Galactic Field Research Articles

X-Ray Census of Millisecond Pulsars in the Galactic Field

Abstract We have conducted a systematic survey for the X-ray properties of millisecond pulsars (MSPs). Currently, there are 47 MSPs with confirmed X-ray detections. We have also placed the upper limits for the X-ray emission from the other 36 MSPs by using the archival data. We have normalized their X-ray luminosities L x and their effective photon indices Γ into a homogeneous data set, which enables us to carry out a detailed statistical analysis. Based on our censored sample, we report a relation of L x ≃ 10 31.05 ( E ˙ / 10 35 ) 1.31 erg s−1 (2–10 keV) for the MSPs. The inferred X-ray conversion efficiency is found to be lower than the previously reported estimate that could be affected by selection bias. L x also correlates/anti-correlates with the magnetic field strength at the light cylinder B LC/characteristic age τ. On the other hand, there is no correlation between L x and their surface magnetic field strength B s . We have further divided the sample into four classes: (i) black-widows, (ii) redbacks, (iii) isolated MSPs, and (iv) other MSP binaries, and compare the properties among them. We noted that while the rotational parameters and the orbital periods of redbacks and black-widows are similar, L x of redbacks are significantly higher than those of black-widows in the 2–10 keV band. Also the Γ of redbacks are apparently smaller than those of black-widows, which indicates that the X-ray emission of redbacks are harder than that of black-widows. This can be explained by the different contribution of intrabinary shocks in the X-ray emission of these two classes.

The Long-term Evolution of Star Clusters Formed with a Centrally Peaked Star Formation Efficiency Profile

Abstract We have studied the long-term evolution of star clusters of the solar neighborhood, starting from their birth in gaseous clumps until their complete dissolution in the Galactic tidal field. We have combined the “local-density-driven cluster formation model” of Parmentier & Pfalzner with direct N-body simulations of clusters following instantaneous gas expulsion. We have studied the relation between cluster dissolution time, t dis, and cluster “initial” mass, M init, defined as the cluster mass at the end of the dynamical response to gas expulsion (i.e., violent relaxation), when the cluster age is 20–30 Myr. We consider the “initial” mass to be consistent with other works that neglect violent relaxation. The model clusters formed with a high star formation efficiency (SFE, i.e., gas mass fraction converted into stars) follow a tight mass-dependent relation, in agreement with previous theoretical studies. However, the low-SFE models present a large scatter in both the “initial” mass and the dissolution time, a shallower mass-dependent relation than high-SFE clusters, and most dissolve within 1 Gyr (cluster teenage mortality). Both groups differ in their structural properties on average. Combining two populations of clusters, high- and low-SFE ones, with domination of the latter, yields a cluster dissolution time for the solar neighborhood in agreement with that inferred from observations, without any additional destructive processes such as giant molecular cloud encounters. An apparent mass-independent relation may emerge for our low-SFE clusters when we neglect low-mass clusters (as expected for extragalactic observations), although more simulations are needed to investigate this aspect.

To Be or Not to Be: EHB Stars and AGB Stars

AbstractThe formation of EHB stars is linked to the lives of AGB stars by indications that such EHB/sdB stars might form in globular clusters with multiple populations linked to AGB evolution. Observations of massive globular clusters, such as ω-Centauri (Bedin et al.2004, Piotto et al.2005) suggest that single EHB stars might form from He-enhanced progenitors (D’Antona et al.2005, D’Antona & Caloi 2008, Lee et al.2005) in environments enriched by AGB ejecta. The studies conducted by Han et al. (2002), Han et al. (2003), and Han et al. (2007) have been able to provide a strong case for the binary formation of EHB/sdB stars in the Galactic field, though binary formation channels in globular clusters is uncertain. Simulations presented here are an extension of the simulations of Han et al. (2002) and Han et al. (2003), for low metallicities to examine the binary EHB population in globular clusters.

Abundance of zinc in the red giants of Galactic globular cluster 47 Tucanae

Aims. We investigate possible relations between the abundances of zinc and the light elements sodium, magnesium, and potassium in the atmospheres of red giant branch (RGB) stars of the Galactic globular cluster 47 Tuc and study connections between the chemical composition and dynamical properties of the cluster RGB stars. Methods. The abundance of zinc was determined in 27 RGB stars of 47 Tuc using 1D local thermal equilibrium (LTE) synthetic line profile fitting to the high-resolution 2dF/HERMES spectra obtained with the Anglo-Australian Telescope (AAT). Synthetic spectra used in the fitting procedure were computed with the SYNTHE code and 1D ATLAS9 stellar model atmospheres. Results. The average 1D LTE zinc-to-iron abundance ratio and its RMS variations due to star-to-star abundance spread determined in the sample of 27 RGB stars is 〈[Zn/Fe]〉1D LTE = 0.11 ± 0.09. We did not detect any statistically significant relations between the abundances of zinc and those of light elements. Neither did we find any significant correlation or anticorrelation between the zinc abundance in individual stars and their projected distance from the cluster center. Finally, no statistically significant relation between the absolute radial velocities of individual stars and the abundance of zinc in their atmospheres was detected. The obtained average [Zn/Fe]1DLTE ratio agrees well with those determined in this cluster in earlier studies and nearly coincides with that of Galactic field stars at this metallicity. All these results suggest that nucleosynthesis of zinc and light elements proceeded in separate, unrelated pathways in 47 Tuc.

Constraining the multiplicity statistics of the coolest brown dwarfs: binary fraction continues to decrease with spectral type

Binary statistics of the latest-type T and Y brown dwarfs are sparse and it is unclear whether the trends seen in the multiplicity properties of their more massive counterparts hold for the very coolest brown dwarfs. We present results from a search for substellar and planetary-mass companions to a sample of 12 ultracool T8$-$Y0 field brown dwarfs with the Hubble Space Telescope/Wide Field Camera 3. We find no evidence for resolved binary companions among our sample down to separations of 0.7$-$2.5 AU. Combining our survey with prior searches, we place some of the first statistically robust constraints to date on the multiplicity properties of the coolest, lowest-mass brown dwarfs in the field. Accounting for observational biases and incompleteness, we derive a binary frequency of $f = 5.5^{+5.2}_{-3.3}$% for T5$-$Y0 brown dwarfs at separations of 1.5$-$1000 AU, for an overall binary fraction of $f_\mathrm{tot} = 8\pm6$%. Modelling the projected separation as a lognormal distribution, we find a peak in separation at $\rho_0 = 2.9^{+0.8}_{-1.4}$ AU with a logarithmic width of $\sigma = 0.21^{+0.14}_{-0.08}$. We infer a mass ratio distribution peaking strongly towards unity, with a power law index of $\gamma = 6.1^{+4.0}_{-2.7}$, reinforcing the significance of the detection of a tighter and higher mass ratio companion population around lower-mass primaries. These results are consistent with prior studies and support the idea of a decreasing binary frequency with spectral type in the Galactic field.

Pulsational instabilities driven by the ∈ mechanism in hot pre-horizontal branch stars

Context. The ∈ mechanism is a self-excitation mechanism of stellar pulsations that acts in regions inside the star where nuclear burning takes place. It has been shown that the ∈ mechanism can excite pulsations in models of hot pre-horizontal branch stars before they settle into the stable helium core-burning phase. Moreover, it has been shown that this mechanism could explain the shortest periods of LS IV-14°116, a mild He-sdBV star. Aims. We aim to study the ∈ mechanism in stellar models appropriate for hot pre-horizontal branch stars to predict their pulsational properties and the instability domain in the log g − log Teff plane. Methods. We performed detailed computations of non-adiabatic non-radial pulsations on stellar models during the helium subflashes just before the helium-core burning phase. Computations were carried out for different values of initial helium composition, metallicity, and envelope mass at the moment of helium flash. Results. We find an instability domain of long-period gravity modes due to the ∈ mechanism in the log g − log Teff plane at roughly 22 000 K ≲ Teff ≲ 50 000 K and 4.67 ≲ log g ≲ 6.15. Consequently, we find instabilities due to the ∈ mechanism on pre-extreme horizontal branch stellar models (Teff ≳ 22 000 K), but not on pre-blue horizontal branch stellar models (Teff ≲ 21 000 K). The periods of excited modes range between ~200 and ~2000 s. Comparison with the three known pulsating He-rich subdwarfs shows that ∈ mechanism can excite gravity modes in stars with similar surface properties (He abundances, log g, log Teff), but in our models it is only able to excite modes in the range of the shortest observed periods. Conclusions. We predict a new instability strip for hot-subdwarf stars of which LS IV-14°116 could be the first inhabitant. Based on simple estimates we expect 1 to 10 stars in the current samples of hot-subdwarf stars to be pulsating by the ∈ mechanism. Our results could constitute a theoretical basis for future searches of pulsators in the Galactic field.

Searching for primordial magnetic fields with CMB B-modes

Was the primordial universe magnetized? The answer to this question would help explain the origin of micro-Gauss strength magnetic fields observed in galaxies. It is also of fundamental importance in developing a complete theory of the early universe. While there can be other signatures of cosmological magnetic fields, a signature in the cosmic microwave background (CMB) would prove their primordial origin. The B-mode polarization of CMB is particularly promising in this regard because there are relatively few other sources of B-modes, and because the vortical modes sourced by the primordial magnetic field (PMF) survive diffusion damping up to a small fraction of the Silk length. At present, the Planck temperature and polarization spectra combined with the B-mode spectrum measured by the South Pole Telescope (SPT) constrain the PMF strength to be no more than ∼1 nano-Gauss (nG). Because of the quartic scaling of the CMB anisotropy spectra with the PMF strength, this bound will not change by much even with the significantly better measurements of the B-mode spectrum by the Stage III and Stage IV CMB experiments. On the other hand, tightening the bound well below the 1 nG threshold would rule out the purely primordial origin (requiring no dynamo action) of galactic fields. Considering Faraday rotation, which converts some of the E-modes into B-modes and scales linearly with the field strength, will help to achieve this goal. As we demonstrate, the upcoming experiments, such as SPT-3G and the Simons Observatory, will be sensitive to fields of ∼0.5 nG strength thanks to the mode-coupling signature induced by Faraday rotation. A future Stage IV ground based experiment or a space probe will be capable of probing fields below 0.1 nG, and would detect a scale-invariant PMF of 0.2 nG strength without de-lensing or subtracting the galactic rotation measure.

Is stellar multiplicity universal? Tight stellar binaries in the Orion Nebula Cluster

We present a survey for the tightest visual binaries among 0.3-2 Msun members the Orion Nebula Cluster (ONC). Among 42 targets, we discovered 13 new 0.025-0.15" companions. Accounting for the Branch bias, we find a companion star fraction (CSF) in the 10-60 au range of 21+8/-5%, consistent with that observed in other star-forming regions (SFRs) and twice as high as among field stars; this excess is found with a high level of confidence. Since our sample is dominated by disk-bearing targets, this indicates that disk disruption by close binaries is inefficient, or has not yet taken place, in the ONC. The resulting separation distribution in the ONC drops sharply outside 60\,au. These findings are consistent with a scenario in which the initial multiplicity properties, set by the star formation process itself, are identical in the ONC and in other SFRs and subsequently altered by the cluster's dynamical evolution. This implies that the fragmentation process does not depend on the global properties of a molecular cloud, but on the local properties of prestellar cores, and that the latter are self-regulated to be nearly identical in a wide range of environments. These results, however, raise anew the question of the origin of field stars as the tight binaries we have discovered will not be destroyed as the ONC dissolves into the galactic field. It thus appears that most field stars formed in regions differ from well-studied SFRs in the Solar neighborhood, possibly due to changes in core fragmentation on Gyr timescales.

Signature of non-isotropic distribution of stellar rotation inclination angles in the Praesepe cluster

The distribution of the stellar rotation axes of 113 main sequence stars in the open cluster Praesepe are examined by using current photometric rotation periods, spectroscopic rotation velocities, and estimated stellar radii. Three different samples of stellar rotation data on spotted stars from the Galactic field and two independent samples of planetary hosts are used as control samples to support the consistency of the analysis. Considering the high completeness of the Praesepe sample and the behavior of the control samples, we find that the main sequence F − K stars in this cluster are susceptible to rotational axis alignment. Using a cone model, the most likely inclination angle is 76° ± 14° with a half opening angle of 47° ± 24°. Non-isotropic distribution of the inclination angles is preferred over the isotropic distribution, except if the rotation velocities used in this work are systematically overestimated. We found no indication of this being the case on the basis of the currently available data.

A Search for Extraterrestrial Intelligence (SETI) toward the Galactic Anticenter with the Murchison Widefield Array

Abstract Following the results of the first systematic modern low-frequency search for extraterrestrial intelligence using the Murchison Widefield Array (MWA), which was directed toward a Galactic Center field, we report a second survey toward a Galactic Anticenter field. Using the MWA in the frequency range 99–122 MHz over a three-hour period, a 625 deg2 field centered on Orion KL (in the general direction of the Galactic Anticenter) was observed with a frequency resolution of 10 kHz. Within this field, 22 exoplanets are known. At the positions of these exoplanets, we searched for narrowband signals consistent with radio transmissions from intelligent civilizations. No such signals were found with a 5σ detection threshold. Our sample is significantly different to the 45 exoplanets previously studied with the MWA toward the Galactic Center, since the Galactic Center sample is dominated by exoplanets detected using microlensing, and hence at much larger distances than the exoplanets toward the Anticenter, found via radial velocity and transit detection methods. Our average effective sensitivity to extraterrestrial transmitter power is therefore much improved for the Anticenter sample. Added to this, our data processing techniques have improved, reducing our observational errors, leading to our best detection limit being reduced by approximately a factor of four compared to our previously published results.

A High-precision Trigonometric Parallax to an Ancient Metal-poor Globular Cluster∗

Abstract Using the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST), we have obtained a direct trigonometric parallax for the nearest metal-poor globular cluster, NGC 6397. Although trigonometric parallaxes have been previously measured for many nearby open clusters, this is the first parallax for an ancient metal-poor population—one that is used as a fundamental template in many stellar population studies. This high-precision measurement was enabled by the HST/WFC3 spatial-scanning mode, providing hundreds of astrometric measurements for dozens of stars in the cluster and also for Galactic field stars along the same sightline. We find a parallax of 0.418 ± 0.013 ± 0.018 mas (statistical, systematic), corresponding to a true distance modulus of 11.89 ± 0.07 ± 0.09 mag (2.39 ± 0.07 ± 0.10 kpc). The V luminosity at the stellar main-sequence turnoff implies an absolute cluster age of 13.4 ± 0.7 ± 1.2 Gyr.

BANYAN. XI. The BANYAN Σ Multivariate Bayesian Algorithm to Identify Members of Young Associations with 150 pc

Abstract BANYAN Σ is a new Bayesian algorithm to identify members of young stellar associations within 150 pc of the Sun. It includes 27 young associations with ages in the range ∼1–800 Myr, modeled with multivariate Gaussians in six-dimensional (6D) XYZUVW space. It is the first such multi-association classification tool to include the nearest sub-groups of the Sco-Cen OB star-forming region, the IC 2602, IC 2391, Pleiades and Platais 8 clusters, and the ρ Ophiuchi, Corona Australis, and Taurus star formation regions. A model of field stars is built from a mixture of multivariate Gaussians based on the Besançon Galactic model. The algorithm can derive membership probabilities for objects with only sky coordinates and proper motion, but can also include parallax and radial velocity measurements, as well as spectrophotometric distance constraints from sequences in color–magnitude or spectral type–magnitude diagrams. BANYAN Σ benefits from an analytical solution to the Bayesian marginalization integrals over unknown radial velocities and distances that makes it more accurate and significantly faster than its predecessor BANYAN II. A contamination versus hit rate analysis is presented and demonstrates that BANYAN Σ achieves a better classification performance than other moving group tools available in the literature, especially in terms of cross-contamination between young associations. An updated list of bona fide members in the 27 young associations, augmented by the Gaia-DR1 release, as well as all parameters for the 6D multivariate Gaussian models for each association and the Galactic field neighborhood within 300 pc are presented. This new tool will make it possible to analyze large data sets such as the upcoming Gaia-DR2 to identify new young stars. IDL and Python versions of BANYAN Σ are made available with this publication, and a more limited online web tool is available at http://www.exoplanetes.umontreal.ca/banyan/banyansigma.php.

A survey for dwarf galaxy remnants around 14 globular clusters in the outer halo

We report the results of a systematic photometric survey of the peripheral regions of a sample of fourteen globular clusters in the outer halo of the Milky Way at distances d_GC>25 kpc from the Galactic centre. The survey is aimed at searching for the remnants of the host satellite galaxies where these clusters could originally have been formed before being accreted onto the Galactic halo. The limiting surface brightness varies within our sample, but reaches muV_lim=30-32 mag arcsec^-2. For only two globular clusters (NGC 7492 and Whiting 1; already suggested to be associated with the Sagittarius galaxy) we detect extended stellar populations that cannot be associated with either the clusters themselves or with the surrounding Galactic field population. We show that the lack of substructures around globular clusters at these Galactocentric distances is still compatible with the predictions of cosmological simulations whereby in the outer halo the Galactic globular cluster system is built up through hierarchical accretion at early epochs.

Abundance of Chemical Elements in RR Lyrae Variables and their Kinematic Parameters

A catalog of the chemical and spatial-kinematic parameters of 415 RR Lyrae variables (Lyrids) in the galactic field is compiled. Spectroscopic determinations of the relative abundances of 13 chemical elements in 101 of the RR Lyrae variables are collected from 25 papers published between 1995 and 2017. The data from different sources are reduced to a single solar abundance scale. The mean weighted chemical abundances are calculated with coefficients inversely proportional to the reported errors. An analysis of the deviations in the published relative abundances in each star from the mean square values calculated from them reveals an absence of systematic biases among the results from the various articles. The rectangular coordinates of 407 of the RR Lyrae variables and the components of the three-dimensional (3D) velocities of 401 of the stars are calculated using data from several sources. The collected data on the abundances of chemical elements produced by various nuclear fusion processes for the RR Lyrae variables of the field, as well as the calculated 3D velocities, can be used for studying the evolution of the Galaxy.

Metal-rich RRc Stars in the Carnegie RR Lyrae Survey

Abstract We describe and employ a stacking procedure to investigate abundances derived from the low signal-to-noise ratio spectra obtained in the Carnegie RR Lyrae Survey (CARRS). We find iron metallicities that extend from [Fe/H] ∼ −2.5 to values at least as large as [Fe/H] ∼ −0.5 in the 274-spectrum CARRS RRc data set. We consider RRc sample contamination by high amplitude solar metallicity δ Scuti stars (HADS) at periods less than 0.3 days, where photometric discrimination between RRc and δ Scuti stars has proven to be problematic. We offer a spectroscopic discriminant, the well-marked overabundance of heavy elements, principally [Ba/H], that is a common, if not universal, characteristic of HADS of all periods and axial rotations. No bona fide RRc stars known to us have verified heavy-element overabundances. Three out of 34 stars in our sample with [Fe/H] > −0.7 exhibit anomalously strong features of Sr, Y, Zr, Ba, and many rare earths. However, carbon is not enhanced in these three stars, and we conclude that their elevated n-capture abundances have not been generated in interior neutron-capture nucleosynthesis. Contamination by HADS appears to be unimportant, and metal-rich RRc stars occur in approximately the same proportion in the Galactic field as do metal-rich RRab stars. An apparent dearth of metal-rich RRc is probably a statistical fluke. Finally, we show that RRc stars have a similar inverse period–metallicity relationship as has been found for RRab stars.

Anomalous Cepheids discovered in a sample of galactic short period Type II Cepheids

We revisited the short period Type II Cepheids (T2Cs), called the BL Herculis (BLHs), in the Galactic Field to derive a homogeneous analysis of their Fourier parameters. Only V-band data were compiled to make sure that it was directly comparable between the known variables of the OGLE-III catalogue and the 59 individual objects classified as short period Type II Cepheids in the General Catalogue of Variable Stars (GCVS) we had in our sample. The derived Fourier parameters were used to make distinction between different classes of variables. From the 59 stars we found 19 BLHs, 19 fundamental mode Anomalous Cepheids (ACs) (8 of them were already known from the Catalina Sky Survey (CSS)), 1 first overtone AC, 2 were found to be possible peculiar W Virginis (pWVir), 11 classical Cepheids (DCEPs), and 7 stars were not pulsating variables at all. As a result we created a list of bright BLH stars in the Galactic Field, and separated the ACs, as well as other objects that were misclassified. The number of true BLHs decreased in our sample by more than 50%. We gathered the metallicity from spectroscopic measurements published in literature. While the number of actual measurements is low, it is highly suggestive that ACs are metal poor. The mean metallicity from 8 measurements in 4 stars (UY Eri having 5 different [Fe/H] data points) is -1.12 dex, but if the higher value metallicity outliers of UY Eri are left out the mean metallicity becomes -1.88 dex, regardless if the AC is in the Milky Way itself or in a cluster. On the other hand, BLHs seem to have a Solar-like metallicity of 0.00 dex averaged from 21 measurements of 10 stars.

On the Magnetic Field Inside the Solar Circle of the Galaxy: on the Possibility of Investigation of Some of Characteristics of the Interstellar Medium Using Pulsars with Large Faraday Rotation Values

To study some characteristics of the interstellar medium, observational data of pulsars with large Faraday rotation values (|RM| > 300 rad / m2) were used. It was suggested and justified that large |RM|values can be due to the contribution of the regions with increased electron concentration, projected on the pulsar. Most likely these are the HII regions, dark nebulae and molecular clouds. In these objects the magnetic field can be oriented in the direction of a large-scale field of the Galaxy, or simply is a deformed extension of the galactic field. It was shown that the Galactic distribution of rotation measures of pulsars with|RM|>300 rad/m2 corresponds to the circular model of the magnetic field of the Galaxy, with the counter-clockwise direction of the magnetic field in the galactocentric circle 5 kpc < R < 7 kpc.

Synergy Theory in Radiobiology.

Customized open-source software is used to characterize, exemplify, compare and critically evaluate mathematical/computational synergy analysis methods currently used in biology, and used or potentially applicable in radiobiology. As examples, we reanalyze some published results on murine Harderian gland tumors and on in vitro chromosome aberrations induced by exposure to single-ion radiations that simulate components of the galactic cosmic ray field. Baseline no-synergy/no-antagonism-mixture dose-effect relationships are calculated for corresponding mixed fields. No new experimental results are presented. Synergy analysis of effects due to a mixed radiation field whose components' individual dose-effect relationships are highly curvilinear should not consist of simply comparing to the sum of the components' effects. Such curvilinearity must often be allowed for in current radiobiology, especially when studying possible non-targeted ("bystander") effects. Consequently, many different synergy analysis theories are currently used in biology to replace simple effect additivity. We give evidence that for most synergy experiments and observations, incremental effect additivity is the most appropriate replacement. It has a large domain of applicability, being useful even when pronounced individual dose-effect relationship curvilinearity is a confounding factor. It allows calculation of 95% confidence intervals for baseline mixture dose-effect relationships taking into account parameter correlations; if non-targeted effects are important this gives much tighter intervals than neglecting the correlations. It always obeys two consistency conditions that simple effect additivity usually fails to obey: a "mixture of mixtures principle" and the standard "sham mixture principle". The mixture of mixtures principle is important in radiobiology because even nominally single-ion radiations are usually mixtures when they strike the biological target, due to intervening material. It is not yet clear whether mixing galactic cosmic ray components sometimes leads to statistically significant synergy for animal tumorigenesis. The substantial limitations of synergy theories are sometimes overlooked, and they warrant further study.

The Relationship of Sodium and Oxygen in Galactic Field RR Lyrae Stars

We analyzed 62 high-resolution spectra of 30 Galactic Field RR Lyrae-type stars with the aim of deriving their atmospheric parameters (Teff, , Vt), metallicity ([Fe/H]), radial velocities, and NLTE abundances of oxygen and sodium. We found that there is no clear anti-correlation between [O/Fe] and [Na/Fe] as is seen in globular clusters. On this basis, we conclude that the majority of field RR Lyrae-type stars should hardly be considered to be remnants of the dissolution of globular clusters.

ε-mechanism driven pulsations in hot subdwarf stars with mixed H-He atmospheres

Abstract The ε mechanism is a self-excitation mechanism of stellar pulsations which acts in regions where nuclear burning takes place. It has been shown that the ε mechanism can excite pulsations in hot pre-horizontal branch stars before they settle into the stable helium core-burning phase and that the shortest periods of LS IV-14º116 could be explained that way.We aim to study the ε mechanism in stellar models appropriate for hot pre-horizontal branch stars to predict their pulsational properties.We perform detailed computations of non-adiabatic non-radial pulsations on such stellar models.We predict a new instability domain of long-period gravity modes in the log g − log Teff plane at roughly 22000 K ≲ Teff ≲ 50000 K and 4.67 ≲ log g ≲ 6.15, with a period range from ~ 200 to ~ 2000 s. Comparison with the three known pulsating He-rich subdwarfs shows that the ε mechanism can excite pulsations in models with similar surface properties except for modes with the shortest observed periods. Based on simple estimates we expect at least 3 stars in the current samples of hot-subdwarf stars to be pulsating by the ε mechanism. Our results could constitute a theoretical basis for future searches of pulsators in the Galactic field.