Kepler photometry has revealed that about half of WDs have periodic, low-level (∼10−4−10−3), optical variations. Hubble Space Telescope (HST) ultraviolet spectroscopy has shown that up to about one half of WDs are actively accreting rocky planetary debris, as evidenced by the presence of photospheric metal absorption lines. We have obtained HST ultraviolet spectra of seven WDs that have been monitored for periodic variations, to test the hypothesis that these two phenomena are causally connected, i.e. that the optical periodic modulation is caused by WD rotation coupled with an inhomogeneous surface distribution of accreted metals. We detected photospheric metals in four out of the seven WDs. However, we found no significant correspondence between the existence of optical periodic variability and the detection of photospheric ultraviolet absorption lines. Thus, the null hypothesis stands, that the two phenomena are not directly related. Some other source of WD surface inhomogeneity, perhaps related to magnetic field strength, combined with the WD rotation, or alternatively effects due to close binary companions, may be behind the observed optical modulation. In addition, we marginally detected molecular hydrogen in WD J1949+4734, only the fourth known WD with detected H2 lines.

For further reading:

  • Na'ama Hallakoun, Dan Maoz, Eric Agol, Warren R. Brown, Patrick Dufour, Jay Farihi, Boris T. Gänsicke, Mukremin Kilic, Alekzander Kosakowski, Abraham Loeb, Tsevi Mazeh, and Fergal Mullally
    “Periodic optical variability and debris accretion in white dwarfs: a test for a causal connection"
    2018, Monthly Notices of the Royal Astronomical Society, 476, 933

The initial mass function of stars in the Milky Way

It has long been a major goal in astrophysics to measure the initial mass-distribution function (IMF) with which stars form. Countless theoretical predictions and interpretations of observations, in many astronomical sub-disciplines, rely on the assumption of an IMF. Even more important, perhaps, it has long been hoped that the observed IMF and its variations, if any, with cosmic time and star-forming environment, could serve as a fossil clue to the poorly understood process of star formation.
Until the last decade, opinions seemed to favour the existence of a universal IMF, even if discord remained regarding the exact details of the IMF's functional form. More recently, however, evidence has been accumulating for IMF variations in at least some extra-galactic environments, particularly in massive early-type galaxies. Data from Gaia DR2 revealed that Milky Way halo stars are divided in the Hertzsprung-Russell diagram (HRD) into two parallel sequences, a "blue" low-metallicity sequence and a "red" higher-metallicity locus. Various studies have shown that the blue halo is largely composed of stars that were accreted from merged galaxies (see the paper below for references). Much, or perhaps all, of the red halo, in turn, is composed of thick-disc stars that were heated by the encounter, i.e. the red halo and the thick disc have essentially the same origin, with the thick disc itself probably being an ancient Milky Way pre-merger structure that was heated and thickened by the merger.
Gaia DR2, for the first time, permits the analysis of large and complete stellar samples selected according to kinematic component and metallicity, ideal for IMF determination. The IMF range below ~1 M is particularly straightforward to probe, as stars in that mass range are still in the main-sequence stage of their evolution, and therefore the current mass function and the IMF are one and the same (i.e. no accounting is needed for stars that have evolved post-main-sequence, which would necessitate assumptions about star-formation history). Thus, except for the need to deal with some observational effects (e.g. completeness, extinction, unresolved binaries), a count of stars as a function of their masses gives an almost direct measurement of the IMF.

A bottom-heavy initial mass function for the accreted blue-halo stars of the Milky Way

Normalized IMFs measured for each of the Galactic components
Normalized IMFs measured for each of the Galactic components, shifted from each other vertically for the sake of clarity. The central values are based on the full extinction- and binarity-corrected subsamples, within 100 pc for the disc samples, and 250 pc for the halo samples. The shaded regions mark the uncertainty of each IMF, based on the extreme values measured for all of the binarity-corrected subsamples of each component at the same distance (Hallakoun & Maoz 2020).

We used Gaia DR2 to measure the IMF of stars within 250 pc and masses in the range 0.2 < m/M < 1.0, separated according to kinematics and metallicity, as determined from Gaia transverse velocity and location on the HRD. The predominant thin-disc population has an IMF similar to traditional (e.g. Kroupa 2001) stellar IMFs, with star numbers per mass interval dN/dm described by a broken power law, mα, and index αhigh~-2 above m~0.5, shallowing to αlow~-1.3 at m~<0.5. Thick-disc stars and stars belonging to the "high-metallicity" or "red-sequence" halo have a somewhat steeper high-mass slope, αhigh~-2.3 (and a similar low-mass slope αlow~-1.1). Halo stars from the "blue sequence", which are characterised by low-metallicity, however, have a distinct, bottom-heavy IMF, well-described by a single power law with α~-2.2 over most of the mass range probed. The IMF of the low-metallicity halo is reminiscent of the Salpeter-like IMF that has been measured in massive early-type galaxies, a stellar population that, like Milky-Way halo stars, has a high ratio of α elements to iron, [α/Fe]. Blue-sequence stars are likely the debris from accretion by the Milky Way, ~10 Gyrs ago, of a moderate-mass galaxy or galaxies. These results hint at a distinct mode of star formation common to two ancient stellar populations—elliptical galaxies and galaxies accreted early-on by ours.

For further reading:

Source code

Some of the code I have written (mainly for the Wise Observatory) is available through my GitHub profile.

pyWise

pyWise is an image reduction pipeline for the Wise Observatory. It is available through GitHub.

ScheduleRTML

ACP Scheduler is a commercial tool used to automatically schedule and execute observing plans for telescopes at the Wise Observatory. The observing plans can be entered manually using the Scheduler's GUI, or imported from Remote Telescope Markup Language (RTML) files. ScheduleRTML is a small python script that I wrote, to facilitate the writing of these RTML plans. It is available through GitHub.

STAM

STAM (Stellar-Track-based Assignment of Mass) is a python package that assigns mass and metallicity to stars based on their location on the Hertzsprung-Russell diagram, using publicly-available stellar evolution tracks. It is available through GitHub.

WiseGCN

WiseGCN is a GCN/TAN (Gamma-ray Coordinates Network/Transient Astronomy Network) handler for use at the Wise Observatory in case of gravitational-wave alerts. It is available through GitHub.

Public Outreach

TAU AstroClub

During my Ph.D., I was one of the organizers of the Tel-Aviv University Astronomy Club (AstroClub for short)—a public outreach organization, operated voluntarily by graduate students of the Department of Astrophysics of Tel-Aviv University. The activities are open to the general public, and include monthly lectures given by leading scientists, sidewalk observations, and open days at the Wise Observatory.
I have also paticipated in the Hebrew translations of Galaxy Zoo and the Astronomy Picture of the Day.

How to pronounce my name?

My first name (Na'ama נעמה) has three syllables, the stress should be on the last one: "nah-ah-MAH". To pronounce my last name (Hallakoun חלקון) ignore the spelling and just say "hal-KON", this is what I do.