193 research outputs found
Toward understanding the B[e] phenomenon: the curious case of IRAS 17449+2320
The existence of classical Be stars, stars that show emission lines due to a disk-like structure around the central object, has been recognize for more than one century. In contrast the history of the B[e] stars is much shorter. Conti suggested calling B-type stars that show low ionization forbidden emission lines as B[e]. Nowadays, we know that B[e] stars are an inhomogeneous group of objects so it is uncorrect to speak about a class of stars but it is better to talk about the B[e] phenomenon.
The B[e] phenomenon appears in early-type emission lines stars with low-excitation lines, forbidden lines, and hot dust detected by an IR excess. The phenomenon is important because appears in very different evolutionary stages of stars (in the intermediate mass pre-main sequence stars HAeB[e], in the main sequence stars MSB[e], in the evolved high-mass stars sgB[e], evolved low-mass stars cPNB[e], symbiotic stars symB[e] etc.). Studying this phenomenon we can better understand the physical mechanisms that appear during the life of a star and correlations between dynamical mechanisms (wind, rotation, eruptions, non radial pulsations, binarity etc.) and evolution.
It is important that B[e] objects harbor dust in a circumstellar disk and may be responsible for at least few percent of the Galactic circumstellar dust.
Dust forming B[e] stars exist in the lower metallicity environments of LMC and SMC so it is reasonable to suggest that they might have been important dust producers in the earlier Universe.
It is difficult to study these stars beacause they are embedded in a geometrically and optically thick circumstellar envelope, but, we can try to infer the physical properties by studying the interaction between the star and its environment.
The object of my analysis is IRAS 17449+2320, it is a 10 mag B[e] object situated in the Hercules constellation at a distance of 700 pc from the Sun.
IRAS 17449+2320 is catalogued as a A0V star and a possible FS CMa object, a subclass of the B[e] phenomenon.
There are observational and physical criteria for the identification of a FS CMa star.
Observational criteria:
- Emission line spectra contain H lines stronger than in Be, HAeBe, and normal supergiants; Fe II and [O I]; sometimes [Fe II] and weak [O III] lines.
-Large IR excess that peaks at 10-30 micrometer and sharply decreases longward.
-Location outside of star forming regions.
-When presents, a secondary companion (i) is typically fainter and cooler (if it is a normal star) than the primary or (ii) is degenerate.
Physical criteria:
-T_eff of the hot star/companion of about 9000- 30000 K (O9 - A2 type stars).
-Luminosity of the hot star/companion log (L/L_sun) approx 2.5 - 4.5
V_pri -V_sec > 2 mag
This is an observational thesis, so a special focus is on the data analysis. I will explain how to reduce spectral data, both slit and échelle, with IRAF software, then I will analyze the spectra taken from the Ondrejov Observatory, the Observatorio Astronomico Nacional San Pedro Martir , McDonald Observatory and the Canada France Hawaii Telescope. In particular, I will analyze line profile variations, expecially for the Balmer lines. Then, theoretical chapters follow, trying to describe the phenomenology behind the physical mechanisms inferred by the spectroscopic studies.
The Balmer lines have broad wings in absorption wide 3600 km/s (FWZI) but the core is composite by emission and absorption features. The variation of Balmer lines is only in the core and in the range of velocity of [-300,300] km/s, this means that the dynamical phenomena that are affecting these lines are playing at a distance lesser than an AU. The wings of Fe II, Na I D, O I and Ca II are broad as the range of velocity where the dynamical phenomena are affecting the Balmer lines. This can suggest that these lines are forming in the same region of the perturbations. Instead the width of [O I] emission lines suggest that [O I] are forming in a wider low density region and it is possible to use the doppler shift of these lines as an indicator of the radial velocity of the system that is RV=(-16\pm 2) km/s.
I found in the Balmer lines, the presence of composite absorption features moving from the blue to the red side with a time scale of few days, changing their velocity of 200 km/s.
This behaviour be explained by the presence of coherent but temporary structures that are orbiting around the star at about 0.2 AU where the radiation has a temperature of about approx 2900 K. The origin of this structure is not known, but some scenarios are the presence of global one-armed (i.e. azimuthal wave number m = 1) oscillations in the equatorial discs and the evaporation of a planet forming a multi-cometary tail.
Line variations are found on a range of timescales: few days for Balmer lines, and month or years for the other lines.
The presence of He I lines suggests that the star is earlier than thought by Miroshnichenko but not earlier than a B8V.
The wings of Balmer lines compared with the ones obtained rotating the spectrum of Vega gives an estimation of the projected rotational velocity. The best match is assuming v_{eq} sin(theta) approx 200 km/s, hence the star is a fast rotator.
FS CMa objects sometimes show binarity, in this case there is no clear evidence of a companion but binarity cannot be ruled out since a brown dwarf or a white dwarf do not produce detectable spectra
The Pristine Dwarf-Galaxy survey -- V. The edges of the dwarf galaxy Hercules
We present a new spectroscopic study of the dwarf galaxy Hercules (d ~ 132
kpc) with data from the Anglo-Australian Telescope and its AAOmega spectrograph
together with the Two Degree Field multi-object system to solve the conundrum
that whether Hercules is tidally disrupting. We combine broadband photometry,
proper motions from Gaia, and our Pristine narrow-band and
metallicity-sensitive photometry to efficiently weed out the Milky Way
contamination. Such cleaning is particularly critical in this kinematic regime,
as both the transverse and heliocentric velocities of Milky Way populations
overlap with Hercules. Thanks to this method, three new member stars are
identified, including one at almost 10rh of the satellite. All three have
velocities and metallicities consistent with that of the main body. Combining
this new dataset with the entire literature cleaned out from contamination
shows that Hercules does not exhibit a velocity gradient (d/dX =
0.1+0.4/-0.2 km s-1 arcmin-1) and, as such, does not show evidence to undergo
tidal disruption
Small-scale stellar haloes: detecting low surface brightness features in the outskirts of Milky Way dwarf satellites
Dwarf galaxies are valuable laboratories for dynamical studies related to
dark matter and galaxy evolution, yet it is currently unknown just how
physically extended their stellar components are. Satellites orbiting the
Galaxy's potential may undergo tidal stripping by the host, or alternatively,
may themselves have accreted smaller systems whose debris populates the dwarf's
own stellar halo. Evidence of these past interactions, if present, is best
searched for in the outskirts of the satellite. However, foreground
contamination dominates the signal at these large radial distances, making
observation of stars in these regions difficult. In this work, we introduce an
updated algorithm for application to Gaia data that identifies candidate member
stars of dwarf galaxies, based on spatial, color-magnitude and proper motion
information, and which allows for an outer component to the stellar
distribution. Our method shows excellent consistency with spectroscopically
confirmed members from the literature despite having no requirement for radial
velocity information. We apply the algorithm to all 60 Milky Way dwarf
galaxy satellites, and we find 9 dwarfs (Bo\"otes 1, Bo\"otes 3, Draco 2, Grus
2, Segue 1, Sculptor, Tucana 2, Tucana 3, and Ursa Minor) that exhibit evidence
for a secondary, low-density outer profile. We identify many member stars which
are located beyond 5 half-light radii (and in some cases, beyond 10). We argue
these distant stars are likely tracers of dwarf stellar haloes or tidal
streams, though ongoing spectroscopic follow-up will be required to determine
the origin of these extended stellar populations.Comment: 24 pages, 13 figures, 4 tables, submitted to MNRA
Stars on the edge: Galactic tides and the outskirts of the Sculptor dwarf spheroidal
Stars far beyond the half-light radius of a galaxy suggest the existence of a
mechanism able to move stars out of the region where most star formation has
taken place. The formation of these "stellar halos" are usually ascribed to the
effects of early mergers or Galactic tides, although fluctuations in the
gravitational potential due to stellar feedback is also a possible candidate
mechanism. A Bayesian algorithm is used to find new candidate members in the
extreme outskirts of the Sculptor dwarf galaxy. Precise metallicities and
radial velocities for two distant stars are measured from their spectra taken
with the Gemini South GMOS spectrograph. The radial velocity, proper motion and
metallicity of these targets are consistent with Sculptor membership. As a
result, the known boundary of the Sculptor dwarf extends now out to an
elliptical distance of half-light radii, which corresponds to a
projected physical distance of kpc. As reported in earlier work, the
overall distribution of radial velocities and metallicities indicate the
presence of a more spatially and kinematically dispersed metal-poor population
that surrounds the more concentrated and colder metal-rich stars. Sculptor's
density profile shows a "kink" in its logarithmic slope at a projected distance
of arcmin (620 pc), which we interpret as evidence that Galactic tides
have helped to populate the distant outskirts of the dwarf. We discuss further
ways to test and validate this tidal interpretation for the origin of these
distant stars.Comment: 10 pages, 4 figures, submitted to MNRA
Could very low-metallicity stars with rotation-dominated orbits have been shepherded by the bar?
The most metal-poor stars (e.g. [Fe/H] ) are the ancient fossils
from the early assembly epoch of our Galaxy, very likely before the formation
of the thick disc. Recent studies have shown that a non-negligible fraction of
them have prograde planar orbits, which makes their origin a puzzle. It has
been suggested that a later-formed rotating bar could have driven these old
stars from the inner Galaxy outward, and transformed their orbits to be more
rotation-dominated. However, it is not clear if this mechanism can explain
these stars as observed in the solar neighborhood. In this paper, we explore
the possibility of this scenario by tracing these stars backwards in an
axisymmetric Milky Way potential with a bar perturber. We integrate their
orbits backward for 6 Gyr under two bar models: one with a constant pattern
speed and another one with a decelerating speed. Our experiments show that,
under the constantly-rotating bar model, the stars of interest are little
affected by the bar and cannot have been shepherded from a spheroidal inner
Milky Way to their current orbits. In the extreme case of a rapidly
decelerating bar, some of the very metal-poor stars on planar and prograde
orbits can be brought from the inner Milky Way, but of them were
nevertheless already rotation-dominated ( 1000 km s
kpc) 6 Gyr ago. The chance of these stars having started with spheroid-like
orbits with small rotation ( 600 km s kpc) is very
low ( 3). We therefore conclude that, within the solar neighborhood, the
bar is unlikely to have shepherded a significant fraction of inner Galaxy
spheroid stars to produce the overdensity of stars on prograde, planar orbits
that is observed today.Comment: submitted to A&A, comments are welcom
Discovery of a new Local Group Dwarf Galaxy Candidate in UNIONS: Bo\"otes V
We present the discovery of Bo\"otes V, a new ultra-faint dwarf galaxy
candidate. This satellite is detected as a resolved overdensity of stars during
an ongoing search for new Local Group dwarf galaxy candidates in the UNIONS
photometric dataset. It has a physical half-light radius of
26.9 pc, a -band magnitude of 4.5 0.4 mag, and
resides at a heliocentric distance of approximately 100 kpc. We use Gaia DR3
astrometry to identify member stars, characterize the systemic proper motion,
and confirm the reality of this faint stellar system. The brightest star in
this system was followed up using Gemini GMOS-N long-slit spectroscopy and is
measured to have a metallicity of [Fe/H] 2.85 0.10 dex and a
heliocentric radial velocity of = 5.1 13.4 km s. Bo\"otes V
is larger (in terms of scale radius), more distant, and more metal-poor than
the vast majority of globular clusters. It is likely that Bo\"otes V is an
ultra-faint dwarf galaxy, though future spectroscopic studies will be necessary
to definitively classify this object.Comment: 13 pages, 7 figures, 3 tables. Accepted for publication in the AAS
Journals. Please note that this paper was submitted in coordination with the
work of William Cerny et al. 2022. These authors independently discovered
this same satellite so our two research groups have coordinated the
submission of these discovery paper
The discovery of the faintest known Milky Way satellite using UNIONS
We present the discovery of Ursa Major III/UNIONS 1, the least luminous known
satellite of the Milky Way, which is estimated to have an absolute V-band
magnitude of mag, equivalent to a total stellar mass of
16 M. Ursa Major III/UNIONS 1 was uncovered in the deep,
wide-field Ultraviolet Near Infrared Optical Northern Survey (UNIONS) and is
consistent with an old ( Gyr), metal-poor ([Fe/H] )
stellar population at a heliocentric distance of 10 kpc. Despite being
compact ( pc) and composed of so few stars, we confirm
the reality of Ursa Major III/UNIONS 1 with Keck II/DEIMOS follow-up
spectroscopy and identify 11 radial velocity members, 8 of which have full
astrometric data from and are co-moving based on their proper motions.
Based on these 11 radial velocity members, we derive an intrinsic velocity
dispersion of km s but some caveats preclude this
value from being interpreted as a direct indicator of the underlying
gravitational potential at this time. Primarily, the exclusion of the largest
velocity outlier from the member list drops the velocity dispersion to
km s, and the subsequent removal of an additional
outlier star produces an unresolved velocity dispersion. While the presence of
binary stars may be inflating the measurement, the possibility of a significant
velocity dispersion makes Ursa Major III/UNIONS 1 a high priority candidate for
multi-epoch spectroscopic follow-ups to deduce to true nature of this
incredibly faint satellite.Comment: 21 pages, 9 figures, 3 tables; Accepted for publication in Ap
The Pristine survey -- XII:Gemini-GRACES chemo-dynamical study of newly discovered extremely metal-poor stars in the Galaxy
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