arXiv subjects

Sigurd S. Jensen

Publications and source records attributed to Sigurd S. Jensen.

4 recordsLinked to original sources

arXiv2026

A tale of two isotopes: Spatial variation in HCN fractionation toward young cores

Context. Isotopic fractionation can serve as a powerful tracer of the chemical evolution during star and planet formation. To accurately interpret observations, it is crucial to identify the dominant pathways of nitrogen and carbon fractionation at different evolutionary stages. Aims. We aim to study nitrogen and carbon fractionation in a sample of young cores at the onset of star formation. Methods. We map H$^{13}$CN and HC$^{15}$N around one starless and three pre-stellar cores. We compute the $N$(H$^{13}$CN)/$N$(HC$^{15}$N) column density ratio across the cores and compare the distribution with $N$(H$_2$) maps from $\textit{Herschel}$/SPIRE. In addition, we calculate $^{14}$N/$^{15}$N maps using the double isotope method for comparison with earlier studies. The results are compared with astrochemical modeling of carbon and nitrogen fractionation for a one-dimensional pre-stellar core model. Results. The computed $N$(H$^{13}$CN)/$N$(HC$^{15}$N) ratio exhibit clear spatial variation across the maps. This variation is correlated with $N$(H$_2$) in three out of four cores. Conclusions. Our analysis reveals a correlation between the H$^{13}$CN/HC$^{15}$N ratios and the $N$(H$_2$) maps. According to the astrochemical model, the correlation is mainly due to variations in the $^{12}$C/$^{13}$C ratio. Consequently, the results caution against applying the double-isotope method to derive $^{14}$N/$^{15}$N ratios without independently assessing possible spatial variations in the $^{12}$C/$^{13}$C ratio. Furthermore, the leading cause of the isotopic variation in the model is not isotope-selective photodissociation, but rather more efficient fractionation through exchange reactions at lower temperatures in the denser regions of the cores.

astro-ph.SR↗

arXiv2021

Imaging the water snowline around protostars with water and HCO$^+$ isotopologues

The water snowline location in protostellar envelopes provides crucial information about the thermal structure and the mass accretion process as it can inform about the occurrence of recent ($\lesssim$1,000 yr) accretion bursts. In addition, the ability to image water emission makes these sources excellent laboratories to test indirect snowline tracers such as H$^{13}$CO$^+$. We study the water snowline in five protostellar envelopes in Perseus using a suite of molecular line observations taken with the Atacama Large Millimeter/submillimeter Array (ALMA) at $\sim$0.2$^{\prime\prime}-$0.7$^{\prime\prime}$ (60--210 au) resolution. B1-c provides a textbook example of compact H$_2^{18}$O ($3_{1,3}-2_{2,0}$) and HDO ($3_{1,2}-2_{2,1}$) emission surrounded by a ring of H$^{13}$CO$^+$ ($J=2-1$) and HC$^{18}$O$^+$ ($J=3-2$). Compact HDO surrounded by H$^{13}$CO$^+$ is also detected toward B1-bS. The optically thick main isotopologue HCO$^+$ is not suited to trace the snowline and HC$^{18}$O$^+$ is a better tracer than H$^{13}$CO$^+$ due to a lower contribution from the outer envelope. However, since a detailed analysis is needed to derive a snowline location from H$^{13}$CO$^+$ or HC$^{18}$O$^+$ emission, their true value as snowline tracer will lie in the application in sources where water cannot be readily detected. For protostellar envelopes, the most straightforward way to locate the water snowline is through observations of H$_2^{18}$O or HDO. Including all sub-arcsecond resolution water observations from the literature, we derive an average burst interval of $\sim$10,000 yr, but high-resolution water observations of a larger number of protostars is required to better constrain the burst frequency.

astro-ph.GA↗

arXiv2018

Explaining the luminosity spread in young clusters: proto and pre-main sequence stellar evolution in a molecular cloud environment

Hertzsprung-Russell diagrams of star forming regions show a large luminosity spread. This is incompatible with well-defined isochrones based on classic non-accreting protostellar evo- lution models. Protostars do not evolve in isolation of their environment, but grow through accretion of gas. In addition, while an age can be defined for a star forming region, the ages of individual stars in the region will vary. We show how the combined effect of a protostellar age spread, a consequence of sustained star formation in the molecular cloud, and time-varying protostellar accretion for individual protostars can explain the observed luminosity spread. We use a global MHD simulation including a sub-scale sink particle model of a star forming region to follow the accretion process of each star. The accretion profiles are used to compute stellar evolution models for each star, incorporating a model of how the accretion energy is distributed to the disk, radiated away at the accretion shock, or incorporated into the outer layers of the protostar. Using a modelled cluster age of 5 Myr we naturally reproduce the lumi- nosity spread and find good agreement with observations of the Collinder 69 cluster, and the Orion Nebular Cluster. It is shown how stars in binary and multiple systems can be externally forced creating recurrent episodic accretion events. We find that in a realistic global molecular cloud model massive stars build up mass over relatively long time-scales. This leads to an important conceptual change compared to the classic picture of non-accreting stellar evolution segmented in to low-mass Hayashi tracks and high-mass Henyey tracks.

astro-ph.SR↗

arXiv2018

Episodic accretion: the interplay of infall and disc instabilities

Using zoom-simulations carried out with the adaptive mesh-refinement code RAMSES with a dynamic range of up to $2^{27} \approx 1.34 \times 10^8$ we investigate the accretion profiles around six stars embedded in different environments inside a (40 pc)$^3$ giant molecular cloud, the role of mass infall and disc instabilities on the accretion profile, and thus on the luminosity of the forming protostar. Our results show that the environment in which the protostar is embedded determines the overall accretion profile of the protostar. Infall on to the circumstellar disc may trigger gravitational disc instabilities in the disc at distances of around ~10 to ~50 au leading to rapid transport of angular momentum and strong accretion bursts. These bursts typically last for about ~10 to a ~100 yr, consistent with typical orbital times at the location of the instability, and enhance the luminosity of the protostar. Calculations with the stellar evolution code mesa show that the accretion bursts induce significant changes in the protostellar proper- ties, such as the stellar temperature and radius. We apply the obtained protostellar properties to produce synthetic observables with RADMC3D and predict that accretion bursts lead to ob- servable enhancements around 20 to 200 $μ$m in the spectral energy distribution of Class 0 type young stellar objects.

astro-ph.SR↗