Observations reveal complex environment of HD 50138

Spectra, visibilities, differential phases and closure
phase for each baseline observation, of the Brγ line (left)
and high-n Pfund lines (right) for HD 50138. The dashed
lines correspond to the systemic velocities of the lines.
Flux is normalized to the continuum. Credit: Koutoulaki et
al., 2018.

Using European Southern Observatory’s Very Large Telescope
(VLT) European astronomers have uncovered a complex
circumstellar environment of the star HD 50138. The finding,
which could provide important clues about the evolutionary
status of this star, was presented in a paper published May 3
on the arXiv pre-print repository.


Located some 1,100 light years away from the Earth, HD 50138 is
a Herbig Be star of spectral type B8, about seven times larger
and six times more massive than our sun. With a lumonisity of
approximately 1,000 solar luminosities, it is one of the
brightest Be in the southern sky.

The evolutionary status of HD 50138 is still unclear. Some
astronomers suggests that it as a pre-main sequence star, while
others propose that it should be considered as an evolved
object close to turn-off from the main sequence. Moreover, it
is also not clear whether HD 50138 is part of the Orion
Monoceros molecular cloud complex or not associated with any
star-forming region. Determining such a possible association
could provide additional hints of the age of this star.

In order to learn more insights into the nature of HD 50138, a
team of researchers led by Maria Koutoulaki of the Dublin
Institute for Advanced Studies in Dublin, Ireland, conducted
near-infrared interferometric observations of the innermost
circumstellar environment of this star. For this purpose they
employed the VLT-Interferometer (VLTI) with the beam combiner
AMBER (Astronomical Multi-BEam combineR). This instrument
allowed the scientists to find that the star’s circumstellar
environment is very complex.

“We report observations of the Herbig B[e] star, HD 50138,
using VLTI/AMBER interferometric observations in the K band at
medium spectral resolution. (…) Our results reveal that the
circumstellar environment is very complex,” the researchers
wrote in the paper.

Study unveils that the spectrum of HD 50138 shows continuum
emission, as well as hydrogen emission lines of bright
Brackett-gamma and faint high-n Pfund. The researchers
underlined that this in contrast to previous studies that
detected only continuum and Brackett-gamma line emission.

“Our observations allowed us to map the size of the continuum
emission and of the Brγ and high-n Pfund lines. The latter is
detected for the first time,” the paper reads.

The researchers estimated that the region of continuum emission
has a projected size between 0.6 and 1.0 AU. When it comes to
hydrogen lines, the projected size is about 0.4 AU. Moreover,
they noted that both continuum and Brackett-gamma lines were
found to trace an asymmetric origin but with opposing
directions of asymmetries.

In concluding remarks, the astronomers wrote that results
regarding and hydrogen lines, including
morphology of the regions indicate complex
circumstellar environment of HD 50138. They added that their
findings also suggest that the studied object is most likely an
evolved star.

“Finally, although we cannot exclude the possibility that HD
50138 is a young star our results point to an evolved source,”
the researchers concluded.

Explore further:

Researchers study complex morphology of the protoplanetary disc
around star MWC 758

More information: The circumstellar environment of
HD50138 revealed by VLTI/AMBER at high angular resolution,
arXiv:1805.01432 [astro-ph.SR] arxiv.org/abs/1805.01432

Abstract
HD50138 is a Herbig B[e] star with a circumstellar disc
detected at IR and mm wavelength. Its brightness makes it a
good candidate for NIR interferometry observations. We aim to
resolve, spatially and spectrally, the continuum and hydrogen
emission lines in the 2.12-2.47 micron region, to shed light on
the immediate circumstellar environment of the star. VLTI/AMBER
K-band observations provide spectra, visibilities, differential
phases, and closure phases along three long baselines for the
continuum, and HI emission in Brγ and five high-n Pfund lines.
By computing the pure-line visibilities, we derive the angular
size of the different line-emitting regions. A simple LTE model
was created to constrain the physical conditions of HI emitting
region. The continuum region cannot be reproduced by a
geometrical 2D elongated Gaussian fitting model. We estimate
the size of the region to be 1 au. We find the Brγ and Pfund
lines come from a more compact region of size 0.4 au. The Brγ
line exhibits an S-shaped differential phase, indicative of
rotation. The continuum and Brγ line closure phase show offsets
of ∼-25±5o and 20±10o, respectively. This is evidence of an
asymmetry in their origin, but with opposing directions. We
find that we cannot converge on constraints for the HI physical
parameters without a more detailed model. Our analysis reveals
that HD50138 hosts a complex circumstellar environment. Its
continuum emission cannot be reproduced by a simple disc
brightness distribution. Similarly, several components must be
evoked to reproduce the interferometric observables within the
Brγ, line. Combining the spectroscopic and interferometric data
of the Brγ and Pfund lines favours an origin in a wind region
with a large opening angle. Finally, our results point to an
evolved source.

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