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Register a new userDiscovery of fossil magnetic fields in the intermediate-mass pre-main sequence stars
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E. Alecian
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It is now well-known that the surface magnetic fields observed in cool, lower-mass stars on the main sequence (MS) are generated by dynamos operating in their convective envelopes. However, higher-mass stars (above 1.5 Msun) pass their MS lives with a small convective core and a largely radiative envelope. Remarkably, notwithstanding the absence of energetically-important envelope convection, we observe very strong (from 300 G to 30 kG) and organised (mainly dipolar) magnetic fields in a few percent of the A and B-type stars on the MS, the origin of which is not well understood. In this poster we propose that these magnetic fields could be of fossil origin, and we present very strong observational results in favour of this proposal.
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We use X-ray and infrared observations to study the properties of three classes of young stars in the Carina Nebula: intermediate-mass (2--8M$_odot$) pre-main sequence stars (IMPS; i.e. intermediate-mass T Tauri stars), late-B and A stars on the zero-age main sequence (AB), and lower-mass T Tauri stars (TTS). We divide our sources among these three sub-classifications and further identify disk-bearing young stellar objects versus diskless sources with no detectable infrared (IR) excess emission using IR (1--8 $mu$m) spectral energy distribution modeling. We then perform X-ray spectral fitting to determine the hydrogen absorbing column density ($N_{rm H}$), absorption-corrected X-ray luminosity ($L_{rm X}$), and coronal plasma temperature ($kT$) for each source. We find that the X-ray spectra of both IMPS and TTS are characterized by similar $kT$ and $N_{rm H}$, and on average $L_{rm X}$/$L_{rm bol} sim4times10^{-4}$. IMPS are systematically more luminous in X-rays (by $sim$0.3 dex) than all other sub-classifications, with median $L_{rm X} = 2.5times10^{31}$ erg s$^{-1}$, while AB stars of similar masses have X-ray emission consistent with TTS companions. These lines of evidence converge on a magneto-coronal flaring source for IMPS X-ray emission, a scaled-up version of the TTS emission mechanism. IMPS therefore provide powerful probes of isochronal ages for the first $sim$10 Myr in the evolution of a massive stellar population, because their intrinsic, coronal X-ray emission decays rapidly after they commence evolving along radiative tracks. We suggest that the most luminous (in both X-rays and IR) IMPS could be used to place empirical constraints on the location of the intermediate-mass stellar birth line.
We present initial result of a large spectroscopic survey aimed at measuring the timescale of mass accretion in young, pre-main-sequence stars in the spectral type range K0 - M5. Using multi-object spectroscopy with VIMOS at the VLT we identified the fraction of accreting stars in a number of young stellar clusters and associations of ages between 1 - 50 Myr. The fraction of accreting stars decreases from ~60% at 1.5 - 2 Myr to ~2% at 10 Myr. No accreting stars are found after 10 Myr at a sensitivity limit of $10^{-11}$ Msun yr-1. We compared the fraction of stars showing ongoing accretion (f_acc) to the fraction of stars with near-to-mid infrared excess (f_IRAC). In most cases we find f_acc < f_IRAC, i.e., mass accretion appears to cease (or drop below detectable level) earlier than the dust is dissipated in the inner disk. At 5 Myr, 95% of the stellar population has stopped accreting material at a rate of > 10^{-11} Msun yr-1, while ~20% of the stars show near-infrared excess emission. Assuming an exponential decay, we measure a mass accretion timescale (t_acc) of 2.3 Myr, compared to a near-to-mid infrared excess timescale (t_IRAC) of 2.9 Myr. Planet formation, and/or migration, in the inner disk might be a viable mechanism to halt further accretion onto the central star on such a short timescale.
[Abridged] The stellar Initial Mass Function (IMF) suggests that sub-solar stars form in very large numbers. Most attractive places for catching low-mass star formation in the act are young stellar clusters and associations, still (half-)embedded in star-forming regions. The low-mass stars in such regions are still in their pre--main-sequence (PMS) evolutionary phase. The peculiar nature of these objects and the contamination of their samples by the evolved populations of the Galactic disk impose demanding observational techniques for the detection of complete numbers of PMS stars in the Milky Way. The Magellanic Clouds, the companion galaxies to our own, demonstrate an exceptional star formation activity. The low extinction and stellar field contamination in star-forming regions of these galaxies imply a more efficient detection of low-mass PMS stars than in the Milky Way, but their distance from us make the application of special detection techniques unfeasible. Nonetheless, imaging with the Hubble Space Telescope yield the discovery of solar and sub-solar PMS stars in the Magellanic Clouds from photometry alone. Unprecedented numbers of such objects are identified as the low-mass stellar content of their star-forming regions, changing completely our picture of young stellar systems outside the Milky Way, and extending the extragalactic stellar IMF below the persisting threshold of a few solar masses. This review presents the recent developments in the investigation of PMS stars in the Magellanic Clouds, with special focus on the limitations by single-epoch photometry that can only be circumvented by the detailed study of the observable behavior of these stars in the color-magnitude diagram. The achieved characterization of the low-mass PMS stars in the Magellanic Clouds allowed thus a more comprehensive understanding of the star formation process in our neighboring galaxies.
Today, one of the greatest challenges concerning the Ap/Bp stars is to understand the origin of their slow rotation and their magnetic fields. The favoured hypothesis for the latter is the fossil field, which implies that the magnetic fields subsist throughout the different evolutionary phases, and in particular during the pre-main sequence phase. The existence of magnetic fields at the pre-main sequence phase is also required to explain the slow rotation of Ap/Bp stars. However, until recently, essentially no information was available about the magnetic properties of intermediate-mass pre-main sequence stars, the so-called Herbig Ae/Be stars. The new high-resolution spectropolarimeter ESPaDOnS, installed in 2005 at the Canada-France-Hawaii telescope, provided the capability necessary to perform surveys of the Herbig Ae/Be stars in order to investigate their magnetism and rotation. These investigations have resulted in the detection and/or confirmation of magnetic fields in 8 Herbig Ae/Be stars, ranging in mass from 2 to nearly 15 solar masses. In this contribution I will present the results of our survey, as well as their implications for the origin and evolution of the magnetic fields and rotation.
I have isolated a population of numerous F stars that appear to be pre-main-sequence (PMS). The candidate PMS stars have been identified using CM diagram, reddening, flux excess in the UV and near-infrared, and luminosity anomaly. Negative luminosity anomaly and excessive UV flux for many of these stars is suggestive of accretion disks, while the NIR excess is indicative in many cases of the presence of dust disk thermal emission. Observed overluminosity of many PMS candidates is consistent with their pre-main-sequence status. The bulk of the PMS candidates is located within 200 pc, exhibiting strong association with regions of star formation that are numerous between ~130 to 180 pc. The number of PMS candidates plummets redward of the spectral type ~ F5. This effect may provide important clues for understanding the evolution of PMS stars in the solar-mass range.
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