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Main Colloquium |
Dr. Pierre Hily-Blant
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UGA/IUF/IPAG, Grenoble
To which extent did the primitive solar system keep memory of its
interstellar past is a key question from planetary science and
astrophysical perspectives. More generally, what is the interstellar
heritage of planetary systems ? Most of our knowledge about the earliest
stages of the solar system comes from isotopic ratios, because they
trace the history of volatile elements. Another such tracer is the
ortho-to-para ratio of relevant species such as water. In this talk, I
will focus on one element namely, nitrogen. Our starting point for the
history of nitrogen in the solar system will be comets for which all
observations converge towards a single value of the 14N/15N isotopic
ratio of ~140. This threefold 15N-enrichment with respect to the
elemental 14N/15N ratio of 441 in the PSN may have several origins: 1/
cometary carriers of the primary reservoir of nitrogen have not been
observed so far, 2/ fractionation in cometary nuclei over the last 4.6
Gyr, 3/ fractionation in the protosolar nebula by e.g. selective
photodissociation of N2, or 4/ fractionation
in the interstellar cloud where the Sun formed. We will discuss these
possibilities in the light of measurements of the nitrogen isotopic
ratios performed recently in prestellar cores, protostars, disks, and
comets. In particular, the direct measurement of the 14N/15N ratio in
the disk orbiting TW Hya performed with ALMA rules out fractionation in
cometar nuclei as an important process. The measured ratio of 323+/-30
in TW Hya is interpreted as the elemental isotopic ratio of nitrogen in
the present-day solar neighbourhood, in excellent agreement with
galactic chemical evolution models. I will also present the recent
direct determination of the HCN/HC15N in the L1498 prestellar core,
suggesting that nitrogen fractionation is not efficient in such a cold
environement, although source-to-source variations cannot be excluded. I
will also present recent models of the ortho:para chemistry in
collapsing prestellar cores. Finally, I will conclude with current
challenges from observational, theoretical, and modeling perspectives.