Selected publications

Agurto-Gangas, C., Pineda, J. E., Szűcs, L., Testi, L., Tazzari, M., Miotello, A., Caselli, P., Dunham, M., Stephens, I. W., Bourke, T. L.
Revealing the dust grain size in the inner envelope of the Class I protostar Per-emb-50
2019, A&A, 623, 147 [link,arxiv]

Bisbas, T. G., van Dishoeck, E. F., Papadopoulos, P. P., Szűcs, L., Bialy, S., Zhang, Z.-Y.
Cosmic-ray Induced Destruction of CO in Star-forming Galaxies
2017, ApJ, 839, 90 [link,arxiv]

Hocuk, S., Szűcs, L., Caselli, P., Cazaux, S., Spaans, M., Esplugues, G. B.
Parameterizing the interstellar dust temperature
2017, A&A, 604, 58 [link,arxiv]

Szűcs, L., Glover, S. C. O., & Klessen, R. S.
How well does CO emission measure the H2 mass of MCs?
2016, MNRAS, 460, 82 [link,arxiv,poster,short]

Szűcs, L., Glover, S. C. O., & Klessen, R. S.
The ¹²CO/¹³CO ratio in turbulent molecular clouds
2014, MNRAS, 445, 4055 [link,arxiv,poster,short]

Olofsson, J., Szűcs, L., Henning, Th., Linz, H., Pascucci, I., & Joergens, V.
The Herschel/PACS view of disks around low-mass stars in Chameleon-I
2013, A&A, 560, 100 [link,short]

Skemer, A. J., Close, L. M., Szűcs, L., Apai, D., Pascucci, I., & Biller, B. A.
Evidence against an edge-on disk around the extrasolar planet, 2MASS1207b
and a new thick-cloud explanation for its underluminosity
2011, ApJ, 732, 107 [link]

Szűcs, L., Apai, D., Pascucci, I., & Dullemond, C. P.
Stellar-mass-dependent disk structure in coeval planet-forming disks
2010, ApJL, 720, 1668 [link]

My papers on ADS

The ¹²CO/¹³CO isotope ratio in turbulent molecular clouds

The line emission of carbon monoxide (CO) isotopes is a frequently used column density (total number of atoms or molecules of a given species in line of sight) and mass tracers in observational studies of the interstellar medium. At intermediate and high densities, however, the emission of the most abundant CO isotope, ¹²CO, saturates and therefore provides only lower limit on the real column density. In these regions the less abundant ¹³CO isotope is used. When the ¹³CO observations are converted to ¹²CO column density a globally constant (i.e. uniform) ¹²CO/¹³CO isotopic ratio is often adopted. A similar assumption is frequently made when simulations are compared to observations of real clouds. We know that this assumption, at least amongst some conditions, is incorrect. This is because isotope selective reactions, such as preferential photodissociation and chemical fractionation are active in certain cloud regions (see e.g. van Dishoeck & Black 1988 or Visser et al. 2009). These reactions, depending on which of them is the more efficient how in the given region, can increase or decrease the ¹²CO/¹³CO ratio. Ignoring these effects may lead to under- or overestimation of the real CO column density.

In this study we aim to test and improve the above mentioned assumption in the context of inferring ¹³CO emission from simulations which neglect isotopic chemistry and of calculating ¹²CO column density from ¹³CO observations. We follow the coupled chemical, thermal and dynamical evolution of isolated, starless molecular clouds with several different metallicities, radiation fields and initial densities.

Fig. 1. Quantitative picture of the isotopic carbon chemistry in molecular clouds. The diffuse gas (gray) is followed by the translucent (yellow and blue) and the dense (orange) regions. Green and red reactions are CO production and destruction pathways, respectively. Font size shows how important a reaction or a species is in a given region. The qualitative picture of CO isotope chemistry can be summarised as follows (see also Fig. 1): The preferred pathways of CO production are the ion-neutral reaction of C⁺ and OH producing HCO⁺, which dissociatively recombines to CO and H, and the neutral-neutral reaction of CH or CH2 with an oxygen atom. These reactions are not isotope-selective and work with varying efficiency through the whole molecular cloud. In the diffuse regions (gray) the photodissociation of CO by interstellar far-ultraviolet photons dominates over the production reactions and most of the carbon is in ionized form. In translucent regions the CO production rates start to compete with the photodissociation. The ¹²CO column density becomes high enough to shield the ¹²CO molecules from the high energy photons. However, ¹³CO, due its lower column density is less effectively shielded. This selective photodissociation leads to an increased isotopic ratio. Further in (blue), due to the shielding by dust absorption and the increasing ¹³CO column density both isotopic species are effectively protected from the dissociating radiation. In this region ionized carbon is still abundant and the chemical fractionation reaction (described by Watson et al. 1976) becomes important. The fractionation reaction at high temperatures (>50 K) could go two ways: one enhances ¹³CO and ¹²C+, while the other enhances ¹²CO and ¹³C⁺. At temperatures typical to the corresponding cloud depths (15-20 K), the former, exothermic reaction is preferred, resulting in more ¹³CO production, and consequently in a reduced isotopic ratio. In these cloud regions the production of ¹³CO and the destruction of ¹²CO are dominated by the chemical fractionation reaction. At the highest column densities (orange) the production and destruction of CO isotopes are overtaken by the non-isotope-selective HCO⁺ recombination and dissociative charge transfer with He⁺, as a result the isotope ratio approaches to the elemental (¹²C/¹³C) ratio. The details isotopic structure of the cloud, however, depends on the physical properties, such as temperature, density and irradiation.

Fig. 2. CO isotopic column density ratio as a function of the ¹²CO column density in case of our 6 simulations (see the legend for the model parameters). The vertical dashed line shows the detection limit of millimetre-wavelength CO emission achieved in the Taurus molecular cloud (Goldsmith et al. 2008). The colour indicates the mass fraction of the cloud with the specific column density and isotope ratio. The black dotted curves represent the mean ratios at given column densities.

We examined the CO isotope ratio in clouds with various physical conditions (see Fig. 2). We find that it shows a close correlation to both the ¹²CO and the ¹³CO column densities, with only a week dependence on the cloud properties. The ratio varies within a factor of 2-3 with the column density. When a uniform ratio is assumed, this results in up to 60 percent error in ¹²CO column density and mass estimates derived from observed ¹³CO column density. The same assumption results in up to 50 percent difference in the intensity of ¹³CO emission derived from ¹²CO number density distributions. We show that this effect can be corrected by the use of a fitting formula derived from our simulations. The proposed formula is consistent with millimetre-wavelength measurements of the CO isotope ratio, but underestimates the ratios from ultraviolet absorption data. The reason for the discrepancy in the latter case might be that that these measurements are not tracing individual clouds, but an accumulation of low density material along the line of sight or that physical properties of the traced clouds are fundamentally different from our simulations (e.g. more similar to PDR regions). The fitting formula nevertheless allows us to obtain more accurate ¹³CO emission maps from simulations and to estimate the isotope ratio -- therefore the ¹²CO column density -- from ¹³CO observations.

Herschel view on disks around low-mass stars

Fig. 1. SED of the M0.5 spectral type star, J11071206-7632232. The solid black line shows the best fitting model, while the solid gray lines show the models within the 68% confidence interval. The understanding of the stellar mass dependence of protoplanetary disk parameters is necessary to complete our picture of disk formation and evolution. Furthermore, depending on how different the disk properties are around low-mass and massive stars, we might find very similar or very different planetary systems as a function of stellar mass. Previous studies found longer disk lifetimes, lower accretion rates, and different organic chemistry in disks around low-mass stars and brown dwarfs, while other studies find stellar mass independent turbulent mixing strength, and similar disk scale heights. To better constrain the (in)dependence of disk parameters a good spectral energy distribution (SED) coverage of these objects is required. While due to larger signal-to-noise ratio, T Tauri disks are relatively well studied at all wavelengths, this is not the case for low-mass star and brown dwarf disks. With the aid of sensitive Herschel observations, Harvey et al. (2012) studied a large sample of brown dwarf disks, and found lower median disk masses than in case of T Tauri disks, while the scale heights and flaring indexes cover similar ranges, independent of the stellar mass.

In this study we aimed to complete the stellar mass spectrum between T Tauri stars and brown dwarfs by studying a large number of young, M dwarfs. We selected 62 coeval, low-mass, disk bearing stars from the Chamaleon-I star forming region, and observed them with Herschel. From the selected objects we managed to detect 17 stars in PACS 70, 100 and 160 micron bands. With the addition of 2MASS, WISE, Spitzer IRAC/MIPS and IRS data we got a well sampled SED between J band and 160 micron. For each object we constructed a grid of radiative transfer models to constrain the disk properties. The considered model parameters are the total dust mass, inner disk radius, flaring index, scale height at 100 AU and the disk inclination. To better model the dust emission features, we introduced a population of large grains (from 0.03 micron to 1 mm), and an independent population of small grains (from 0.01 micron to 0.1 micron), with their own scale heights.

Fig. 2. left: Disk-to-stellar mass ratio as the function of stellar mass collected from the literature and from this work. Filled symbols correspond to masses derived from far-IR observations (and SED fitting), while the rest is from sub-mm measurements. right: flaring index as function of mid- to far-IR colour. Higher flaring index results in a flatter SED in the mid- to far-IR wavelength range. We find such correlation between the observed colour and the fitted flaring index; [11.56]-[100] = 0.5 seems to separate flat and flared disks well.

One important result from the modelling is that most of the detected disks -- in contrast with brown dwarf disks -- are not fully optically thin at far-IR wavelengths, thus only upper limit on the total dust mass could be determined. The model grid also helps us to translate the detection limits to disk mass and flaring indexes: we find that the undetected sources should have disk masses lower than 10^-5 M_⊙ and/or flaring indexes around 1.1. When combining our results with disk mass measurements from the literature (see Fig. 2.), we find that disk mass estimates based on far-IR emission (including our work) tend to result in lower disk masses than those determined by sub-mm emission. In the case of more massive disks, this could be a consequence of optically thick emission and/or it could be because far-IR observations might miss cool dust in the outer disk. In case of low-mass disks (e.g. around brown dwarfs) the far-IR emission is less effected by optical depth effects, thus it could be used as a dust mass tracer.