Magnetic field measurements in a sample of Class I and Flat-Spectrum protostars observed with SPIRou

Abstract

Magnetic fields play a crucial role throughout stellar evolution, regulating angular momentum, channelling accretion, and launching jets and outflows. While the magnetic properties of Classical T Tauri Stars (CTTS) are well characterised, those of their progenitors, Class I and Flat-Spectrum (FS) protostars, remain poorly constrained due to observational challenges linked to their embedded nature. We aim to detect and characterise large-scale magnetic fields in a sample of Class I and FS protostars, which are expected to host strong dynamo-generated fields. Using SPIRou, a high-resolution near-infrared spectropolarimeter, we analysed polarised spectra and applied the Least Squares Deconvolution (LSD) technique to extract magnetic signatures and measure longitudinal fields from Stokes V profiles. We report new detections of large-scale magnetic fields in 5 FS protostars. Including the previously known magnetic FS protostar V347 Aur, 40% of our sample (15 objects) is confirmed to be magnetic. These stars exhibit clear Zeeman signatures, with longitudinal field strengths ranging from ~80 to ~200 G. The remaining targets show no detectable Stokes V signature, with upper limits on dipolar fields between 500 G and >5 kG. These results indicate that Class I and FS protostars can host large-scale magnetic fields, possibly weaker than in CTTS, supporting the idea that magnetic processes are already active during the main accretion phase and may influence star-disk interactions from the earliest stages.

Figures (10)

• Figure 1: Upper panel: order 32 of the spectrum of VV CrA SW without normalisation (black full line). In blue are the data points selected to fit the continuum using a sigma-clipping of 3 and 2 iterations. The spline fit of order 3 is plotted in red with a number of knots of 2. The two regions with emission lines were manually removed using the interactive tool. Lower panel: the same spectrum after normalisation. The dashed red line shows the continuum normalised to 1.
• Figure 2: LSD profiles of V806 Tau. Different colours are used for the different observations. The Y-axes are I/Ic, N/Ic, and V/Ic, where Ic is the intensity of the stellar continuum. The $I$ profile represents the total intensity. Stokes $V$ shows a clear Zeeman signature, which corresponds to an actual magnetic detection because the signal is significantly non-null compared to the surrounding noise and the $N$ profile remains flat in the velocity domain.
• Figure 3: The distribution of projected rotational velocity for each star of the sample as a function of effective temperature. Magnetic stars are represented in red and undetected magnetic stars in black. For the magnetic stars, the size of the symbols are proportional to the maximum absolute values of the longitudinal magnetic field. For the non-detected magnetic stars, it is proportional to the minimum of the 3$\sigma$ uncertainty on $B_{\ell}$.
• Figure 4: Veiling in the K band as a function of the veiling in the H band for all observations of the sample. The size of the symbol is proportional to the uncertainty of the longitudinal magnetic field and each symbol corresponds to a different target. Magnetic stars are represented in red and stars without magnetic detections in black.
• Figure 5: Projected rotational velocities $v\sin i$, as a function of effective temperatures measured in this work (blue), and in the work of Flores2024 (red). The connected points with the dashed-line indicate the common stars in both samples.